Generator Wattage Calculator
This generator wattage calculator adds up running and starting watts for what you need to power, so you know the minimum generator size.
- Free
- No sign-up
- Runs in your browser
- Works on your phone
Your appliances
Leave starting watts blank for anything without a motor (lights, electronics, chargers), since those draw the same watts to start as they do running.
Minimum generator size
Add appliances to see your minimum generator size.
Motor-driven appliances (refrigerators, pumps, air conditioners) need extra surge capacity for a moment when they switch on, which is why the calculator asks for starting watts separately.
How this calculator works
Total running watts is every appliance's running wattage added together. That's your continuous load. The minimum generator size is that running total plus the single largest starting-watts figure among your appliances, since only one motor typically surges on at a time. Adding every appliance's starting watts together would significantly overestimate what you actually need.
Worked example
Say you want to run a refrigerator (700 W running, 2,100 W starting), a sump pump (1,200 W running, 2,500 W starting), and 300 W of lights and electronics.
- Running watts: 700 + 1,200 + 300 = 2,200 W.
- Largest starting figure: the sump pump's 2,500 W.
- Minimum generator size: 2,200 + 2,500 = 4,700 W.
Those figures fall within the ranges in the table below; use your own appliances' nameplate numbers for a real answer.
Common appliance running / starting watts
| Appliance | Running | Starting (total surge) |
|---|---|---|
| Refrigerator (ENERGY STAR, mid-size) | 300-930 W | 900-2,790 W |
| Sump pump (3/4 HP) | 1,200 W | 2,500-4,500 W |
| Well pump (1/2 HP) | 1,000 W | 2,100-4,000 W |
| Window air conditioner (10,000 BTU) | 1,800 W | 2,700 W |
| Furnace fan blower (1/2 HP) | 800 W | 2,350 W |
| Space heater | 1,500 W | 2,000 W |
| Microwave | 1,000 W | 0 W |
| Lights & electronics (per device) | varies | 0 W |
Source: Jackery appliance wattage reference chart, a commercial source rather than a government one. Check your own appliance's nameplate for its exact rating.
The sizing method here (total running watts plus the single largest starting-watts figure among your appliances) follows standard generator sizing guidance, since typically only one motor-driven appliance surges on at a time. Preset running/starting watt figures for common backup-power appliances come from Jackery's appliance wattage reference chart, a commercial source rather than a government one. Last checked September 2026. Cedrick Reese built this generator calculator and rechecks the preset running and starting watts against the chart above.
Frequently asked questions
What's the difference between running watts and starting watts?
Running watts is the steady power an appliance draws once it's operating normally. Starting (or surge) watts is the extra power a motor-driven appliance briefly needs for a second or two when it first switches on, commonly 2 to 3 times its running wattage for things like a refrigerator, sump pump, or air conditioner.
Do I add up the starting watts of every appliance?
No. The standard method is to add up everyone's running watts, then add only the single largest starting-watts figure among your appliances on top of that total, since motor-driven appliances don't all surge on at exactly the same moment in normal use. Adding every starting watt figure together would significantly oversize the generator you need.
What size generator do I need for a refrigerator and a few other essentials?
It depends entirely on what you're running. A refrigerator alone often needs a generator in the 1,000-3,000 watt surge range; add a sump pump, a window AC, some lights, and a few electronics, and the total climbs quickly. Use the calculator above with your specific appliances for a real number.
Is it safe to run a generator right at its maximum rated wattage?
It's generally safer to leave some headroom rather than running a generator at its absolute maximum continuously. Many guides suggest sizing for at least 10-20% above your calculated total where practical, and any whole-house backup setup should be planned with a licensed electrician, including a transfer switch to avoid backfeeding the grid.
Related tools
This calculator provides estimates based on the figures you enter and is meant as a starting point, not a final specification. Consult a licensed electrician for any permanent or whole-house backup power installation, including proper transfer switch equipment.