diff --git a/src/HPWH.cc b/src/HPWH.cc
index 3a709c34..cfda079f 100644
--- a/src/HPWH.cc
+++ b/src/HPWH.cc
@@ -2681,6 +2681,8 @@ void HPWH::addExtraHeat(std::vector<double> &nodePowerExtra_W,double tankAmbient
 			heatSources[i].perfMap.clear();
 			heatSources[i].energyInput_kWh = 0.0;
 			heatSources[i].energyOutput_kWh = 0.0;
+
+			break; // Only add extra heat to the first "extra" heat source found.
 		}
 	}
 }
diff --git a/src/HPWHHeatSources.cc b/src/HPWHHeatSources.cc
index 6730168c..6773b53c 100644
--- a/src/HPWHHeatSources.cc
+++ b/src/HPWHHeatSources.cc
@@ -433,7 +433,8 @@ void HPWH::HeatSource::addHeat(double externalT_C,double minutesToRun) {
 
 	case CONFIG_EXTERNAL:
 		//Else the heat source is external. SANCO2 system is only current example
-		//capacity is calculated internal to this function, and cap/input_BTUperHr, cop are outputs
+		//capacity is calculated internal to this functio
+		// n, and cap/input_BTUperHr, cop are outputs
 		this->runtime_min = addHeatExternal(externalT_C,minutesToRun,cap_BTUperHr,input_BTUperHr,cop);
 		break;
 	}
@@ -513,7 +514,10 @@ void HPWH::HeatSource::getCapacity(double externalT_C,double condenserTemp_C,dou
 		std::vector<double> target{externalT_F,Tout_F,condenserTemp_F};
 		btwxtInterp(input_BTUperHr,cop,target);
 	} else {
-		if(perfMap.size() > 1) {
+		if(perfMap.empty()) {
+			input_BTUperHr = 0.;
+			cop = 0.;
+		} else if(perfMap.size() > 1) {
 			double COP_T1,COP_T2;    			   //cop at ambient temperatures T1 and T2
 			double inputPower_T1_Watts,inputPower_T2_Watts; //input power at ambient temperatures T1 and T2
 
diff --git a/test/testHeatingLogics.cc b/test/testHeatingLogics.cc
index b166bada..cc8f0a20 100644
--- a/test/testHeatingLogics.cc
+++ b/test/testHeatingLogics.cc
@@ -20,6 +20,7 @@ void testChangeToStateofChargeControlled(string& input);
 void testSetStateOfCharge(string& input);
 
 void testSetStateOfCharge(string& input, double coldWater_F, double minTUse_F, double tankTAt76SoC);
+void testExtraHeat();
 
 const std::vector<string> hasHighShuttOffVectSP = { "Sanden80", "QAHV_N136TAU_HPB_SP", \
 	"ColmacCxA_20_SP", "ColmacCxV_5_SP", "NyleC60A_SP", "NyleC60A_C_SP", "NyleC185A_C_SP", "TamScalable_SP" };
@@ -62,6 +63,7 @@ int main(int, char*) {
 		testCanNotSetEnteringWaterShutOff(hpwhStr);
 	}
 
+	testExtraHeat();
 	return 0;
 }
 
@@ -264,3 +266,31 @@ void testSetStateOfCharge(string& input, double coldWater_F, double minTUse_F, d
 	hpwh.runOneStep(0, externalT_C, externalT_C, HPWH::DR_ALLOW);
 	ASSERTFALSE(compressorIsRunning(hpwh));
 }
+
+/*Test adding extra heat to a tank for one minute*/
+void testExtraHeat() {
+	HPWH hpwh;
+	getHPWHObject(hpwh, "StorageTank");
+
+	const double ambientT_C = 20.;
+	const double externalT_C = 20.;
+	const double inletVol2_L = 0.;
+	const double inletT2_C = 0.;
+
+	double extraPower_W = 1000.;
+	std::vector<double> nodePowerExtra_W = {extraPower_W};
+
+	//
+	hpwh.setUA(0.);
+	hpwh.setTankToTemperature(20.);
+
+	double Q_init = hpwh.getTankHeatContent_kJ();
+	hpwh.runOneStep(0, ambientT_C, externalT_C, HPWH::DR_LOC, inletVol2_L, inletT2_C, &nodePowerExtra_W);
+	double Q_final = hpwh.getTankHeatContent_kJ();
+
+	double dQ_actual_kJ = (Q_final - Q_init) * 1.055055853 / 1.055; // Correct for approx. BU->kJ conversion.
+
+	double dQ_expected_kJ = extraPower_W * 60. / 1.e3; // 1 min
+
+	ASSERTTRUE(cmpd(dQ_actual_kJ, dQ_expected_kJ));
+}
\ No newline at end of file