feat: Finishes economic balance point.

Sources/ApiController/Extensions/HeatingBalancePoint.swift

@@ -1,6 +1,7 @@
 import CoreModels
 import Foundation
 import Logging
+import OpcostAnalysis
 import Routes
 
 public extension HeatingBalancePoint.Request {
@@ -10,6 +11,42 @@ public extension HeatingBalancePoint.Request {
     case let .thermal(request):
       logger.debug("Calculating thermal balance point: \(request)")
       return try await request.respond(logger: logger)
+    case let .economic(request):
+      logger.debug("Calculating economic balance point: \(request)")
+      return try await request.respond(logger: logger)
+    }
+  }
+}
+
+private extension HeatingBalancePoint.Request.Economic {
+
+  func respond(logger: Logger) async throws -> HeatingBalancePoint.Response {
+    try validate()
+    let fuelCostPerMMBTU = fuelType.costPerMMBTU(afue: fuelAFUE, costPerUnit: fuelCostPerUnit)
+    logger.debug("Fuel cost per mmBTU: \(fuelCostPerMMBTU)")
+    let electricCostPerMMBTU = 1_000_000 / 3412 * costPerKW
+    let electricFuelRatio = electricCostPerMMBTU / fuelCostPerMMBTU
+    let balancePointTemperature = (electricFuelRatio - 1.8273) / 0.0413
+    let copAtBalancePoint = 0.0413 * balancePointTemperature + 1.8273
+
+    return .economic(.init(
+      balancePointTemperature: balancePointTemperature,
+      fuelCostPerMMBTU: fuelCostPerMMBTU,
+      electricCostPerMMBTU: electricCostPerMMBTU,
+      copAtBalancePoint: copAtBalancePoint,
+      electricFuelRatio: electricFuelRatio
+    ))
+  }
+
+  func validate() throws {
+    guard fuelCostPerUnit > 0 else {
+      throw ValidationError(message: "Fuel cost should be greater than 0.")
+    }
+    guard fuelAFUE > 0, fuelAFUE < 100 else {
+      throw ValidationError(message: "AFUE is outside of range 0-100%.")
+    }
+    guard costPerKW > 0 else {
+      throw ValidationError(message: "Electric cost per KW should be greater than 0.")
     }
   }
 }
@@ -144,6 +181,35 @@ extension ClimateZone {
     case .seven: return -15
     }
   }
+
+  var averageHeatingLoadHours: Double {
+    switch self {
+    case .one: return 1000
+    case .two: return 2250
+    case .three: return 3750
+    case .four: return 5250
+    case .five: return 6750
+    case .six: return 8250
+    case .seven: return 10000
+    }
+  }
+}
+
+private extension HeatingBalancePoint.FuelType {
+  var btuPerUnit: Double {
+    switch self {
+    case .naturalGas: return 100_000
+    case .propane: return 91333
+    case .oil: return 138_690
+    }
+  }
+
+  func costPerMMBTU(afue: Double, costPerUnit: Double) -> Double {
+    if self == .naturalGas {
+      return costPerUnit * 10 / afue * 100
+    }
+    return (1_000_000 / btuPerUnit) * costPerUnit / afue * 100
+  }
 }
 
 private func thermalBalancePoint(