diff --git a/src/graph.rs b/src/graph.rs
index 579e15c..6d8b0c5 100644
--- a/src/graph.rs
+++ b/src/graph.rs
@@ -13,6 +13,7 @@ mod formulas;
 pub mod iterators;
 
 use crate::{ComponentGraphConfig, Edge, Node};
+pub use formulas::Formula;
 use petgraph::graph::{DiGraph, NodeIndex};
 use std::collections::HashMap;
 
diff --git a/src/graph/formulas.rs b/src/graph/formulas.rs
index dedc9f1..0802ebf 100644
--- a/src/graph/formulas.rs
+++ b/src/graph/formulas.rs
@@ -12,9 +12,12 @@ use crate::Node;
 
 mod expr;
 mod fallback;
+mod formula;
 mod generators;
 mod traversal;
 
+pub use formula::Formula;
+
 /// Formulas for various microgrid metrics.
 impl<N, E> ComponentGraph<N, E>
 where
@@ -22,32 +25,32 @@ where
     E: Edge,
 {
     /// Returns a string representing the consumer formula for the graph.
-    pub fn consumer_formula(&self) -> Result<String, Error> {
+    pub fn consumer_formula(&self) -> Result<Formula, Error> {
         generators::consumer::ConsumerFormulaBuilder::try_new(self)?.build()
     }
 
     /// Returns a string representing the grid formula for the graph.
-    pub fn grid_formula(&self) -> Result<String, Error> {
+    pub fn grid_formula(&self) -> Result<Formula, Error> {
         generators::grid::GridFormulaBuilder::try_new(self)?.build()
     }
 
     /// Returns a string representing the producer formula for the graph.
-    pub fn producer_formula(&self) -> Result<String, Error> {
+    pub fn producer_formula(&self) -> Result<Formula, Error> {
         generators::producer::ProducerFormulaBuilder::try_new(self)?.build()
     }
 
     /// Returns a string representing the battery formula for the graph.
-    pub fn battery_formula(&self, battery_ids: Option<BTreeSet<u64>>) -> Result<String, Error> {
+    pub fn battery_formula(&self, battery_ids: Option<BTreeSet<u64>>) -> Result<Formula, Error> {
         generators::battery::BatteryFormulaBuilder::try_new(self, battery_ids)?.build()
     }
 
     /// Returns a string representing the CHP formula for the graph.
-    pub fn chp_formula(&self, chp_ids: Option<BTreeSet<u64>>) -> Result<String, Error> {
+    pub fn chp_formula(&self, chp_ids: Option<BTreeSet<u64>>) -> Result<Formula, Error> {
         generators::chp::CHPFormulaBuilder::try_new(self, chp_ids)?.build()
     }
 
     /// Returns a string representing the PV formula for the graph.
-    pub fn pv_formula(&self, pv_inverter_ids: Option<BTreeSet<u64>>) -> Result<String, Error> {
+    pub fn pv_formula(&self, pv_inverter_ids: Option<BTreeSet<u64>>) -> Result<Formula, Error> {
         generators::pv::PVFormulaBuilder::try_new(self, pv_inverter_ids)?.build()
     }
 
@@ -55,7 +58,7 @@ where
     ///
     /// This formula uses the `COALESCE` function to return the first non-null
     /// value from the components with the provided IDs.
-    pub fn coalesce(&self, component_ids: BTreeSet<u64>) -> Result<String, Error> {
+    pub fn coalesce(&self, component_ids: BTreeSet<u64>) -> Result<Formula, Error> {
         generators::generic::CoalesceFormulaBuilder::try_new(self, component_ids)?.build()
     }
 
@@ -63,7 +66,7 @@ where
     pub fn ev_charger_formula(
         &self,
         ev_charger_ids: Option<BTreeSet<u64>>,
-    ) -> Result<String, Error> {
+    ) -> Result<Formula, Error> {
         generators::ev_charger::EVChargerFormulaBuilder::try_new(self, ev_charger_ids)?.build()
     }
 }
diff --git a/src/graph/formulas/formula.rs b/src/graph/formulas/formula.rs
new file mode 100644
index 0000000..e99202e
--- /dev/null
+++ b/src/graph/formulas/formula.rs
@@ -0,0 +1,54 @@
+// License: MIT
+// Copyright © 2025 Frequenz Energy-as-a-Service GmbH
+
+//! This module defines the `Formula` struct and its operations.
+
+use super::expr::Expr;
+
+// This struct represents the microgrid metric formulas that are generated by
+// traversing the component graph.
+//
+// `Formula` objects can be added or subtracted from each other, and they can
+// converted to a string representation, before they are passed to an evaluator.
+#[derive(Debug, Clone)]
+pub struct Formula {
+    expr: Expr,
+}
+
+impl Formula {
+    pub(crate) fn new(expr: Expr) -> Self {
+        Formula { expr }
+    }
+}
+
+impl std::ops::Add for Formula {
+    type Output = Self;
+
+    fn add(self, rhs: Self) -> Self::Output {
+        Formula {
+            expr: self.expr + rhs.expr,
+        }
+    }
+}
+
+impl std::ops::Sub for Formula {
+    type Output = Self;
+
+    fn sub(self, rhs: Self) -> Self::Output {
+        Formula {
+            expr: self.expr - rhs.expr,
+        }
+    }
+}
+
+impl std::fmt::Display for Formula {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        self.expr.fmt(f)
+    }
+}
+
+impl From<Formula> for String {
+    fn from(formula: Formula) -> Self {
+        formula.expr.to_string()
+    }
+}
diff --git a/src/graph/formulas/generators/battery.rs b/src/graph/formulas/generators/battery.rs
index 045566c..e30822f 100644
--- a/src/graph/formulas/generators/battery.rs
+++ b/src/graph/formulas/generators/battery.rs
@@ -6,6 +6,8 @@
 use std::collections::BTreeSet;
 
 use crate::component_category::CategoryPredicates;
+use crate::graph::formulas::expr::Expr;
+use crate::graph::formulas::Formula;
 use crate::{ComponentGraph, Edge, Error, Node};
 
 pub(crate) struct BatteryFormulaBuilder<'a, N, E>
@@ -47,14 +49,14 @@ where
     /// This is the sum of all battery_inverters in the graph. If the
     /// battery_ids are provided, only the batteries with the given ids are
     /// included in the formula.
-    pub fn build(self) -> Result<String, Error> {
+    pub fn build(self) -> Result<Formula, Error> {
         if self.inverter_ids.is_empty() {
-            return Ok("0.0".to_string());
+            return Ok(Formula::new(Expr::number(0.0)));
         }
 
         self.graph
             .fallback_expr(self.inverter_ids, false)
-            .map(|expr| expr.to_string())
+            .map(Formula::new)
     }
 
     fn find_inverter_ids(
@@ -104,7 +106,7 @@ mod tests {
         builder.connect(grid, grid_meter);
 
         let graph = builder.build(None)?;
-        let formula = graph.battery_formula(None)?;
+        let formula = graph.battery_formula(None)?.to_string();
         assert_eq!(formula, "0.0");
 
         // Add a battery meter with one inverter and one battery.
@@ -115,7 +117,7 @@ mod tests {
         assert_eq!(meter_bat_chain.component_id(), 2);
 
         let graph = builder.build(None)?;
-        let formula = graph.battery_formula(None)?;
+        let formula = graph.battery_formula(None)?.to_string();
         assert_eq!(formula, "COALESCE(#3, #2, 0.0)");
 
         // Add a second battery meter with one inverter and two batteries.
@@ -125,18 +127,22 @@ mod tests {
         assert_eq!(meter_bat_chain.component_id(), 5);
 
         let graph = builder.build(None)?;
-        let formula = graph.battery_formula(None)?;
+        let formula = graph.battery_formula(None)?.to_string();
         assert_eq!(formula, "COALESCE(#3, #2, 0.0) + COALESCE(#6, #5, 0.0)");
 
-        let formula = graph.battery_formula(Some(BTreeSet::from([4])))?;
+        let formula = graph
+            .battery_formula(Some(BTreeSet::from([4])))?
+            .to_string();
         assert_eq!(formula, "COALESCE(#3, #2, 0.0)");
 
-        let formula = graph.battery_formula(Some(BTreeSet::from([7, 8])))?;
+        let formula = graph
+            .battery_formula(Some(BTreeSet::from([7, 8])))?
+            .to_string();
         assert_eq!(formula, "COALESCE(#6, #5, 0.0)");
 
         let formula = graph
-            .battery_formula(Some(BTreeSet::from([4, 8, 7])))
-            .unwrap();
+            .battery_formula(Some(BTreeSet::from([4, 8, 7])))?
+            .to_string();
         assert_eq!(formula, "COALESCE(#3, #2, 0.0) + COALESCE(#6, #5, 0.0)");
 
         // Add a third battery meter with two inverters with two connected batteries.
@@ -146,7 +152,7 @@ mod tests {
         assert_eq!(meter_bat_chain.component_id(), 9);
 
         let graph = builder.build(None)?;
-        let formula = graph.battery_formula(None)?;
+        let formula = graph.battery_formula(None)?.to_string();
         assert_eq!(
             formula,
             concat!(
@@ -157,8 +163,8 @@ mod tests {
         );
 
         let formula = graph
-            .battery_formula(Some(BTreeSet::from([12, 13])))
-            .unwrap();
+            .battery_formula(Some(BTreeSet::from([12, 13])))?
+            .to_string();
         assert_eq!(
             formula,
             "COALESCE(#11 + #10, #9, COALESCE(#11, 0.0) + COALESCE(#10, 0.0))"
@@ -171,7 +177,7 @@ mod tests {
         assert_eq!(meter_pv_chain.component_id(), 14);
 
         let graph = builder.build(None)?;
-        let formula = graph.battery_formula(None)?;
+        let formula = graph.battery_formula(None)?.to_string();
         assert_eq!(
             formula,
             concat!(
@@ -208,7 +214,7 @@ mod tests {
             allow_unspecified_inverters: true,
             ..Default::default()
         }))?;
-        let formula = graph.battery_formula(None)?;
+        let formula = graph.battery_formula(None)?.to_string();
         assert_eq!(
             formula,
             concat!(
@@ -220,22 +226,26 @@ mod tests {
         );
 
         let formula = graph
-            .battery_formula(Some(BTreeSet::from([19, 21])))
-            .unwrap();
+            .battery_formula(Some(BTreeSet::from([19, 21])))?
+            .to_string();
         assert_eq!(
             formula,
             "COALESCE(#20 + #18, #17, COALESCE(#20, 0.0) + COALESCE(#18, 0.0))"
         );
 
-        let formula = graph.battery_formula(Some(BTreeSet::from([19]))).unwrap();
+        let formula = graph
+            .battery_formula(Some(BTreeSet::from([19])))?
+            .to_string();
         assert_eq!(formula, "COALESCE(#18, 0.0)");
 
-        let formula = graph.battery_formula(Some(BTreeSet::from([21]))).unwrap();
+        let formula = graph
+            .battery_formula(Some(BTreeSet::from([21])))?
+            .to_string();
         assert_eq!(formula, "COALESCE(#20, 0.0)");
 
         let formula = graph
-            .battery_formula(Some(BTreeSet::from([4, 12, 13, 19])))
-            .unwrap();
+            .battery_formula(Some(BTreeSet::from([4, 12, 13, 19])))?
+            .to_string();
         assert_eq!(
             formula,
             concat!(
diff --git a/src/graph/formulas/generators/chp.rs b/src/graph/formulas/generators/chp.rs
index ebf2f44..803ee1e 100644
--- a/src/graph/formulas/generators/chp.rs
+++ b/src/graph/formulas/generators/chp.rs
@@ -6,6 +6,8 @@
 use std::collections::BTreeSet;
 
 use crate::component_category::CategoryPredicates;
+use crate::graph::formulas::expr::Expr;
+use crate::graph::formulas::Formula;
 use crate::{ComponentGraph, Edge, Error, Node};
 
 pub(crate) struct CHPFormulaBuilder<'a, N, E>
@@ -43,9 +45,9 @@ where
     ///
     /// This is the sum of all CHPs in the graph. If the chp_ids are provided,
     /// only the CHPs with the given ids are included in the formula.
-    pub fn build(self) -> Result<String, Error> {
+    pub fn build(self) -> Result<Formula, Error> {
         if self.chp_ids.is_empty() {
-            return Ok("0.0".to_string());
+            return Ok(Formula::new(Expr::number(0.0)));
         }
 
         for id in &self.chp_ids {
@@ -59,7 +61,7 @@ where
 
         self.graph
             .fallback_expr(self.chp_ids, false)
-            .map(|expr| expr.to_string())
+            .map(Formula::new)
     }
 }
 
@@ -78,7 +80,7 @@ mod tests {
         builder.connect(grid, grid_meter);
 
         let graph = builder.build(None)?;
-        let formula = graph.chp_formula(None)?;
+        let formula = graph.chp_formula(None)?.to_string();
         assert_eq!(formula, "0.0");
 
         // Add a chp meter with one chp
@@ -89,7 +91,7 @@ mod tests {
         assert_eq!(meter_chp_chain.component_id(), 2);
 
         let graph = builder.build(None)?;
-        let formula = graph.chp_formula(None)?;
+        let formula = graph.chp_formula(None)?.to_string();
         assert_eq!(formula, "COALESCE(#3, #2, 0.0)");
 
         // Add a battery meter with one inverter and two batteries.
@@ -99,7 +101,7 @@ mod tests {
         assert_eq!(meter_bat_chain.component_id(), 4);
 
         let graph = builder.build(None)?;
-        let formula = graph.chp_formula(None)?;
+        let formula = graph.chp_formula(None)?.to_string();
         assert_eq!(formula, "COALESCE(#3, #2, 0.0)");
 
         // Add a chp meter with two CHPs.
@@ -109,7 +111,7 @@ mod tests {
         assert_eq!(meter_chp_chain.component_id(), 8);
 
         let graph = builder.build(None)?;
-        let formula = graph.chp_formula(None)?;
+        let formula = graph.chp_formula(None)?.to_string();
         assert_eq!(
             formula,
             concat!(
@@ -118,7 +120,9 @@ mod tests {
             )
         );
 
-        let formula = graph.chp_formula(Some(BTreeSet::from([10, 3]))).unwrap();
+        let formula = graph
+            .chp_formula(Some(BTreeSet::from([10, 3])))?
+            .to_string();
         assert_eq!(formula, "COALESCE(#3, #2, 0.0) + COALESCE(#10, 0.0)");
 
         // add a meter direct to the grid with three CHPs
@@ -128,7 +132,7 @@ mod tests {
         assert_eq!(meter_chp_chain.component_id(), 11);
 
         let graph = builder.build(None)?;
-        let formula = graph.chp_formula(None)?;
+        let formula = graph.chp_formula(None)?.to_string();
         assert_eq!(
             formula,
             concat!(
@@ -143,8 +147,8 @@ mod tests {
         );
 
         let formula = graph
-            .chp_formula(Some(BTreeSet::from([3, 9, 10, 12, 13])))
-            .unwrap();
+            .chp_formula(Some(BTreeSet::from([3, 9, 10, 12, 13])))?
+            .to_string();
         assert_eq!(
             formula,
             concat!(
@@ -155,8 +159,8 @@ mod tests {
         );
 
         let formula = graph
-            .chp_formula(Some(BTreeSet::from([3, 9, 10, 12, 13, 14])))
-            .unwrap();
+            .chp_formula(Some(BTreeSet::from([3, 9, 10, 12, 13, 14])))?
+            .to_string();
         assert_eq!(
             formula,
             concat!(
@@ -170,7 +174,9 @@ mod tests {
             ),
         );
 
-        let formula = graph.chp_formula(Some(BTreeSet::from([10, 14]))).unwrap();
+        let formula = graph
+            .chp_formula(Some(BTreeSet::from([10, 14])))?
+            .to_string();
         assert_eq!(formula, "COALESCE(#10, 0.0) + COALESCE(#14, 0.0)");
 
         // Failure cases:
diff --git a/src/graph/formulas/generators/consumer.rs b/src/graph/formulas/generators/consumer.rs
index 0e17a05..cfa07ae 100644
--- a/src/graph/formulas/generators/consumer.rs
+++ b/src/graph/formulas/generators/consumer.rs
@@ -6,7 +6,10 @@
 use std::collections::{BTreeMap, BTreeSet};
 
 use super::super::expr::Expr;
-use crate::{component_category::CategoryPredicates, ComponentGraph, Edge, Error, Node};
+use crate::{
+    component_category::CategoryPredicates, graph::formulas::Formula, ComponentGraph, Edge, Error,
+    Node,
+};
 
 pub(crate) struct ConsumerFormulaBuilder<'a, N, E>
 where
@@ -35,7 +38,7 @@ where
     }
 
     /// Generates the consumer formula for the given node.
-    pub fn build(mut self) -> Result<String, Error> {
+    pub fn build(mut self) -> Result<Formula, Error> {
         let mut all_meters = None;
         while let Some(meter_id) = self.unvisited_meters.pop_first() {
             let consumption = self.component_consumption(meter_id)?;
@@ -60,9 +63,9 @@ where
         };
 
         match (all_meters, other_grid_successors) {
-            (Some(lhs), Some(rhs)) => Ok((lhs + rhs).to_string()),
-            (None, Some(expr)) | (Some(expr), None) => Ok(expr.to_string()),
-            (None, None) => Ok("0.0".to_string()),
+            (Some(lhs), Some(rhs)) => Ok(Formula::new(lhs + rhs)),
+            (None, Some(expr)) | (Some(expr), None) => Ok(Formula::new(expr)),
+            (None, None) => Ok(Formula::new(Expr::number(0.0))),
         }
     }
 
@@ -138,7 +141,7 @@ mod tests {
         builder.connect(grid, inv_bat_chain);
 
         let graph = builder.build(None)?;
-        let formula = graph.consumer_formula()?;
+        let formula = graph.consumer_formula()?.to_string();
         assert_eq!(formula, "0.0");
 
         Ok(())
@@ -154,7 +157,7 @@ mod tests {
         builder.connect(grid, grid_meter);
 
         let graph = builder.build(None)?;
-        let formula = graph.consumer_formula()?;
+        let formula = graph.consumer_formula()?.to_string();
         assert_eq!(formula, "MAX(0.0, #1)");
 
         // Add a battery meter with one battery inverter and one battery to the
@@ -165,7 +168,7 @@ mod tests {
         assert_eq!(meter_bat_chain.component_id(), 2);
 
         let graph = builder.build(None)?;
-        let formula = graph.consumer_formula()?;
+        let formula = graph.consumer_formula()?.to_string();
         // Formula subtracts the battery meter from the grid meter, and the
         // battery inverter from the battery meter.
         assert_eq!(
@@ -180,7 +183,7 @@ mod tests {
         assert_eq!(meter_pv_chain.component_id(), 5);
 
         let graph = builder.build(None)?;
-        let formula = graph.consumer_formula()?;
+        let formula = graph.consumer_formula()?.to_string();
         assert_eq!(
             formula,
             concat!(
@@ -212,7 +215,7 @@ mod tests {
         assert_eq!(meter.component_id(), 11);
 
         let graph = builder.build(None)?;
-        let formula = graph.consumer_formula()?;
+        let formula = graph.consumer_formula()?.to_string();
         assert_eq!(
             formula,
             concat!(
@@ -234,7 +237,7 @@ mod tests {
             disable_fallback_components: true,
             ..Default::default()
         }))?;
-        let formula = graph.consumer_formula()?;
+        let formula = graph.consumer_formula()?.to_string();
         assert_eq!(
             formula,
             concat!(
@@ -258,7 +261,7 @@ mod tests {
         assert_eq!(dangling_meter.component_id(), 15);
 
         let graph = builder.build(None)?;
-        let formula = graph.consumer_formula()?;
+        let formula = graph.consumer_formula()?.to_string();
         assert_eq!(
             formula,
             concat!(
@@ -297,7 +300,7 @@ mod tests {
         assert_eq!(meter_bat_chain.component_id(), 1);
 
         let graph = builder.build(None)?;
-        let formula = graph.consumer_formula()?;
+        let formula = graph.consumer_formula()?.to_string();
         // Formula subtracts inverter from battery meter, or shows zero
         // consumption if either of the components have no data.
         assert_eq!(formula, "COALESCE(MAX(0.0, #1 - #2), 0.0)");
@@ -315,7 +318,7 @@ mod tests {
         assert_eq!(dangling_meter_2.component_id(), 7);
 
         let graph = builder.build(None)?;
-        let formula = graph.consumer_formula()?;
+        let formula = graph.consumer_formula()?.to_string();
         assert_eq!(
             formula,
             concat!(
@@ -334,7 +337,7 @@ mod tests {
         builder.connect(grid, inv_bat_chain);
 
         let graph = builder.build(None)?;
-        let formula = graph.consumer_formula()?;
+        let formula = graph.consumer_formula()?.to_string();
         assert_eq!(
             formula,
             concat!(
@@ -358,7 +361,7 @@ mod tests {
         assert_eq!(chp.component_id(), 11);
 
         let graph = builder.build(None)?;
-        let formula = graph.consumer_formula()?;
+        let formula = graph.consumer_formula()?.to_string();
         assert_eq!(
             formula,
             concat!(
@@ -388,7 +391,7 @@ mod tests {
         builder.connect(grid, grid_meter_3);
 
         let graph = builder.build(None)?;
-        let formula = graph.consumer_formula()?;
+        let formula = graph.consumer_formula()?.to_string();
         assert_eq!(formula, "MAX(0.0, #1) + MAX(0.0, #2) + MAX(0.0, #3)");
 
         // Add two solar inverters with two grid meters as predecessors.
@@ -403,7 +406,7 @@ mod tests {
         assert_eq!(meter_pv_chain_2.component_id(), 6);
 
         let graph = builder.build(None)?;
-        let formula = graph.consumer_formula()?;
+        let formula = graph.consumer_formula()?.to_string();
         assert_eq!(
             formula,
             concat!(
@@ -426,7 +429,7 @@ mod tests {
         assert_eq!(meter.component_id(), 8);
 
         let graph = builder.build(None)?;
-        let formula = graph.consumer_formula()?;
+        let formula = graph.consumer_formula()?.to_string();
         assert_eq!(
             formula,
             concat!(
@@ -444,7 +447,7 @@ mod tests {
         builder.connect(grid_meter_1, meter_bat_chain);
 
         let graph = builder.build(None)?;
-        let formula = graph.consumer_formula()?;
+        let formula = graph.consumer_formula()?.to_string();
         assert_eq!(
             formula,
             concat!(
diff --git a/src/graph/formulas/generators/ev_charger.rs b/src/graph/formulas/generators/ev_charger.rs
index cb00eff..7cabda6 100644
--- a/src/graph/formulas/generators/ev_charger.rs
+++ b/src/graph/formulas/generators/ev_charger.rs
@@ -6,6 +6,8 @@
 use std::collections::BTreeSet;
 
 use crate::component_category::CategoryPredicates;
+use crate::graph::formulas::expr::Expr;
+use crate::graph::formulas::Formula;
 use crate::{ComponentGraph, Edge, Error, Node};
 
 pub(crate) struct EVChargerFormulaBuilder<'a, N, E>
@@ -46,9 +48,9 @@ where
     ///
     /// This is the sum of all EV chargers in the graph. If the ev_charger_ids are provided,
     /// only the EV chargers with the given ids are included in the formula.
-    pub fn build(self) -> Result<String, Error> {
+    pub fn build(self) -> Result<Formula, Error> {
         if self.ev_charger_ids.is_empty() {
-            return Ok("0.0".to_string());
+            return Ok(Formula::new(Expr::number(0.0)));
         }
 
         for id in &self.ev_charger_ids {
@@ -62,7 +64,7 @@ where
 
         self.graph
             .fallback_expr(self.ev_charger_ids, false)
-            .map(|expr| expr.to_string())
+            .map(Formula::new)
     }
 }
 
@@ -81,7 +83,7 @@ mod tests {
         builder.connect(grid, grid_meter);
 
         let graph = builder.build(None)?;
-        let formula = graph.ev_charger_formula(None)?;
+        let formula = graph.ev_charger_formula(None)?.to_string();
         assert_eq!(formula, "0.0");
 
         // Add a EV charger meter with one EV charger.
@@ -92,7 +94,7 @@ mod tests {
         assert_eq!(meter_ev_charger_chain.component_id(), 2);
 
         let graph = builder.build(None)?;
-        let formula = graph.ev_charger_formula(None)?;
+        let formula = graph.ev_charger_formula(None)?.to_string();
         assert_eq!(formula, "COALESCE(#3, #2, 0.0)");
 
         // Add a battery meter with one inverter and two batteries.
@@ -102,7 +104,7 @@ mod tests {
         assert_eq!(meter_bat_chain.component_id(), 4);
 
         let graph = builder.build(None)?;
-        let formula = graph.ev_charger_formula(None)?;
+        let formula = graph.ev_charger_formula(None)?.to_string();
         assert_eq!(formula, "COALESCE(#3, #2, 0.0)");
 
         // Add a EV charger meter with two EV chargers.
@@ -112,7 +114,7 @@ mod tests {
         assert_eq!(meter_ev_charger_chain.component_id(), 8);
 
         let graph = builder.build(None)?;
-        let formula = graph.ev_charger_formula(None)?;
+        let formula = graph.ev_charger_formula(None)?.to_string();
         assert_eq!(
             formula,
             concat!(
@@ -122,8 +124,8 @@ mod tests {
         );
 
         let formula = graph
-            .ev_charger_formula(Some(BTreeSet::from([10, 3])))
-            .unwrap();
+            .ev_charger_formula(Some(BTreeSet::from([10, 3])))?
+            .to_string();
         assert_eq!(formula, "COALESCE(#3, #2, 0.0) + COALESCE(#10, 0.0)");
 
         // add a meter direct to the grid with three EV chargers
@@ -133,7 +135,7 @@ mod tests {
         assert_eq!(meter_ev_charger_chain.component_id(), 11);
 
         let graph = builder.build(None)?;
-        let formula = graph.ev_charger_formula(None)?;
+        let formula = graph.ev_charger_formula(None)?.to_string();
         assert_eq!(
             formula,
             concat!(
@@ -148,8 +150,8 @@ mod tests {
         );
 
         let formula = graph
-            .ev_charger_formula(Some(BTreeSet::from([3, 9, 10, 12, 13])))
-            .unwrap();
+            .ev_charger_formula(Some(BTreeSet::from([3, 9, 10, 12, 13])))?
+            .to_string();
         assert_eq!(
             formula,
             concat!(
@@ -161,8 +163,8 @@ mod tests {
         );
 
         let formula = graph
-            .ev_charger_formula(Some(BTreeSet::from([3, 9, 10, 12, 13, 14])))
-            .unwrap();
+            .ev_charger_formula(Some(BTreeSet::from([3, 9, 10, 12, 13, 14])))?
+            .to_string();
         assert_eq!(
             formula,
             concat!(
@@ -177,8 +179,8 @@ mod tests {
         );
 
         let formula = graph
-            .ev_charger_formula(Some(BTreeSet::from([10, 14])))
-            .unwrap();
+            .ev_charger_formula(Some(BTreeSet::from([10, 14])))?
+            .to_string();
         assert_eq!(formula, "COALESCE(#10, 0.0) + COALESCE(#14, 0.0)");
 
         // Failure cases:
diff --git a/src/graph/formulas/generators/generic/coalesce.rs b/src/graph/formulas/generators/generic/coalesce.rs
index 16f88ce..0feeec7 100644
--- a/src/graph/formulas/generators/generic/coalesce.rs
+++ b/src/graph/formulas/generators/generic/coalesce.rs
@@ -7,7 +7,10 @@
 
 use std::collections::BTreeSet;
 
-use crate::{graph::formulas::expr::Expr, ComponentGraph, Edge, Error, Node};
+use crate::{
+    graph::formulas::{expr::Expr, Formula},
+    ComponentGraph, Edge, Error, Node,
+};
 
 pub(crate) struct CoalesceFormulaBuilder {
     component_ids: BTreeSet<u64>,
@@ -34,10 +37,10 @@ impl CoalesceFormulaBuilder {
 
     /// Generates a formula that uses the `COALESCE` function to return the first
     /// non-null value from the provided component IDs.
-    pub fn build(self) -> Result<String, Error> {
+    pub fn build(self) -> Result<Formula, Error> {
         if self.component_ids.len() == 1 {
             if let Some(component_id) = self.component_ids.into_iter().next() {
-                return Ok(Expr::Component { component_id }.to_string());
+                return Ok(Formula::new(Expr::component(component_id)));
             } else {
                 return Err(Error::internal(
                     "Failed to create expression for single component ID.",
@@ -50,7 +53,7 @@ impl CoalesceFormulaBuilder {
                 .map(|component_id| Expr::Component { component_id })
                 .collect(),
         );
-        Ok(expr.to_string())
+        Ok(Formula::new(expr))
     }
 }
 
@@ -71,14 +74,14 @@ mod tests {
         builder.connect(grid, grid_meter_2);
 
         let graph = builder.build(None)?;
-        let formula = graph.coalesce(BTreeSet::from([1, 2]))?;
+        let formula = graph.coalesce(BTreeSet::from([1, 2]))?.to_string();
         assert_eq!(formula, "COALESCE(#1, #2)");
-        let formula = graph.coalesce(BTreeSet::from([1]))?;
+        let formula = graph.coalesce(BTreeSet::from([1]))?.to_string();
         assert_eq!(formula, "#1");
-        let formula = graph.coalesce(BTreeSet::from([]));
+        let formula = graph.coalesce(BTreeSet::from([])).unwrap_err();
         assert_eq!(
             formula,
-            Err(Error::missing_parameters("No component IDs specified."))
+            Error::missing_parameters("No component IDs specified.")
         );
 
         Ok(())
diff --git a/src/graph/formulas/generators/grid.rs b/src/graph/formulas/generators/grid.rs
index 95c8edc..168627c 100644
--- a/src/graph/formulas/generators/grid.rs
+++ b/src/graph/formulas/generators/grid.rs
@@ -3,7 +3,10 @@
 
 //! This module contains the methods for generating grid formulas.
 
-use crate::{ComponentGraph, Edge, Error, Node};
+use crate::{
+    graph::formulas::{expr::Expr, Formula},
+    ComponentGraph, Edge, Error, Node,
+};
 
 pub(crate) struct GridFormulaBuilder<'a, N, E>
 where
@@ -27,7 +30,7 @@ where
     /// The grid formula is the sum of all components connected to the grid.
     /// This formula can be used for calculating power or current metrics at the
     /// grid connection point.
-    pub fn build(self) -> Result<String, Error> {
+    pub fn build(self) -> Result<Formula, Error> {
         let mut expr = None;
         for comp in self.graph.successors(self.graph.root_id)? {
             let comp = self.graph.fallback_expr([comp.component_id()], true)?;
@@ -37,8 +40,8 @@ where
             };
         }
         Ok(expr
-            .map(|e| e.to_string())
-            .unwrap_or_else(|| "0.0".to_string()))
+            .map(Formula::new)
+            .unwrap_or_else(|| Formula::new(Expr::number(0.0))))
     }
 }
 
@@ -59,7 +62,7 @@ mod tests {
         builder.connect(grid_meter, meter_bat_chain);
 
         let graph = builder.build(None)?;
-        let formula = graph.grid_formula()?;
+        let formula = graph.grid_formula()?.to_string();
         assert_eq!(formula, "#1");
 
         // Add an additional dangling meter, and a PV chain and a battery chain
@@ -76,7 +79,7 @@ mod tests {
         assert_eq!(meter_pv_chain.component_id(), 9);
 
         let graph = builder.build(None)?;
-        let formula = graph.grid_formula()?;
+        let formula = graph.grid_formula()?.to_string();
         assert_eq!(
             formula,
             "#1 + #5 + COALESCE(#6, #7, 0.0) + COALESCE(#9, #10, 0.0)"
@@ -89,7 +92,7 @@ mod tests {
         assert_eq!(pv_inverter.component_id(), 11);
 
         let graph = builder.build(None)?;
-        let formula = graph.grid_formula()?;
+        let formula = graph.grid_formula()?.to_string();
         assert_eq!(
             formula,
             "#1 + #5 + COALESCE(#6, #7, 0.0) + COALESCE(#9, #10, 0.0) + COALESCE(#11, 0.0)"
diff --git a/src/graph/formulas/generators/producer.rs b/src/graph/formulas/generators/producer.rs
index 12a0db7..36fb850 100644
--- a/src/graph/formulas/generators/producer.rs
+++ b/src/graph/formulas/generators/producer.rs
@@ -5,6 +5,7 @@
 
 use super::super::expr::Expr;
 use crate::component_category::CategoryPredicates;
+use crate::graph::formulas::Formula;
 use crate::{ComponentGraph, Edge, Error, Node};
 
 pub(crate) struct ProducerFormulaBuilder<'a, N, E>
@@ -28,7 +29,7 @@ where
     ///
     /// The production formula is the sum of all the PV and CHP components in
     /// the graph.
-    pub fn build(self) -> Result<String, Error> {
+    pub fn build(self) -> Result<Formula, Error> {
         let mut expr = None;
         for component_id in self.graph.find_all(
             self.graph.root_id,
@@ -51,8 +52,8 @@ where
             };
         }
         Ok(expr
-            .map(|e| e.to_string())
-            .unwrap_or_else(|| "0.0".to_string()))
+            .map(Formula::new)
+            .unwrap_or_else(|| Formula::new(Expr::number(0.0))))
     }
 
     /// Returns a formula expression for just the production part of the given
@@ -78,14 +79,14 @@ mod tests {
         builder.connect(grid, grid_meter);
 
         let graph = builder.build(None)?;
-        let formula = graph.producer_formula()?;
+        let formula = graph.producer_formula()?.to_string();
         assert_eq!(formula, "0.0");
 
         let meter_pv_chain = builder.meter_pv_chain(2);
         builder.connect(grid_meter, meter_pv_chain);
 
         let graph = builder.build(None)?;
-        let formula = graph.producer_formula()?;
+        let formula = graph.producer_formula()?.to_string();
         assert_eq!(
             formula,
             "MIN(0.0, COALESCE(#4 + #3, #2, COALESCE(#4, 0.0) + COALESCE(#3, 0.0)))"
@@ -96,7 +97,7 @@ mod tests {
         builder.connect(grid, meter_chp_chain);
 
         let graph = builder.build(None)?;
-        let formula = graph.producer_formula()?;
+        let formula = graph.producer_formula()?.to_string();
         assert_eq!(
             formula,
             concat!(
@@ -110,7 +111,7 @@ mod tests {
         builder.connect(grid, chp);
 
         let graph = builder.build(None)?;
-        let formula = graph.producer_formula()?;
+        let formula = graph.producer_formula()?.to_string();
         assert_eq!(
             formula,
             concat!(
@@ -125,7 +126,7 @@ mod tests {
         builder.connect(grid_meter, pv_inverter);
 
         let graph = builder.build(None)?;
-        let formula = graph.producer_formula()?;
+        let formula = graph.producer_formula()?.to_string();
         assert_eq!(
             formula,
             concat!(
@@ -141,7 +142,7 @@ mod tests {
         builder.connect(grid_meter, meter_bat_chain);
 
         let graph = builder.build(None)?;
-        let formula = graph.producer_formula()?;
+        let formula = graph.producer_formula()?.to_string();
         assert_eq!(
             formula,
             concat!(
@@ -161,7 +162,7 @@ mod tests {
         builder.connect(grid_meter, meter);
 
         let graph = builder.build(None)?;
-        let formula = graph.producer_formula()?;
+        let formula = graph.producer_formula()?.to_string();
         assert_eq!(
             formula,
             concat!(
diff --git a/src/graph/formulas/generators/pv.rs b/src/graph/formulas/generators/pv.rs
index 8065b35..2af7fe1 100644
--- a/src/graph/formulas/generators/pv.rs
+++ b/src/graph/formulas/generators/pv.rs
@@ -6,6 +6,8 @@
 use std::collections::BTreeSet;
 
 use crate::component_category::CategoryPredicates;
+use crate::graph::formulas::expr::Expr;
+use crate::graph::formulas::Formula;
 use crate::{ComponentGraph, Edge, Error, Node};
 
 pub(crate) struct PVFormulaBuilder<'a, N, E>
@@ -46,9 +48,9 @@ where
     ///
     /// This is the sum of all PV inverters in the graph. If the pv_inverter_ids are provided,
     /// only the PV inverters with the given ids are included in the formula.
-    pub fn build(self) -> Result<String, Error> {
+    pub fn build(self) -> Result<Formula, Error> {
         if self.pv_inverter_ids.is_empty() {
-            return Ok("0.0".to_string());
+            return Ok(Formula::new(Expr::number(0.0)));
         }
 
         for id in &self.pv_inverter_ids {
@@ -62,7 +64,7 @@ where
 
         self.graph
             .fallback_expr(self.pv_inverter_ids, false)
-            .map(|expr| expr.to_string())
+            .map(Formula::new)
     }
 }
 
@@ -81,7 +83,7 @@ mod tests {
         builder.connect(grid, grid_meter);
 
         let graph = builder.build(None)?;
-        let formula = graph.pv_formula(None)?;
+        let formula = graph.pv_formula(None)?.to_string();
         assert_eq!(formula, "0.0");
 
         // Add a PV meter with one PV inverter.
@@ -92,7 +94,7 @@ mod tests {
         assert_eq!(meter_pv_chain.component_id(), 2);
 
         let graph = builder.build(None)?;
-        let formula = graph.pv_formula(None)?;
+        let formula = graph.pv_formula(None)?.to_string();
         assert_eq!(formula, "COALESCE(#3, #2, 0.0)");
 
         // Add a battery meter with one inverter and two batteries.
@@ -102,7 +104,7 @@ mod tests {
         assert_eq!(meter_bat_chain.component_id(), 4);
 
         let graph = builder.build(None)?;
-        let formula = graph.pv_formula(None)?;
+        let formula = graph.pv_formula(None)?.to_string();
         assert_eq!(formula, "COALESCE(#3, #2, 0.0)");
 
         // Add a PV meter with two PV inverters.
@@ -112,7 +114,7 @@ mod tests {
         assert_eq!(meter_pv_chain.component_id(), 8);
 
         let graph = builder.build(None)?;
-        let formula = graph.pv_formula(None)?;
+        let formula = graph.pv_formula(None)?.to_string();
         assert_eq!(
             formula,
             concat!(
@@ -121,7 +123,7 @@ mod tests {
             )
         );
 
-        let formula = graph.pv_formula(Some(BTreeSet::from([10, 3]))).unwrap();
+        let formula = graph.pv_formula(Some(BTreeSet::from([10, 3])))?.to_string();
         assert_eq!(formula, "COALESCE(#3, #2, 0.0) + COALESCE(#10, 0.0)");
 
         // add a meter direct to the grid with three PV inverters
@@ -131,7 +133,7 @@ mod tests {
         assert_eq!(meter_pv_chain.component_id(), 11);
 
         let graph = builder.build(None)?;
-        let formula = graph.pv_formula(None)?;
+        let formula = graph.pv_formula(None)?.to_string();
         assert_eq!(
             formula,
             concat!(
@@ -146,8 +148,8 @@ mod tests {
         );
 
         let formula = graph
-            .pv_formula(Some(BTreeSet::from([3, 9, 10, 12, 13])))
-            .unwrap();
+            .pv_formula(Some(BTreeSet::from([3, 9, 10, 12, 13])))?
+            .to_string();
         assert_eq!(
             formula,
             concat!(
@@ -159,8 +161,8 @@ mod tests {
         );
 
         let formula = graph
-            .pv_formula(Some(BTreeSet::from([3, 9, 10, 12, 13, 14])))
-            .unwrap();
+            .pv_formula(Some(BTreeSet::from([3, 9, 10, 12, 13, 14])))?
+            .to_string();
         assert_eq!(
             formula,
             concat!(
@@ -174,7 +176,9 @@ mod tests {
             )
         );
 
-        let formula = graph.pv_formula(Some(BTreeSet::from([10, 14]))).unwrap();
+        let formula = graph
+            .pv_formula(Some(BTreeSet::from([10, 14])))?
+            .to_string();
         assert_eq!(formula, "COALESCE(#10, 0.0) + COALESCE(#14, 0.0)");
 
         // Failure cases:
diff --git a/src/lib.rs b/src/lib.rs
index d844bee..748c8b5 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -58,7 +58,7 @@ mod component_category;
 pub use component_category::{BatteryType, ComponentCategory, EvChargerType, InverterType};
 
 mod graph;
-pub use graph::{iterators, ComponentGraph};
+pub use graph::{iterators, ComponentGraph, Formula};
 
 mod graph_traits;
 pub use graph_traits::{Edge, Node};