Generator Size Calculator
Running load plus one surge — surges are not additive.
Work out Generator Size. Running load plus one surge — surges are not additive. Says what it does not answer, as well as what it does.
Same as running for anything without a motor.
Lights, electronics — anything with no starting surge.
Generator size
4,800 W
2,000 W running + 2,000 W surge, plus 20%
Surges are not additive — motors do not all start at the same instant, so the requirement is the running total plus the single largest surge. Adding every surge would put this at 5,500 W, oversizing by 1,500 W and costing considerably more for capacity you cannot use.
How the Generator Size Calculator works
The rule that decides generator sizing: starting surges do not add up. Motors draw several times their running current for a second or two at start, but a fridge and a well pump do not start at the same instant — so the requirement is total running load plus the single largest surge. Summing every surge buys a generator two or three times bigger than needed; ignoring surges entirely buys one that stalls when the compressor kicks in.
Also known as: what size generator do I need · generator wattage calculator · backup generator sizing · generator starting watts calculator
The rule that decides it
Total running watts, plus the single largest starting surge, plus headroom. Not the sum of every surge.
Motors draw several times their running current for a second or two at start, while overcoming inertia. But a fridge compressor and a well pump do not start at the same instant, and the probability of every motor in a house starting simultaneously is negligible.
Sizing for that coincidence buys a generator two or three times larger than needed. Ignoring surges entirely buys one that stalls the moment the compressor kicks in.
In practice
A fridge at 700 W running and 2,200 starting, a pump at 1,000 running and 3,000 starting, and 300 W of lights.
Running total: 2,000 W. The surges are 1,500 and 2,000, and only the larger counts — so the peak requirement is 4,000 W. With 20% headroom that is 4,800 W.
Add both surges instead and you get 5,500 W before headroom, pushing you to a 6,600 W generator. That is 1,800 W of capacity you are buying and will never use, at a real difference in price, weight and fuel consumption.
Where the figure deceives
Starting multipliers vary by motor type. A capacitor-start induction motor pulls harder than a modern inverter-driven compressor, and the nameplate is the reliable source rather than any general ratio.
Generators are also rated two ways — a surge rating they can hold briefly and a running rating they can sustain. Sizing against the surge rating leaves nothing in reserve for the continuous load.
Altitude and temperature derate output too. A generator at 5,000 feet delivers noticeably less than its sea-level rating, which is not in this calculation.
And running any generator continuously near its maximum shortens its life, which is what the headroom is really for.
Acting on it
List what genuinely needs to run at once. Most backup scenarios need far less than a whole house, and the list is where the sizing decision actually happens.
Take starting watts from the nameplate rather than a rule of thumb, and note that resistive loads have none at all.
For a whole-house standby unit, ask about load management. Shedding non-essential circuits when a large motor starts prevents the simultaneous case rather than paying to survive it.
Frequently asked questions
What size generator do I need?
Add the running watts of everything you want powered at once, then add the largest single starting surge among them, then add headroom. Not the sum of all the surges.
Why not add all the starting watts?
Because motors start at different moments and for only a second or two. The chance of every motor starting simultaneously is negligible, and sizing for it is expensive — often two or three times the generator you actually need.
What is the difference between running and starting watts?
Running watts are the steady draw; starting watts are the brief surge as a motor overcomes inertia, typically two to three times running for an induction motor. Resistive loads like heaters and lights have no surge at all.
How much headroom should I leave?
Around twenty percent is common. Running a generator continuously at its maximum shortens its life and leaves nothing for a load you forgot, and generators are usually rated for a surge they cannot sustain.
Does it matter for a whole-house standby unit?
Yes, though those are usually sized with a load-management device that sheds non-essential circuits when a large motor starts. That changes the arithmetic by preventing the simultaneous case rather than paying for it.
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