Generator Size Calculator

How the generator size is worked out

Add up the running watts of everything that will be on at the same time. Then add the largest single start-up surge: the extra a motor (fridge compressor, sump or well pump, furnace blower, AC) draws for a second or two as it starts. Motors rarely start at the same instant, so only the biggest surge is added. That total is the starting (peak) watts the generator must reach.

We then add 20% headroom on running watts, because generators shouldn't run flat out for hours, and derate for altitude: engines lose roughly 3.5% of output per 1,000 ft above sea level (manufacturers' manuals typically say 3–3.5%). A soft starter on an air conditioner cuts its start-up surge by about two-thirds (makers claim 60–70%).

Some calculators show running watts only, which can leave you with a generator that won't start your fridge. Others add up every surge, which oversizes. Running total plus the largest surge is the standard method used in retailer and manufacturer worksheets.

What is the difference between running and starting watts?

Running watts are what an appliance draws once it is going. Motors in fridges, pumps, furnace blowers and air conditioners draw two to three times more for a second or two while they start. A generator has to cover everything that is running plus the biggest of those start-up surges.

What size generator do I need to run a refrigerator and a sump pump?

With typical values (refrigerator 700 W running, 2,200 W starting; 1/3 HP sump pump 800 W running, 1,300 W starting) you need about 1,500 W running and 3,000 W starting. With 20% headroom that is a generator rated about 1,900 W running and 3,000 W starting, such as a 3,000–3,500 W inverter class.

Can I plug a generator into a wall outlet to power the house?

No. Backfeeding through an outlet can electrocute you, your neighbors and utility line workers, and can start a fire. Use extension cords rated for the load, or have a licensed electrician install a transfer switch or interlock.

How far from the house should a generator be?

The CPSC says at least 20 feet from the house, with the exhaust pointed away from windows, doors and vents. Never run it in a garage, basement or crawlspace, even with the door open, and fit battery-powered carbon monoxide alarms. Many newer models have a CO shut-off; look for one that meets ANSI/PGMA G300 or UL 2201.

What about a whole-house or standby generator?

Above about 10 kW, or with central AC, a well pump and electric appliances, you are into standby generator territory. Sizing that is a load calculation under the electrical code (NEC Article 220) plus a permitted install, so ask a licensed electrician for a quote.

Sources: appliance values from the Home Depot generator worksheet (refrigerator 700/2,200 W), Honda's wattage calculator (ENERGY STAR refrigerator 192/1,200 W), the American Industrial generator wattage guide, and typical worksheet values; CPSC portable generator guidance.