Generator Size Calculator
Size a backup generator by adding your appliances' running watts and the largest single starting surge to find the running and peak (starting) wattage you need.
How to use this tool
- List the appliances you'll run at the same time and their running watts.
- Enter the total running watts of that list.
- Find the single appliance with the biggest starting surge and enter it.
- Add an optional safety margin for headroom or future loads.
- Read the running and starting watts your generator must meet or exceed.
Size a backup generator correctly. Add up the running watts of what you need to power, plus the biggest startup surge, to find the running and starting wattage your generator must provide.
Formula
A generator must cover the steady running load plus the worst single startup surge happening on top of it:
Running watts needed = Total running watts × (1 + Safety%)
Starting watts needed = Running watts needed + Largest single surge
Only one motor is assumed to start at a time, so just the biggest surge is added.
How it works
Sizing a backup generator means matching two numbers: the running (rated) wattage it can sustain, and the higher starting (surge or peak) wattage it can deliver for a few seconds. Motor-driven appliances — refrigerators, well pumps, air conditioners, sump pumps — draw a brief inrush several times their running watts when they start.
This calculator adds the running watts of everything you intend to power simultaneously, then adds the single largest starting surge among those appliances. The assumption is that you won't start two big motors at the exact same instant, so only the biggest surge needs headroom above the running total — the standard way to size generators without massively oversizing them.
Read each appliance's running and starting watts from its label or the reference table below; resistive loads like lights, heaters, and microwaves have no surge (starting equals running). The optional safety margin adds headroom on the running load for future loads or to avoid running the generator at full output continuously. Pick a generator whose rated and surge ratings both meet or exceed the results.
Worked example
4,000 W of running load with a 2,000 W largest surge
- Running watts needed = 4,000 W × (1 + 0%) = 4,000 W.
- Add the single largest starting surge: 4,000 W + 2,000 W = 6,000 W.
- Choose a generator rated at least 4,000 W running and 6,000 W starting.
Running watts needed 4,000 W | Starting watts needed 6,000 W
Typical running and starting watts of common backup loads
| Appliance | Running watts | Starting (surge) watts |
|---|---|---|
| Refrigerator | 700 W | 2,200 W |
| Sump pump (1/2 hp) | 1,050 W | 2,150 W |
| Window AC (10,000 BTU) | 1,200 W | 1,800 W |
| Furnace blower (1/2 hp) | 800 W | 1,600 W |
| Well pump (1 hp) | 2,000 W | 4,000 W |
| Microwave | 1,000 W | 1,000 W |
| Lights + electronics | 600 W | 600 W |
Key terms
- Running (rated) watts
- The continuous power a generator can supply indefinitely — must cover the sum of all loads running at once.
- Starting (surge) watts
- The brief peak power a generator can deliver for a few seconds to start motor-driven appliances.
- Inrush / surge current
- The momentary spike in power an electric motor draws at startup, often two to three times its running draw.
- Resistive load
- A load with no motor — lights, heaters, microwaves — whose starting watts equal its running watts (no surge).
Frequently asked questions
- What size generator do I need for my house?
- Add the running watts of everything you'll power at once, then add the largest single starting surge. Essentials (fridge, lights, a few outlets, furnace fan) often need 3,000–5,000 running watts; whole-home backup with AC and well pump can need 7,500–10,000 or more.
- Why are starting watts higher than running watts?
- Appliances with electric motors draw a brief inrush current at startup, often two to three times their running draw. The generator must supply that surge momentarily, so its starting rating must exceed the running total plus the biggest surge.
- Should I add a safety margin?
- A 10–25% margin on the running load is prudent so the generator isn't always at full output and you have room for added loads. This tool's safety-margin field lets you build that headroom into the result.