Pick a starting point, then add or remove what you'll run. It allows for the start-up surge from fridges, air con, pumps and power tools, which is what catches most people out.
Everything set to On is added up. A generator shouldn't run above about 80% of its rating for long, so the continuous rating needs to be at least the running load ÷ 0.8.
Motors draw 2–4 times their running power for a second or two. The generator must hold the running load plus the biggest single surge. This allows a conservative 10% surge above the generator's rating, since surge figures vary a lot between brands.
Fuel use is estimated for a petrol generator at your running load. Real figures vary by brand. Inverter generators use much less fuel at light load because the engine slows down.
A typical rooftop caravan air conditioner runs at about 1300 W but needs around 2800 W for a moment when the compressor starts. With a few small loads running too, a 3000 to 3500 W generator is usually needed. Fitting a soft starter to the air conditioner cuts the start-up surge enough for many to run on a 2000 to 2200 W inverter generator.
Running watts are what an appliance uses once it is going. Motors in fridges, air conditioners, pumps, compressors and saws draw two to four times that for a second or two as they start. A generator must cover all running loads plus the biggest start-up surge.
No. Back-feeding through a power point with a double-ended lead is illegal and can kill line workers or start a fire. A licensed electrician needs to fit a generator inlet and a changeover or transfer switch so the house is disconnected from the grid while the generator runs.
Inverter generators are quieter, use less fuel at light load and give clean power that is safe for laptops, TVs and chargers, which makes them the best choice for camping and caravans up to about 3.5 kW. Larger conventional or open-frame generators cost less per watt and suit home backup and worksites.