Generator Size Calculator
Short answer: size a generator by running watts plus the single largest starting surge. Add the running watts of everything you’ll run at once, then add only the biggest motor’s starting watts on top (one motor starts at a time). Backup essentials need about 1,500 running / 2,500 surge watts (a 2,000–3,000W unit); add a well or sump pump and you need ~3,350 running / 5,350 surge (a 5,000–7,500W unit). This free calculator runs that math on real manufacturer wattage data.
Pick the appliances you want to back up and get the running watts and the starting (surge) watts you actually need — or switch modes and enter a generator’s rated and surge watts to see whether it will carry your load. The wattage figures are typical ranges drawn from Honda Power Equipment’s and DonRowe’s published appliance charts; every appliance you add shows its own numbers so nothing is hidden.
Check what you want to power at the same time, and set how many of each. The calculator adds the running watts and the single largest starting surge.
Both numbers are printed on the generator’s spec sheet. Then check the same appliances above — we compare your generator to that load.
How the calculator works
It uses the standard manufacturer method, the same one Honda spells out in its sizing guide:
| Step | What it does |
|---|---|
| Running watts | Add the running watts of every appliance you select (× quantity). This is the steady load your generator carries continuously. |
| Starting surge | Find the appliance with the largest starting spike, and add only that one surge on top of the running total — because only one motor starts at a time. |
| Recommended size | The smallest common generator whose rated watts cover the running total and whose surge (we assume ~20% over rated, typical for portables) covers the starting peak. |
| Generator type | Small clean loads → inverter; mid loads → portable; whole-house with central AC / electric heat → standby. |
The exact formula: runningNeeded = ∑(appliance running watts × qty), then startingNeeded = runningNeeded + max(appliance starting watts − that appliance’s running watts). A generator fits when its rated ≥ runningNeeded and its surge ≥ startingNeeded.
The mistake to avoid: sizing on running watts alone
The classic error is adding up only the running watts and buying a generator at that number. It boots fine on the test bench, then the well pump or AC compressor kicks in, the brief surge exceeds what the generator can deliver, and it stalls or trips. A 5,000W running figure means nothing if the surge from a single 1/2 hp well pump pushes the instantaneous demand to over 7,000W. Always size for the surge, not just the steady draw — see running vs starting watts explained.
Frequently asked questions
What size generator do I need to run my house during an outage?
The essentials — fridge, gas furnace blower, lights, a TV — need about 1,500 running watts and a ~2,500W surge, so a 2,000–3,000W unit. Add a well or sump pump and the surge passes 5,000W, moving you to a 5,000–7,500W generator. See outage picks.
What is the difference between running watts and starting watts?
Running watts is the steady draw once an appliance is on; starting watts is the brief surge a motor needs the instant it switches on, often 2–3× its running watts. Full explanation here.
How do I calculate what size generator I need?
Sum the running watts of everything running at once, then add only the single largest starting surge on top (one motor starts at a time). That gives the running watts and peak surge your generator must supply.
Will a 5,000 watt generator run my whole house?
It runs the essentials plus a well or sump pump, but not central AC or an electric water heater. Central AC alone can need ~3,800 running and a ~5,800W surge. A whole house with central AC and electric heat needs a 10,000–22,000W standby unit. See whole-house picks.
Related: What size generator do I need? (full guide) · Running vs starting watts · Best for a whole house · Best for a power outage
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