Generator Sizing Calculator

What size generator do I need?

Sizing a generator means meeting two different demands at once: the steady running watts your appliances draw continuously, and the brief starting (surge) watts that motors and compressors spike to at the instant they switch on. A generator that can handle your running total but not the surge will stall or trip when the fridge or pump kicks in — which is why surge, not running load, often decides the size you need.

Choosing a generator size is the single most expensive decision to get wrong in backup power: too small and it stalls the moment your fridge and pump both want to start, leaving you no better off in an outage; too large and you’ve overspent on a noisy, thirsty machine that loafs inefficiently and burns extra fuel. The trap is that the right size isn’t simply the sum of your appliance wattages — it’s driven by the brief surges that motors demand at start-up, which can dwarf their steady draw. Once you understand running watts versus starting watts and how to combine them correctly, sizing becomes straightforward, and you can buy exactly the capacity you need with sensible headroom and nothing wasted.

Running watts vs starting watts

Resistive loads — lights, heaters, kettles — draw the same starting and running watts. But anything with a motor (fridges, freezers, well and sump pumps, air-conditioners, power tools) demands a surge at start-up that's typically around three times its running wattage, sometimes more. A 700 W fridge compressor can briefly need 2,000 W or more to start. The generator has to supply that momentary peak or the appliance won't start and the generator may overload.

How to size correctly

1. List every load you want to run during an outage, with its running watts.
2. Add the running watts together — this is your continuous demand.
3. Add the surge of the single largest motor load, not every surge at once. Appliances rarely all start at the same instant, so the correct peak is your total running watts plus the biggest single starting surge.
4. Add ~20% headroom for safety, future loads and altitude/temperature derating.

A common mistake is adding every appliance's surge watts together, which hugely overstates the generator you need; the realistic peak is running total plus the largest one surge.

Typical sizes

For most homes, 5,000–8,000 W of output covers essential appliances and circuits during an outage. Smaller homes backing up just a fridge, lights and electronics can manage with less; whole-home coverage including central air, electric heating or a well pump can need 10,000–20,000 W and usually a permanently installed standby generator with a transfer switch rather than a portable unit. Be honest about whether you need to run the whole house or just keep essentials alive — the latter is far cheaper and is what most households actually do.

Solar generators and fuel

Battery-based "solar generators" are sized the same way on the wattage side — the inverter must cover your running load plus the largest surge — but you also need enough stored energy (kWh) for the hours you want to run, and enough solar to recharge. For fuel generators, remember that fuel can be hard to obtain during a prolonged outage, so plan storage accordingly; for natural-gas standby units that constraint disappears.

Matching the generator to a transfer switch and your panel

Sizing the wattage is only part of a safe backup setup. To power household circuits rather than running extension cords, a generator connects through a transfer switch that safely isolates your home from the grid — this prevents dangerous "backfeed" that could injure utility workers, and it's a code requirement in most places. The transfer switch (manual or automatic) is sized to the generator and the circuits you intend to back up, so it's worth deciding early which circuits matter: a whole-home switch costs more than one feeding a handful of essential circuits. Fuel type also shapes the decision — petrol is convenient but stores poorly and can be scarce in a prolonged outage; propane stores well; natural gas removes the storage problem entirely but ties you to the gas supply. Standby units start automatically within seconds of an outage, while portables must be wheeled out, fuelled and started manually. Factoring in the transfer switch, fuel and start method alongside raw wattage is what turns a generator from a box in the garage into reliable backup power.

Frequently asked questions

For essential appliances, 5,000–8,000 W suits most homes. Whole-home coverage with central air or electric heat often needs 10,000–20,000 W and a standby unit. Size from your actual load list plus the largest single surge, with ~20% headroom.

Its compressor motor draws a brief inrush current at start-up, often about three times its running wattage. The generator must supply that momentary surge, so motor loads usually dictate the size more than steady loads do.

No — that overstates the requirement. Add all running watts, then add only the single largest starting surge, since appliances don't all start at the same instant. That sum is your realistic peak demand.

Around 20% above your calculated peak, to cover future loads, voltage stability, and the output loss generators suffer at high altitude or in hot weather. Running a generator constantly at its limit shortens its life.