Backup generator tools

Sizing. Enter every appliance you want backed up with its running watts, then add only the largest single starting surge. Worked example: 3,000 running watts plus a 1,500-watt largest surge means at least 4,500 watts of generator. Common running figures to plug in: fridge/freezer 600–800 W, microwave 1,200 W, washing machine 750 W, sump pump 1,500 W, water heater 3,000–4,500 W, electric furnace 5,000–25,000 W, window AC 600–1,500 W, central AC 2,000–4,000 W.

Running cost. Fuel is the number people forget. Bob Vila's estimates put a 21 kW natural gas unit near $90 a day, a 20 kW diesel near $175, and a 20 kW propane near $220. Multiply by the outage length you're actually planning for.

Quote comparison. The most valuable tool is a spreadsheet with five rows: unit, install labor, transfer switch, pad and permits. Published ranges to check against are $1,500–$5,000 labor, roughly $1,200 transfer switch labor, $50–$75 per square foot of concrete, and $50–$200 per permit.

Treat any dealer's online sizing tool as a starting point, not an answer. They're built to produce a purchase, and they tend to round up. Do the arithmetic yourself before the sales call.

How to choose

Add one starting surge, not all of them

Motors rarely start simultaneously. Adding every surge is the most common way sizing tools inflate a recommendation.

Use nameplate watts from your own appliances

Generic tables are estimates. Your furnace's data plate is the real number.

Model a realistic outage length

Nationally, 2024 averaged about 663 minutes of outage per customer including major events. Run your fuel cost against the length you actually expect locally.

Sanity-check any tool that recommends the top of the range

If a calculator lands on 26–32 kW ($6,500–$15,000 for the unit alone), make it show you which loads drove that.

Keep 10 percent headroom, not 100 percent

Target running at no more than about 90 percent of rated capacity. Beyond that you're paying for capacity you'll never use.