Generator Size Calculator
Select the appliances you need to power to get a recommended generator size.
How this is calculated
Total running watts sums the steady-state draw of every selected appliance. Peak starting demand takes the worst case of any single unit's motor-starting surge happening while everything else is already running — not the sum of every appliance starting at once, which would badly oversize the recommendation. A 20% safety margin is then added on top.
That safety margin exists because generators shouldn't be run continuously at their absolute maximum rated output — sustained operation above roughly 75–80% of rated capacity accelerates wear, runs the engine hotter, and leaves no headroom for voltage fluctuations or appliances drawing slightly more than their nameplate figures suggest. A generator sized with this margin has room to handle real-world variability without being pushed to its limit on every use. To estimate your typical household electricity usage in the first place, see the Home Electricity Consumption Estimator, and for the underlying power relationship behind wattage figures, see the Electrical Power calculator.
Frequently asked questions
- Why is the recommended size less than the sum of every starting watt?
- Because in practice, appliances don't all switch on at the exact same instant. Generators only need to survive the single worst moment — one motor starting while everything else is already running steady-state — not every motor surging simultaneously. Sizing for the naive sum of all starting watts would recommend a generator far bigger (and more expensive) than necessary.
- Why do motors need extra starting watts?
- Motors (fridges, pumps, air conditioners) draw a brief surge — often 2-3x their running wattage — to overcome initial mechanical resistance when starting up. Once running, they settle down to their steady running wattage.
- What is the 20% safety margin for?
- A buffer above the calculated peak demand, so the generator isn't running at its absolute limit — extending its lifespan and leaving headroom for voltage fluctuations or slightly higher real-world draw than the nameplate figures suggest.
- Can I use this in my own app?
- Yes — every calculator on Stupidly Clever has a matching REST API and MCP tool that runs the same underlying logic.
- How is generator size calculated?
- Total running watts sums the steady-state draw of every selected appliance. Peak demand adds the single largest starting-watt surge on top of that running total, modelling the worst realistic moment — one motor starting while everything else already runs steadily. A 20% safety margin is added to that peak for the final recommended size.
- What is starting vs running wattage?
- Running wattage is the steady power an appliance draws once operating normally. Starting (or surge) wattage is the brief spike — often 2–3× the running wattage — that motor-driven appliances like fridges, pumps, and air conditioners draw for a fraction of a second when first switching on.
- What is a surge factor and why does it matter?
- The surge factor is the ratio between an appliance's starting wattage and its running wattage. It matters because a generator must be sized to handle the single largest surge on top of everything else already running — undersizing for surge, even if the generator could handle everything's running wattage combined, can trip the generator or damage the motor trying to start.
- What fuel type should I choose?
- Gasoline generators are widely available and affordable but have a shorter shelf life for stored fuel. Propane burns cleaner and stores indefinitely but generally provides less power per gallon-equivalent. Diesel generators are common for larger, longer-duration backup needs due to fuel efficiency and durability. The right choice depends on how often you'll use it, how long you need to run it, and local fuel availability.