The single biggest generator-sizing mistake is adding up the running wattage of every appliance and stopping there — ignoring that motors, compressors, and pumps momentarily draw far more power the instant they start than they do while running. A generator sized only for steady-state running load will stall, trip its breaker, or shut down the first time your fridge compressor or AC kicks on under load.
Running watts vs starting (surge) watts
Every appliance has a running wattage — what it draws once it's up and operating normally. Motor-driven appliances also have a starting wattage, which can be 2-3× the running figure for a fraction of a second as the motor overcomes initial mechanical resistance. Purely resistive loads (lights, heaters, most electronics) don't have this surge — what you see on the label is what you get continuously.
Why you only add the largest single surge, not all of them
In real-world use, appliances rarely all start at the exact same instant — your fridge compressor cycling on won't usually coincide perfectly with your AC compressor starting. The standard, practical approach is: size the generator for the total running load of everything running simultaneously, plus the starting surge of only the single largest motor-driven item, since that's the realistic worst-case moment the generator needs to survive.
Typical starting surge multipliers
| Appliance type | Typical surge vs running watts |
|---|---|
| Refrigerator / freezer compressor | ~2–3× |
| Window or split AC unit | ~2–3× |
| Well pump / submersible pump | ~2–4× |
| Power tools (drills, saws) | ~2× |
| Lights, TVs, most electronics | ~1× (no meaningful surge) |
Why the safety headroom matters
Running a generator flat-out at its maximum rated capacity for extended periods accelerates wear and increases the risk of nuisance shutdowns from small load fluctuations. Sizing with 25-35% headroom above your calculated peak demand keeps the generator operating in a healthier range and gives you room to add a load later without immediately overloading it.
To work out the loads a generator (or inverter) needs to carry, start with the daily load calculator. If the generator is a backup within a solar setup, the inverter calculator and off-grid system wizard help you size the rest.
Frequently asked questions
Do I need to size for every appliance in the house running at once?
No — size for what will realistically run simultaneously during an outage. Most homes don't run every appliance at once; a generator sized for realistic simultaneous use is both cheaper and more practical than one sized for a scenario that never happens.
What happens if I undersize the generator?
The generator will typically overload and shut down, or its breaker will trip, the moment the largest motor tries to start — usually right when you need power most.
Should I size a generator to also charge my solar batteries?
Yes — if the generator will also run a battery charger or hybrid inverter's charging function, add that charger's rated input wattage to your running load total before sizing.