The short answer: add up the running watts of what you must power, then add the single largest starting spike on top. Most households cover their essentials with a 3,500 to 7,500 watt generator, while backing up a whole home, central air and all, takes a 10,000 to 22,000 watt standby unit. Sizing is where people waste the most money in both directions, so here is how to get it right.
Every generator carries two numbers, and you need both. Running watts is the steady power a device draws once it is on. Starting watts, sometimes called surge or peak, is the brief jolt that anything with a motor, a fridge, a furnace blower, a well pump, demands for a fraction of a second when it switches on. A generator must supply the combined running watts of everything you have on at once, plus the biggest single starting surge among them. Miss the surge and the generator bogs down or trips exactly when a motor tries to start.
Before any math, separate essentials from everything. For most outages the real list is short: keep food cold, keep the heat or a fan running, keep phones and the internet alive, and keep a few lights and any medical device on. That list runs comfortably on a mid-size generator. Whole-home backup, where you never notice the grid is down, is a different and far more expensive goal. Be honest about which one you are planning for.
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator | 100 to 200 | 800 to 1,200 |
| Furnace blower | 600 to 800 | 1,200 to 1,500 |
| Sump pump | 800 to 1,050 | 1,500 to 2,000 |
| Well pump (1/2 hp) | 1,000 | 2,000 to 3,000 |
| Window AC (10,000 BTU) | 1,000 to 1,200 | 1,800 |
| Lights, phones, wifi | 50 to 300 | none |
Say your essentials are a fridge, the furnace blower, lights, and devices: about 200 plus 700 plus 300, or roughly 1,200 running watts. Now add the largest single surge, the furnace blower at about 1,500 watts, which lands you near 2,700 watts at the worst moment. Add a 20 percent buffer and you want a generator rated around 3,300 running watts or more. That is how a modest 3,500 to 5,000 watt unit comfortably covers a typical essentials list.
A small inverter generator around 2,000 watts handles a fridge plus a few small loads, and is quiet and portable. A mid-size 3,500 to 5,000 watt unit covers a full essentials list for most homes. A large portable of 7,500 to 9,500 watts can run most of a typical house if you stagger heavy loads. And a permanently installed standby generator of 10,000 to 22,000 watts backs up the whole home automatically, including central air, with no setup during the outage.
Sizing is only half the job. A fuel-burning generator must run outdoors, well away from windows and doors, because carbon monoxide is deadly, and it must never be plugged straight into a wall outlet, which backfeeds the grid and can kill a lineworker. If you want to power household circuits, have an electrician install a transfer switch. We cover this in full in the generator safety guide below.
If your loads are modest and you want something silent and safe to run indoors, a battery power station may fit better than a fuel generator, with no fumes and no fuel to store. Compare the two honestly in our solar-versus-gas guide, and use the sizing guides to match capacity to your needs.
Most households can cover the essentials, a refrigerator, lights, the furnace blower, internet, phone charging, and a few outlets, with a 3,500 to 7,500 watt generator. Backing up a whole home, including central air conditioning or electric heat, generally needs a 10,000 to 22,000 watt standby generator.
Running watts is the steady power a device needs once it is on. Starting watts is the brief, larger spike that motor-driven appliances need at the moment they start. Your generator has to cover the total running watts of everything on at once, plus the single largest starting spike.
A large portable, around 7,500 to 9,500 watts, can run most of a typical home if you manage which loads run at once. Running everything simultaneously, including central air, usually requires a permanently installed standby generator sized to the home and wired through a transfer switch.
No. An oversized generator wastes fuel and runs inefficiently at light loads, while an undersized one trips or stalls. Size it to the loads you actually need plus a sensible buffer, not to the biggest number you can afford.
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