How we size backup power
Every number on this site comes from one transparent calculation. Here it is in full.
The formula
For a given set of appliances, the peak power your generator must survive is:
peak = (sum of all running watts) + (single largest startup surge)
We then apply a 20% safety margin and round up to the nearest 500W class to get the recommended unit size.
Why this beats the common shortcuts
Many calculators either sum only running watts (which undersizes — the generator stalls the moment a well pump kicks on) or sum the startup surge of every appliance (which oversizes — you buy and fuel far more generator than you need). Neither reflects what actually happens during an outage.
In reality, all your appliances draw their running watts continuously, but only one large motor hits its startup inrush at any given instant. So the realistic worst case is your full running load plus the single biggest surge — usually a well pump, AC compressor, or refrigerator.
Surge multipliers we use
| Load type | Startup surge | Examples |
|---|---|---|
| Resistive / electronic | 1.0× | Heaters, kettles, lights, TVs, chargers |
| Light motor | 2.0× | Small fans, dishwasher, dryer drum |
| Induction motor | 2.5× | Sump pump, washing machine, furnace fan |
| Compressor / heavy motor | 3.0× | Fridge, AC, well pump, heat pump |
Running wattages are typical mid-range residential values; real nameplate values vary, so verify against your own equipment. Surge multipliers follow standard electrical practice for motor inrush (NEMA guidance and manufacturer locked-rotor data).
Common questions
Why not just add up all the watts?
Summing only running watts undersizes the generator — it stalls when a motor starts. Summing every appliance's surge oversizes it — you overpay for capacity you never use. The correct figure is running total plus the single largest startup surge.
Why only one surge?
Motors draw their inrush for a fraction of a second at startup. Two large motors cold-starting in the exact same instant is rare, and good practice staggers them. Sizing for the single largest surge plus full running load is the realistic worst case.
What safety margin do you use?
We add 20% headroom above the calculated peak, then round up to the nearest 500W class. This covers nameplate variation and keeps the unit off its limit.