Generator Runtime Calculator
How long a tank will last.
Special
Whole House Generator Size Calculator
Pick the appliances you want to keep running during an outage and see the running load, the starting surge from motors like the air conditioner and well pump, and the standby generator size that covers both.
How it works
- Tick the appliances you want powered at the same time during an outage.
- Choose the size of your central air conditioner or heat pump and whether it has a soft starter, and your well pump if you have one.
- Add lighting and plug loads and any other equipment in watts.
- Choose natural gas or propane — most standby generators produce less power on natural gas — and read the recommended size.
Formula
Running load = Σ running watts Peak = running load + (largest starting watts − its running watts) Required output = max(running load ÷ 0.8, peak) Recommended = smallest standard size × fuel factor ≥ required output
Motors draw a brief surge several times their running power when they start. Assuming they start one at a time, the peak is the running total plus the single biggest surge. The generator should also run no more than about 80% loaded continuously. Air conditioning is estimated at 1,200 running watts per ton with a starting surge of 3× (1.7× with a soft starter); natural gas output is taken as 90% of the propane rating. Standard sizes: 10, 14, 16, 18, 20, 22, 24, 26, 32, 38 and 48 kW.
Example
A rural home on propane wants to keep the refrigerator, a freezer, a ¾ hp well pump, the furnace blower, a 4-ton heat pump with a soft starter and 2,000 W of lights and outlets running.
Inputs
- Refrigerator
- Yes
- Freezer
- Yes
- Sump pump
- No
- Microwave
- Yes
- Lighting, electronics & outlets
- 2,000 W
- Gas furnace blower
- Yes
- Central AC / heat pump
- 4 ton
- AC has a soft starter
- Yes
- Well pump
- ¾ hp
- Electric water heater
- No
- Electric dryer
- No
- Electric range
- No
- EV charger
- None
- Other running watts
- 0 W
- Other starting watts
- 0 W
- Generator fuel
- Propane
Result
Recommended standby generator16 kWEstimated range 16–18 kW
Running load11.4 kW
Peak starting load14.7 kW
Minimum output on propane14.7 kW
Whole-house vs. essential-circuit backup
A generator that can run everything at once is usually larger than needed. Many installations use an automatic transfer switch with load management: large loads such as a second air conditioner, an electric dryer or an EV charger are shed while the generator is busy and allowed back when capacity frees up. That can cut the required size by a third or more.
The biggest single decision is usually air conditioning. A 4- or 5-ton unit without a soft starter can dominate the size; with a soft starter the same system may fit a smaller generator.
What an installer will check
A licensed electrician or generator dealer will review the actual nameplate data, perform a load calculation, and confirm the gas meter and piping can supply the generator at full load. Local code governs the transfer switch, placement clearances and permits. Use this estimate to understand the range before getting quotes, not as a design.
Frequently asked questions
What size generator do I need to run my whole house?
Many average homes with gas heat and a 3-ton AC land in the 14–22 kW range. All-electric homes or homes with large AC, electric heat or EV charging often need 24 kW or more unless load management is used.
What is the difference between running and starting watts?
Running watts are what an appliance uses continuously. Starting (surge) watts are the brief extra power a motor needs to get going — often two to three times its running watts.
Can a 22 kW generator run a 4-ton air conditioner?
Often yes, especially with a soft starter, but it depends on what else runs at the same time. Enter your loads above to check the peak.
Why is the natural gas rating lower?
Natural gas contains less energy per cubic foot than propane vapor, so most air-cooled standby generators produce roughly 10% less on natural gas.
Does this replace a professional load calculation?
No. It is a planning estimate. Final sizing, transfer switch selection and installation should be done by a licensed electrician or installer following local code.