Generator Sizing

Notes

• The starting surge sizes the generator, not the running load. This house draws 13.4 kW steadily, but when the air conditioner or well pump starts, demand spikes to 24.4 kW for a second or two. A generator that cannot swallow that sags, and the sag is what damages motors and drops the whole panel.
• Surges do not add up, and this is where people overbuy. Two motors almost never start in the same instant, so you size for the largest single surge, not the sum. Adding every appliance's starting watts together produces a number roughly twice what the house needs — and a generator twice the size costs more to buy, more to install, and burns more fuel at part load.
• A load management module would let a 18 kW unit do this job. It sheds the air conditioner or the water heater for the few seconds another large load starts, so the generator never sees the combined peak. It costs a few hundred dollars against several thousand for the next size up, and it is almost never offered unprompted. Ask about it explicitly.
• A soft starter on the air conditioner is the other lever. It cuts compressor inrush by roughly two thirds for a couple of hundred dollars, and on a house whose peak is set by the AC that frequently drops the generator a whole size.
• Fuel matters more than the nameplate for a long outage. Natural gas runs indefinitely but delivers roughly 10% less output than propane. Propane means a tank, and a 500-gallon tank at half load runs a 20 kW unit somewhere around two to three days — not the week people assume.
• The transfer switch is a separate decision and a separate price. A whole-house automatic switch costs more than an essential-circuits one, and it is what determines whether you choose loads in advance or the generator carries everything. Budget it alongside the machine rather than discovering it in the quote.