The two most common generator mistakes we see are buying a 5.5kVA petrol set to run a whole house, and buying a 60kVA diesel to run a small shop. One of them will not start the fridge. The other will burn fuel idling at ten percent load and wear itself out doing it.
Working out the right size is not complicated. It just needs twenty minutes with a pen.
Step one: list what must actually run
Not what you would like to run. What must run. Write down every appliance or machine and its wattage, which is usually on a plate on the back or underneath. Lights and phone chargers are trivial. Fridges, freezers, geysers, pumps, compressors and anything with a heating element are not.
Add them up. That is your running load.
Step two: add the starting load
Anything with an electric motor draws far more current for the first second or two than it does once it is spinning. A fridge compressor, a borehole pump, an air compressor or a three-phase motor can pull three to six times its running current on start-up.
So take the biggest motor on your list and add roughly three times its running wattage on top of your total. If several motors could start at the same moment, add for the two largest. This is the number that catches people out.
Step three: convert to kVA and add headroom
Generators are rated in kVA, appliances in watts. As a working rule for mixed domestic and light commercial load, divide your watt total by 800 to get kVA. Then add about 20 percent headroom so the set is not running flat out.
A worked example. Say your must-run load is 3 200W and the biggest motor is a 750W borehole pump. Starting allowance is roughly 2 250W. Total 5 450W, divided by 800 is about 6.8kVA, plus 20 percent gives about 8kVA. So a 5.5kVA petrol set will not do it, and an 8 to 10kVA set will.
Petrol or diesel
Petrol sets are cheaper to buy, lighter, and fine for intermittent use: a few hours a week, a market stall, a site tool. They are noisy and they drink fuel.
Diesel sets cost more up front, run far more cheaply per hour, last longer and handle sustained load better. If the set is running most days, diesel wins on total cost inside a year. If it runs occasionally, petrol is the sensible buy.
Single phase or three phase
Households and most small shops are single phase. Workshops with a lathe, a compressor or a large motor are usually three phase. Do not buy a three-phase set for a single-phase property just because it looked like a bargain. Check what comes into your distribution board first.
The part everyone forgets
A generator needs a changeover switch, proper earthing and somewhere ventilated to sit. Running an extension cord from a genset into a plug point back-feeds your wiring and can kill somebody working on the line. Budget for a qualified electrician to install a changeover, and get a certificate of compliance for it.
Send us your load list and we will tell you what size you actually need, including whether you need less than you thought.