Generator size depends on your heat pump's starting watts, not running watts
A heat pump needs a generator large enough to handle its starting power, which is two to three times higher than the power it uses once it is running. Most residential heat pumps draw 15 to 20 amps at startup but only 8 to 12 amps while operating. This means a 5-ton heat pump that uses 12,000 watts continuously may need a 30,000 to 40,000-watt generator to start reliably.
The difference between starting watts and running watts is the reason undersized generators fail with heat pumps. When you plug a heat pump into a generator that cannot handle the startup surge, the generator shuts down or the heat pump does not start at all. You need to know both numbers before you buy.
Portable generators under 10,000 watts will not start most heat pumps. Standby generators of 20,000 watts or larger are the practical minimum for a single heat pump system, and 30,000 to 50,000 watts if you want to run other appliances at the same time.
Key Takeaways
- Heat pumps require two to three times more power at startup than during normal operation, so check the nameplate for both starting and running watts before choosing a generator size.
- A 5-ton heat pump typically needs a 30,000 to 40,000-watt generator; a 3-ton unit needs 20,000 to 25,000 watts.
- Portable generators under 10,000 watts will not reliably start a heat pump, even if they claim to handle the running wattage.
- If you want to run other appliances—lights, a refrigerator, a water heater—add their wattage to the heat pump's starting watts to find the total generator size you need.
How to find your heat pump's starting and running watts
The nameplate on your heat pump lists the amperage and voltage, not the wattage directly. You calculate watts by multiplying amps by volts. Most residential heat pumps run on 240 volts. If your heat pump draws 20 amps at startup, multiply 20 × 240 to get 4,800 watts—but that is only the heat pump itself. The compressor and fan motor together create a surge that can be two to three times that figure.
The safest approach is to look up your specific model online or call the manufacturer with the model number from the nameplate. They will tell you the locked rotor amps (LRA) or starting amps, which is what you need. Multiply LRA by 240 to get starting watts. Then multiply the running amps (also called full load amps, or FLA) by 240 to get running watts.
If you cannot find the nameplate or the model number, a licensed HVAC technician can measure the amperage with a clamp meter while the heat pump runs and starts. This costs $100 to $200 but gives you exact numbers before you spend thousands on a generator.
Generator sizing by heat pump capacity
Heat pump size is measured in tons, and larger units draw more power. A 3-ton heat pump (the smallest common residential size) typically needs 15,000 to 25,000 watts. A 5-ton unit needs 25,000 to 40,000 watts. A 10-ton commercial unit needs 50,000 watts or more. These ranges account for the difference between starting and running power.
The actual wattage varies by manufacturer and efficiency rating. A high-efficiency unit may use less power than an older or lower-efficiency model of the same size. This is why checking the nameplate matters more than relying on the ton rating alone.
| Heat Pump Size | Typical Running Watts | Typical Starting Watts | Minimum Generator Size |
|---|---|---|---|
| 3 ton | 9,000–12,000 | 15,000–20,000 | 20,000–25,000 |
| 5 ton | 12,000–15,000 | 25,000–35,000 | 30,000–40,000 |
| 10 ton | 20,000–25,000 | 40,000–60,000 | 50,000–70,000 |
Portable versus standby generators for heat pumps
Portable generators are cheaper upfront but rarely large enough for a heat pump. Most portable units max out at 10,000 to 12,000 watts, which is enough for the running power of a small heat pump but not the startup surge. They also run on gasoline, which means you need to refuel them every 8 to 12 hours of operation, and they are loud—typically 80 to 90 decibels.
Standby generators are permanently installed outside your home, run on natural gas or propane, and automatically switch on when power fails. They are quieter (70 to 80 decibels) and can run indefinitely as long as fuel is available. A standby unit large enough for a heat pump costs $5,000 to $15,000 installed, but it requires no manual intervention and no refueling during an outage.
If you own a heat pump and want backup power, a standby generator is the only practical choice. Portable generators work for other appliances—sump pumps, refrigerators, space heaters—but not for the startup demand of a heat pump compressor.
Running other appliances on the same generator
If you want to run lights, a refrigerator, or a water heater while the heat pump is running, you must add their wattage to the heat pump's starting watts. A refrigerator uses about 600 running watts and 1,200 starting watts. A microwave uses 1,000 watts. An electric water heater uses 4,000 to 5,000 watts. These loads add up quickly.
Most people with a standby generator size it to run the heat pump plus essential appliances—lights, refrigerator, and water heater—but not everything at once. If you want to run the heat pump, the water heater, and the refrigerator simultaneously, you need a generator large enough for all three starting surges combined. This often means 50,000 to 70,000 watts or larger.
A simpler approach is to use a transfer switch that lets you choose which circuits the generator powers. You can run the heat pump on one circuit and the refrigerator on another, but not both at full power if your generator is undersized. An electrician can install a transfer switch for $1,000 to $2,000.
Fuel type and runtime considerations
Standby generators run on natural gas (if your home has a gas line) or propane (stored in a tank). Natural gas is convenient because it is always available, but propane gives you independence from the utility. A propane tank sized for a 30,000-watt generator running continuously lasts about 10 to 14 days before refueling.
If you expect long outages, propane is riskier because you may run out before power is restored. Natural gas is safer in that respect, but if the gas line is damaged in the same event that knocked out power, you lose both. Many people with propane generators keep a second tank on hand so they can swap it out without waiting for a delivery.
Portable generators run on gasoline, which is cheaper per gallon but requires frequent refueling and storage. Gasoline also degrades over time, so you need to add stabilizer if you store it for more than a month. For a heat pump, the fuel type matters less than the generator size, but natural gas standby units are the most convenient for long-term backup.
Installation and permitting
A standby generator must be installed by a licensed electrician and usually requires a building permit from your city or county. The permit process takes one to four weeks and costs $100 to $500. The generator itself must be placed at least 5 to 10 feet from windows and doors (rules vary by location) so exhaust does not enter the home.
The electrician will run a gas line from your meter to the generator, install a disconnect switch between the generator and the main electrical panel, and connect the generator to a transfer switch that routes power to the circuits you choose. This entire installation takes one to two days and costs $2,000 to $5,000 on top of the generator price.
Portable generators do not require a permit, but they must be placed outside and at least 20 feet from windows and doors. They also need a heavy-duty extension cord rated for outdoor use, which adds another $50 to $150 to the cost.
Frequently Asked Questions
Can I use a portable generator to start my heat pump and then switch to the grid?
No. A portable generator cannot reliably start a heat pump because the startup surge exceeds what most portable units can deliver. Even if it did start, you would need to manually switch the power, which is not practical during an outage. A standby generator with an automatic transfer switch is the only safe option.
What if my generator is slightly undersized?
An undersized generator will shut down or fail to start the heat pump. Some generators have a soft-start feature that reduces the startup surge, but this is not standard. Do not buy a generator smaller than the starting watts your heat pump requires; the cost difference between a 25,000-watt and a 30,000-watt unit is small compared to the cost of replacing a failed generator.
Do I need a generator for a heat pump if I have solar panels?
Solar panels alone will not help during a grid outage unless you also have a battery system. Most solar systems shut down when the grid fails to protect utility workers. A battery system (like Tesla Powerwall) can store energy and power a heat pump, but batteries are expensive and have limited capacity. A generator is a more practical backup for extended outages.
How often should I run my standby generator to keep it in working order?
Manufacturers recommend running a standby generator under load for 20 to 30 minutes once a month. This keeps the fuel system clean and the engine in working condition. Many people run theirs during a scheduled maintenance window or when they test their transfer switch.
Can I use a generator with an inverter instead of a standard generator?
Inverter generators produce cleaner power and are quieter than standard portable generators, but they are still limited by wattage. Most inverter generators max out at 6,000 to 8,000 watts, which is not enough for a heat pump startup. Inverter technology does not solve the power problem; it only improves power quality for sensitive electronics.