Running watts and starting watts
Every appliance has a running wattage, the power it uses once it is on. Appliances with electric motors, such as refrigerators, pumps, furnace blowers and air conditioners, also need a short starting surge, two to three times their running watts for a second or two.
A generator must handle the total running load plus the largest surge, because motors rarely start at exactly the same moment.
- Running load = sum of the running watts of everything that will be on
- Starting margin = the largest single extra starting surge
- Recommended size = (running load + starting margin) × (1 + safety margin)
Worked example
During an outage you want to run a refrigerator (700 W, 2,200 W to start), a sump pump (1,050 W, 2,150 W), a furnace blower (875 W, 2,350 W), plus TV and internet (300 W) and LED lights (300 W).
- Running load: 700 + 1,050 + 875 + 300 + 300 = 3,225 W.
- Extra surges: refrigerator 1,500 W, sump pump 1,100 W, furnace 1,475 W. The largest is 1,500 W.
- Before margin: 3,225 + 1,500 = 4,725 W.
- With a 20% margin: 5,670 W, or about 7.1 kVA at a power factor of 0.8.
A 6,000 to 7,500 W portable generator fits this house.
Typical generator sizes
| Size | Typical use |
|---|---|
| 2,000 to 3,500 W (inverter) | Camping, RV, fridge plus lights and phones |
| 5,000 to 8,000 W (portable) | Essentials: fridge, sump pump, furnace, lights |
| 9,000 to 12,000 W (large portable) | Adds a well pump, window AC or electric water heater |
| 14 to 26 kW (standby) | Most or all of a home, including central air |
Why add a margin
Generators should not run at full load for hours. A 20 to 25% margin keeps the engine at a comfortable load, covers appliances you forgot, and accounts for power loss at high altitude and in hot weather.
Safety first
- Never run a generator indoors, in a garage or near windows. Carbon monoxide kills hundreds of people in the US every year. Place it at least 20 feet from the house and install CO alarms.
- Use a transfer switch or interlock installed by an electrician to connect to home circuits. Back-feeding through an outlet can electrocute utility workers.
- Use heavy-duty extension cords rated for the load when plugging appliances in directly.
Check the nameplate on each appliance for its exact wattage; the defaults in this calculator are typical values.