Residential Heat Pump Service Load Calculator

A residential heat pump installation can change the calculated peak load on an existing electrical service even when it replaces electric resistance heat. The calculation produces three planning values:

• Net added heat pump load
• Planned peak with heat pump
• Entered service capacity margin

These results support an early service-load review when the heat-pump package loads, the existing residential peak, the electric heat being replaced, and an available service-capacity value are already known. The output can identify whether the proposed equipment appears to fit within the entered capacity before a full dwelling load calculation, equipment nameplate review, feeder assessment, or service upgrade decision.

A positive capacity margin indicates that the entered service capacity exceeds the calculator’s planned peak. A negative result indicates that the planned peak exceeds the entered service capacity and requires a more detailed electrical review.

Heat-Pump Package Load

The calculation first combines the electrical loads associated with the planned heat-pump system:

\(\displaystyle \text{Heat-Pump Package Load} = \text{Indoor heat pump load} + \text{Auxiliary heat load} + \text{Outdoor unit load}\)

The calculation uses the following input fields:

Input	Electrical use
Indoor heat pump load (kW)	Entered indoor heat-pump equipment load for this planning estimate
Auxiliary heat load (kW)	Entered supplemental or auxiliary electric heating load
Outdoor unit load (kW)	Entered outdoor condensing-unit or heat-pump unit load
Replaced electric heating load (kW)	Existing electric heating load removed or displaced by the heat-pump project
Existing residential peak (kW)	Existing peak residential load used as the starting point for the planning calculation
Entered coincidence factor (%)	Entered percentage applied to the net added heat-pump load
Entered service capacity (kW)	Entered service capacity used for the margin comparison

The package load is not automatically the service increase. Where the project removes an existing electric heating load, the calculator subtracts Replaced electric heating load (kW) from the proposed heat-pump package load.

\(\displaystyle \text{Net added heat pump load} = (\text{Indoor heat pump load} + \text{Auxiliary heat load} + \text{Outdoor unit load}) - \text{Replaced electric heating load}\)

This avoids treating an equipment replacement as though all new heat-pump and auxiliary heat load is being added on top of the existing home load.

Planned Residential Peak

The calculation applies Entered coincidence factor (%) to the net added heat-pump load, then adds that coincident portion to Existing residential peak (kW).

\(\displaystyle \text{Planned Peak With Heat Pump} = \text{Existing residential peak} + \left( \text{Net added heat pump load} \times \frac{\text{Entered coincidence factor}}{100} \right)\)

The coincidence factor is an entered planning assumption. A 70% value means the calculation adds 70% of the net added heat-pump load to the existing peak, rather than adding the entire net added load.

This approach can be useful during preliminary equipment planning, particularly when assessing whether a proposed heat-pump conversion may affect service loading. It does not establish which HVAC loads are noncoincident for a code calculation, nor does it determine an applicable demand factor, feeder rating, service rating, conductor ampacity, or overcurrent protective device size.

Service Capacity Margin

The final comparison is:

\(\displaystyle \text{Entered service capacity margin} = \text{Entered service capacity} - \text{Planned peak with heat pump}\)

The margin is expressed in kW because both the planned peak and entered capacity are entered in kW.

Margin result	Planning interpretation
Positive kW margin	The entered service capacity is greater than the calculated planned peak
Zero kW margin	The entered service capacity equals the calculated planned peak
Negative kW margin	The calculated planned peak exceeds the entered service capacity

A favorable kW margin does not by itself confirm that the service equipment, meter base, service conductors, panelboard, feeder, branch circuit, grounding and bonding system, or utility equipment can support the installation.

Calculation Example

Using the values shown in the calculator:

Input	Value
Indoor heat pump load (kW)	3.6 kW
Auxiliary heat load (kW)	5.0 kW
Outdoor unit load (kW)	0.8 kW
Replaced electric heating load (kW)	2.0 kW
Existing residential peak (kW)	11.0 kW
Entered coincidence factor (%)	70%
Entered service capacity (kW)	24.0 kW

First, calculate the proposed heat-pump package load:

\(\displaystyle 3.6 + 5.0 + 0.8 = 9.4\text{ kW}\)

Then subtract the replaced electric heating load:

\(\displaystyle 9.4 - 2.0 = \mathbf{7.4\text{ kW}}\)

The calculated net added heat-pump load is 7.4 kW.

Apply the entered 70% coincidence factor:

\(\displaystyle 7.4 \times 0.70 = 5.18\text{ kW}\)

Add that coincident load to the existing residential peak:

\(\displaystyle 11.0 + 5.18 = \mathbf{16.18\text{ kW}}\)

The calculated planned peak with heat pump is 16.18 kW.

Finally, compare the planned peak with entered service capacity:

\(\displaystyle 24.0 - 16.18 = \mathbf{7.82\text{ kW}}\)

The entered service capacity margin is 7.82 kW.

Service and Feeder Verification

This heat-pump service-load calculation is a planning estimate based solely on the entered kW values and coincidence factor. It does not size a service, feeder, branch circuit, raceway, or equipment grounding conductor.

The installation decision still requires verification of the actual electrical system, including:

• Heat-pump indoor unit, outdoor unit, and auxiliary heat manufacturer nameplate data.
• Minimum circuit ampacity, maximum overcurrent protection, branch-circuit requirements, and disconnecting means for each piece of equipment.
• Service and feeder conductor ampacity, including conductor material, AWG or kcmil size, insulation temperature rating, terminal rating, ambient correction factor, adjustment factor, and the number of current-carrying conductors where applicable.
• Panelboard bus rating, main breaker rating, service disconnect rating, meter equipment rating, and available panel spaces.
• Existing loads not reflected in the entered Existing residential peak (kW), including electric vehicle charging, cooking equipment, water heating, dryers, pools, spas, workshops, or future loads.
• Voltage drop across branch circuits and feeders, especially where outdoor equipment is remote from the service equipment or panelboard.
• Utility requirements and the applicable AHJ review.

Use the calculated Entered service capacity margin to prioritize the next level of design verification. A small or negative margin generally calls for a complete load calculation and a field review of the service, feeder, and distribution equipment before equipment is selected or installed.