The right generator size depends on what you actually want to run at the same time, not just the square footage of the house. A small home backup setup that only covers essentials often falls somewhere around 5,000 to 8,000 running watts. If you want to run more loads at once, such as a well pump, furnace blower, sump pump, refrigerator, lights, internet equipment, and a microwave, many homes need something in the 7,500 to 12,000 watt range. Whole-house backup with central air, electric water heating, or large appliances can push the need much higher, sometimes 15,000 watts or more.
The best way to size it is to list every item you want powered during an outage and check both the running watts and starting watts. Starting watts matter because motors draw a much larger surge when they kick on. A refrigerator might use only a few hundred running watts but need more than 1,000 watts to start. A furnace blower, sump pump, or air conditioner can have an even bigger startup surge. If the generator cannot handle those startup spikes, the breaker may trip even if the running load looks fine on paper.
A lot of people make the mistake of adding up every appliance in the house and buying a generator far larger than they really need. Others go too small and then wonder why the generator struggles when the refrigerator and furnace start together. A practical approach is to choose a “must-have” list first. For example, fridge, freezer, several LED lights, modem/router, TV, phone chargers, and furnace blower might only require around 3,000 to 5,000 running watts, but the starting surge can raise the needed capacity significantly. If you also want to run a microwave or coffee maker occasionally, you should build in extra headroom.
For comfort and reliability, it is smart to size the generator with some margin, usually about 20 to 25 percent above your expected peak load. That gives you room for startup surges and keeps the generator from running at its limit all the time. If you are using a portable generator, also remember that the advertised wattage may be peak watts, not continuous watts. Continuous output is what matters for long outages.
If you are unsure, the safest next step is to calculate your essential loads and then talk to an electrician or generator installer about a transfer switch and load management. That way you can match the generator to your actual needs instead of guessing.
