How to Use This Tool
Tick everything that has to run during an outage, then read the third card. The first two cards are the two wrong answers, shown so you can see how far apart they are.
Why motors need more to start than to run
An induction motor at rest has no back-EMF opposing the supply, so the initial current is several times the running figure. It lasts a second or two. A fridge that runs at 150 W can draw 1,200 W at that instant; a well pump running at 1,000 W can pull 3,000.
Resistive loads — lights, kettles, heaters, most electronics — have no surge at all. Their starting and running figures are the same, which is why a house full of LED lighting is easy to back up and a house on a well is not.
running starting surge Refrigerator 150 1,200 +1,050 Sump pump 800 2,000 +1,200 Furnace fan 600 1,800 +1,200 Well pump 1,000 3,000 +2,000 Lights 300 300 0 Router and TV 200 200 0 running total 3,050 W every starting watt added 8,500 W oversized running + largest surge only 5,050 W correct
One surge, not all of them
Starting surges last a second or two. For every motor in the house to surge in the same second, they would all have to switch on simultaneously, which does not happen in practice — and if you are worried about it, staggering start-up manually is free while a bigger generator is not.
So the number to size against is the total running load plus the single largest surge. That guarantees the generator can carry everything continuously and still absorb the worst individual start.
Generators are advertised by the bigger number
A unit sold as "5,500 watt" is usually quoting its surge or peak rating, and its continuous running rating is nearer 4,500. The running rating must exceed your running total, and the surge rating must exceed your running total plus the largest surge. Comparing your requirement against the headline number is how people end up one size short.
Fuel changes the output
The same engine produces roughly 10 per cent less on propane and 15 per cent less on natural gas than on petrol, because those fuels carry less energy per unit volume delivered. A dual-fuel unit rated 7,500 W on petrol may only manage 6,750 on propane, so size against the fuel you will actually use.
What this does not cover
How the generator connects to the house is a separate and more serious question. A transfer switch or interlock is required to prevent backfeeding the utility supply, which can kill line workers. That is an electrician's job everywhere, and running a generator indoors or in an attached garage is fatal because of carbon monoxide.
