Heat Pump Balance Point Calculator
Two lines decide what a heat pump costs to run in winter, and they run in opposite directions. As it gets colder the house needs more heat, and as it gets colder the heat pump can deliver less. Where they cross is the balance point. Above it the heat pump carries the house on its own at a coefficient of performance around three; below it the resistance elements make up the difference at a coefficient of exactly one. This calculator finds the crossing, sizes the backup for the gap rather than for the whole house, and puts a price on the difference.
- Balance point
- 30 °F Above 30 °F the heat pump carries the house on its own. Below it, backup heat makes up the difference, and every BTU from a resistance element costs about 2.8 times what the same BTU costs from the heat pump.
- Design heating load
- 57,915 BTU/hr At 20 °F, for 1,800 ft² with good insulation. The load falls linearly as the outdoor temperature rises, reaching zero at 65 °F where the house needs no heat at all.
- Heat pump capacity at 20 °F
- 38,400 BTU/hr 60,000 BTU/hr rated at 47 °F × 64%. A standard model loses capacity steadily as it gets colder, which is why the two curves cross.
- Backup heat needed at design
- 19,515 BTU/hr That is 5.7 kW of resistance strip, or about 2 × 5 kW elements. Size the backup for this figure rather than for the whole load — strips sized to heat the house unaided are the usual reason a heat pump installation draws enormous current on the coldest morning.
- Cost of heat, two ways
- $17.79 vs $49.82 per million BTU At 17¢ per kWh, a COP of 2.8 delivers heat at 17.79 dollars per million BTU. Resistance heat has a COP of exactly 1 and costs 49.82 — 2.8 times as much for the same heat. Every hour spent below the balance point is billed at that rate.
- Why raising the thermostat makes it worse
- Emergency heat is not a boost A large setback recovery asks for more heat than the balance point allows, so the control brings in the strips to catch up. Overnight setback on a heat pump often costs more than it saves for exactly this reason, and the emergency heat switch takes the compressor out entirely rather than helping it.
- Moving the balance point
- Envelope or equipment Insulation and air sealing lower the load line, which moves the crossing to a colder temperature. A cold-climate model raises the capacity line at the cold end and does the same. On this house, a cold-climate model would push the balance point substantially lower for the same rated capacity.
- What this does not model
- Defrost and cycling Real output is reduced by defrost cycles in humid cold weather, and part-load behaviour differs between single-stage and inverter equipment. The capacity curve here is typical published behaviour; the submittal for the specific model carries the extended capacity table that governs.
Two lines, opposite directions
A building loses heat in proportion to the difference between inside and outside, so its heating load is a straight line: zero at around sixty-five degrees, where internal gains cover the losses, rising steadily as the outdoor temperature falls.
A heat pump moves heat rather than making it, and there is less heat in the outdoor air to move as it gets colder. Its capacity is therefore also a line, but sloping the other way — highest when it is mild, lowest when it is cold.
The two cross at one temperature. Above it the heat pump has spare capacity; below it there is a shortfall, and something else has to cover it. That crossing is the balance point, and it is the single most useful number about how a heat pump will behave in a particular house.
A balance point in the mid-twenties or below means the backup runs rarely and the winter bill looks like a heat pump bill. A balance point around forty means the strips are running most of the winter, and the bill looks like electric resistance heating with extra equipment.
Size the backup for the gap
The instinct is to size resistance backup to heat the whole house, on the grounds that the heat pump might fail. That produces enormous strip banks, huge electrical service requirements, and a control that is quick to bring them in.
What the backup actually has to cover is the difference between the load and the heat pump output at the design temperature — the vertical gap between the two lines at the coldest expected condition. That is usually a fraction of the total load.
Oversized strips also change the economics of every cold hour, because a control with a lot of resistance heat available tends to use it. Staged strips, brought in one element at a time by outdoor temperature, keep the compressor doing as much of the work as it can.
The other reason to keep the strips small is the service. Fifteen kilowatts of resistance heat is over sixty amps at two hundred and forty volts, and that has to be in the load calculation for the panel whether it runs or not.
The setback trap
Setting a thermostat back overnight saves energy in a gas-heated house because the furnace recovers at full output and full efficiency regardless.
A heat pump recovers at whatever capacity the outdoor temperature allows, which on a cold morning is not much. Asking for a four degree recovery at six in the morning demands more heat than the balance point permits, so the control brings in the resistance elements to catch up — at three times the cost per BTU.
The result is that aggressive setback on a heat pump frequently costs more than it saves. Small setbacks, or none, with a thermostat that ramps recovery gradually rather than demanding it immediately, work better.
The emergency heat switch is a related misunderstanding. It does not add heat to the compressor — it takes the compressor out of the circuit entirely and heats the house on resistance alone. It is for when the heat pump has failed, not for when it is cold.
What this is based on
- Building heat load proportional to indoor–outdoor temperature difference, zero at 65 °F
- Heat pump extended capacity tables — output against outdoor temperature, rated at 47 °F
- Resistance heating — 3,412 BTU per kWh, COP of 1
A screening estimate, not a Manual J load calculation. The capacity curve is typical published behaviour and varies substantially between models; use the extended capacity table from the equipment submittal. Defrost cycles reduce real output in humid cold weather and are not modelled here.
Frequently asked questions
What is a heat pump balance point?
The outdoor temperature at which the heat pump’s falling capacity crosses the building’s rising heat load. Above it the heat pump carries the house alone; below it backup heat makes up the difference at roughly three times the cost per BTU.
What is a good balance point?
The mid-twenties Fahrenheit or lower means backup runs rarely and the winter bill behaves like a heat pump bill. A balance point around forty means the resistance elements are doing much of the work all winter.
How much backup heat do I need?
The gap between the building load and the heat pump output at your design temperature — usually a fraction of the total load, not all of it. Strips sized to heat the whole house unaided drive up the electrical service and get used more than they should.
Should I set my heat pump thermostat back at night?
Usually not by much. A heat pump recovers at whatever capacity the outdoor temperature allows, so a large morning recovery brings in the resistance elements at three times the cost. Small setbacks, or a thermostat that ramps recovery gradually, work better.
What does the emergency heat setting do?
It takes the compressor out of the circuit and heats the house on resistance elements alone. It is for a failed heat pump, not for cold weather — using it because it is cold simply replaces the cheapest heat in the system with the most expensive.