Generator Sizing Calculator
Size a backup generator from your running and starting watts, accounting for the largest motor surge plus a safety margin. Calculates in your browser.
Recommended generator
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Show the math
The worst case is one motor starting while everything else is already running:
peak W = Σ running W + max(starting − running)
recommended W = peak × (1 + headroom)
Starting (surge) watts apply to motors, fridges, pumps, AC. Resistive loads like lights and heaters have no surge. Leave starting blank to match running. All math runs in your browser.
What this does
A generator sizing calculator finds the backup generator size you need from your running watts plus the largest motor start-up surge and a safety margin.
How to use it
- Enter the running watts of each load.
- Mark the largest starting surge.
- Apply a safety margin.
- Read the required generator size.
How it works
Motors like fridges, pumps, and AC can draw 2–3× their running watts at start-up; resistive loads like lights and heaters have no surge.
size = running watts + largest surge + margin
Understanding your result
The size must cover your running watts plus the surge of the largest motor starting, because fridges, pumps, and air conditioning can pull two to three times their running draw for a moment. Undersizing causes stalls and can damage both the generator and the appliances, while a big oversize wastes fuel, so aim for headroom rather than a generous round-up.
Example
5,000 running watts plus a 2,000 W surge peaks at 7,000 W, so with the default 20% margin you would look for about an 8.4 kW generator.
Last updated .
Frequently asked questions
What size generator do I need?
Add up all running watts, then add the single biggest start-up surge (a motor starting while everything else runs) and apply a safety margin.
What are starting watts?
The brief surge a motor draws at start-up — fridges, pumps and AC can need 2–3× their running watts. Resistive loads like lights and heaters have no surge.
Is my data stored?
No. Everything is calculated locally in your browser.
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