Sizing a standby generator without overpaying
Most people buy twice the generator they need, then complain about fuel. Here is how to work out the real number.
Start with what must stay on
Write the list before you look at any product: fridge and freezer, heating controls and pump, a few lighting circuits, internet, and whatever your work depends on. A house that keeps the essentials alive is usually in the 5–8 kW range. A house that runs everything including electric cooking and air conditioning is 15–22 kW. A workshop is decided by its largest motor, not by the sum of its sockets.
Running watts and starting watts
Anything with a motor — a compressor, a pump, a fridge, an air conditioner — draws a surge of two to three times its running figure for a second or two when it starts. Add the running watts of everything that will be on at once, then add the single largest starting surge on top. That total, not the sum of the surges, is the size you need.
Pure sine wave, and when it matters
Inverter generators produce a clean waveform, usually under 3% total harmonic distortion. That matters for anything with a circuit board: diagnostic equipment, modern boilers, computers, medical devices, variable-speed tools. Conventional generators are cheaper per watt and perfectly fine for heaters, lights, pumps and power tools.
Fuel and run time
Quoted run times are usually measured at 25% or 50% load. At full load, expect roughly half of that. Dual-fuel machines let you run on bottled gas, which stores for years without going stale, while petrol degrades in months unless you treat it. For anything intended to sit unused between emergencies, the fuel that survives storage is worth more than a few extra watts.
Transfer switch, not extension leads
Feeding a house through a socket is how you electrocute the person repairing the line outside. A transfer switch isolates you from the grid and is a legal requirement in most places. Budget for it at the same time as the generator, not afterwards.
Noise and placement
A sound-attenuated standby unit sits around 52–65 dB at a few metres; an open-frame site generator is far louder. Whatever you buy, it runs outdoors, never in a garage or under an overhang, with clear air on all sides and the exhaust pointing away from windows and doors.