Generator calculator
Generator Load Calculator
Add up the running watts and starting surge of everything you want to power, then carry the result into the size calculator.
- Free to use
- Formula shown
- Worked example
Estimate how much generator power your appliances and equipment need before you choose a generator.
Select the appliances you want to run, adjust the quantities, and the calculator adds up your running load, the largest startup surge and your estimated peak demand. You do not need to know every wattage first: where you have no figure, it uses typical values for common household equipment.
Generator Load Calculator
Select the equipment you expect to run together. We'll estimate your running load, startup surge and peak demand.
Add Your Appliances
Common U.S. loads are shown first. Use the nameplate or manufacturer data when available.
Demand Preview
Your estimate updates as you build your load. Click Calculate Load to review and confirm the demand.
How to use the calculator
- Choose your loads. Start from a preset (essential home backup, home with sump pump, job site, RV) or pick appliances one by one. Add a custom appliance if you have the wattage from its label or manual.
- Review your demand. The preview updates as you add equipment and shows three numbers: running load, largest additional startup surge and estimated peak demand.
- Size your generator. Carry the result into the Generator Size Calculator, which adds a reserve and rounds up to a practical generator rating.
What the three results mean
| Result | What it is | How it is worked out |
|---|---|---|
| Running load | Continuous power of everything you run at once | Sum of the running watts of the selected equipment |
| Largest startup surge | Extra power needed for a moment when the biggest motor or compressor starts | The single largest additional starting watts among the selected loads |
| Estimated peak demand | The most the generator has to deliver at once | Running load plus the largest startup surge |
Running watts vs starting watts
Running watts are what equipment draws while it operates normally. Starting watts are the brief extra draw when a motor or compressor starts, usually two to three times the running figure for a second or two. Refrigerators, freezers, sump and well pumps, air conditioners, furnace blowers, compressors and power tools all have a startup surge. Lights, heaters and electronics do not.
Worked example
| Equipment | Running watts | Additional startup |
|---|---|---|
| Refrigerator | 700 W | 2,200 W |
| Sump pump | 800 W | 1,300 W |
| LED lights | 100 W | 0 W |
| Wi-Fi router | 15 W | 0 W |
| TV | 120 W | 0 W |
| Result | 1,735 W running | 2,200 W largest surge |
Estimated peak demand is 1,735 + 2,200 = 3,935 W. That does not mean any 3,935 W generator will do. Compare the generator’s rated (running) output, its surge output, its voltage and your equipment’s actual requirements.
Why the whole list matters
Sizing from the largest appliance alone is how people end up with an undersized generator. A refrigerator, lights, router, sump pump, furnace and TV each look small, but they run together. The calculator looks at the full group you expect to operate instead of one appliance.
120 V and 240 V equipment
Most household electronics and many appliances use 120 V. Larger equipment such as some central air conditioners, well pumps, electric ranges and dryers needs 240 V. If your list includes 240 V equipment, make sure the generator has a compatible 120/240 V outlet. Wattage alone does not tell you whether a generator can safely run a particular appliance, so always check the equipment nameplate and the generator’s specifications.
How accurate are the wattage estimates?
Appliance figures are typical values for common equipment in good working condition, aligned with manufacturer specifications where those are available. Age, efficiency and how often something cycles all change the real number. Use the estimates for planning, and replace them with the label or manual figure for your own equipment when you can.
The starting-demand assumption
Peak demand uses the single largest additional startup surge, which suits a straightforward planning estimate. It does not model every possible startup sequence. If several large motors can start at the same moment, the real requirement may be higher. For a permanent standby system or a complex installation, have a qualified professional verify the generator capacity.
What to compare when choosing a generator
- Rated (running) watts: the continuous power it can supply.
- Starting (surge) watts: the extra power available to start motors.
- Output voltage: 120 V only, or 120/240 V if you have 240 V equipment.
- Fuel type: gasoline, propane, diesel or dual fuel.
- Transfer-switch compatibility: needed for safe connection to a home’s wiring.
- Manufacturer specifications: check the official documents rather than a single peak-watt headline number.
Frequently asked questions
How do I calculate the wattage needed for a generator?
Add the running watts of the equipment you expect to operate, then account for the extra startup demand of motor-driven appliances. This calculator does that automatically from the equipment you select.
Do I add all the starting watts together?
Not usually. A simple estimate adds only the largest additional startup demand while the other loads are already running. If several large motors can start together, the real requirement may be higher.
How many watts does a typical home need?
There is no single number. A small set of essential loads may need only a few thousand watts, while a home with central air conditioning, a well pump or electric heating needs far more. Calculate the specific equipment you want to run.
Can I use this for a whole-house generator?
Yes, as a planning tool: include the major loads the generator will supply. A permanent standby system should still be checked against the home’s electrical service, the transfer equipment, the generator specifications and local installation requirements.
Can I use it for an RV or a job site?
Yes. Choose the RV Basic or Job Site preset and adjust the equipment to match what you actually plan to run.
Does a generator need both 120 V and 240 V output?
Only if your equipment needs it. If your list includes 240 V equipment, the generator must provide compatible 240 V output in the right configuration.
Keep planning
The next calculators most people need after this one.
Ready to choose a size?
Your load total feeds straight into the size calculator.
Calculators give planning estimates from typical values. Check your equipment’s nameplate and manual, and have a licensed electrician confirm anything connected to your home’s wiring. See our terms.