What size generator do I need?

How this actually works

Most online generator calculators add up appliance wattages. That is not how sizing is done, and it is why those tools routinely over-recommend. This one runs the NEC dwelling-unit load calculation — the same method your electrician and your inspector use.

It starts from general lighting and receptacle load based on your square footage, adds the required kitchen and laundry circuits, then applies NEC demand factors: not everything in a house runs at once, and the code says by how much you may discount that. Cooking appliances get Table 220.55 treatment, which is why a single 8.5 kW range counts as 6.8 kW rather than 8.5.

Part III or Part IV?

The code permits two methods and you may use either. Part IV (the optional method) lumps general and fixed loads together and takes the first 10 kW at 100% and the remainder at 40%. Part III (the standard method) discounts each category separately. They give different answers on the same house, and the smaller result is the one worth using. The tool defaults to Part IV — switch it in Advanced settings and compare.

Two constraints, not one

Running load is only half the problem. A motor — your A/C compressor, usually — draws several times its running current for the moment it starts, and a generator that comfortably carries your house can still stall on that inrush. So the tool also checks locked-rotor amps against each unit's starting capability and sizes to whichever constraint is tougher. It tells you which one drove the answer.

If you have the A/C data plate to hand, enter the actual LRA in Advanced settings. A real nameplate figure always beats our estimate, even when it is lower.

Fuel and elevation

Generators are rated differently on natural gas and propane — propane usually yields slightly more output — so the fuel you pick changes which unit fits. Elevation derates output by about 3.5% per 1,000 feet; Houston is near sea level, so it rarely matters here, but the field is there if you are sizing somewhere else.

Load management

Tick "load managed" on a big intermittent load like an A/C or an EV charger and it drops out of the NEC calculation — that is what a load-management module buys you, and it can drop you a whole generator size. It still counts toward starting current, because a module sheds a load after it is running, not during the inrush.

What it cannot tell you

This is an estimate for getting you into the right bracket, not a stamped load calculation. It does not know your actual panel, your service size, whether your gas line can carry the demand, or where a unit can legally sit on your lot. The default wattages are typical figures — override them with your nameplate values wherever you have them.

Commercial buildings need a proper load study rather than this — see commercial generators.

Got a number and want it checked? Call (346) 420-5744 or request a free quote — we will size it properly on site at no charge.