Backup Generator Sizing Calculator
What size generator do I need? Running load plus the starting surge of the biggest motor — the step almost every free calculator leaves out, and the reason so many generators stall the first time the pump kicks in.
What are you backing up?
This changes the starting load list
Filled in from your country
V
Advanced / Custom Input — the site
Generators are sold in kVA at this figure.
Headroom for loads you add later.
%
How much above its rating the set can hold for a second or two.
%
The loads
Tick “motor” for anything with a compressor, pump or fan — those are the ones that surge on start. Tick “essential” for what you would keep if you bought a smaller set.
Advanced / Custom Input — the load list
Replaces the load list with a worked starting point.
Applied to non-essential loads only. 100% is the worst case.
%
How the biggest motor starts
This is often the cheapest way to drop a whole generator size
On the motor nameplate. Left on Auto it is picked by size.
Advanced / Custom Input — motor starting
Motors start at a very poor power factor, typically 0.3 to 0.5.
Overrides the code letter. Leave at 0 to use the letter.
kVA/hp
Most specifications limit this to 15 or 20 per cent.
%
Fuel
Availability varies a lot by country
Filled in from your country — put your own in
/L
Per outage, for the running-cost figure
h
Advanced / Custom Input — fuel and runtime
Consumption is roughly proportional to this.
%
For the runtime figure. Mains gas needs no tank.
L
Filled in from the fuel type.
L/kWh
Result
List your loads, then press “Work out my generator”.
Estimate only — verify with a licensed electrician/engineer and the manufacturer’s own sizing software before ordering. Transient voltage dip depends on the specific alternator.
1
W
hp
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