Home Generator Sizing: Running Watts, Starting Watts and the Transfer Switch
Size a home backup generator from running watts and starting watts, not from the headline wattage alone. A worked example with a fridge, well pump, furnace fan and lights shows why the largest motor decides the peak rating.
Step-by-Step Procedure
List only the loads you will actually run together
Write down each essential load, its running watts and, for motors, its starting watts. A generator sized for every circuit in the house at once will be far larger and more expensive than one sized for the loads you will run during an outage.
Add running watts for the simultaneous load
Add the running watts of all loads that will run at the same time. This is the minimum running-watt rating the generator must meet, before any headroom.
Add the largest motor surge, not every surge
Motors draw several times their running watts for a few seconds at startup. Add the extra surge of the largest motor only, because motors rarely all start in the same second. Extra surge equals starting watts minus running watts for that motor.
Add headroom and check both ratings
Multiply the peak result by about 1.20 to 1.25 for headroom, then check the generator against both numbers: its running rating must cover the simultaneous running load, and its peak rating must cover the startup peak. The headline number on the box is usually the peak rating, not the running rating.
Plan the connection, fuel and carbon monoxide safety
Use a transfer switch or a listed interlock kit, never backfeed through a dryer outlet, and run the generator outdoors away from windows and doors. Plan fuel storage and runtime for the loads you chose, and check local rules for standby installation. This guide is a planning estimate only; standby installation should be done by a licensed electrician.
Worked Practical Example
Fridge, well pump, furnace fan and lights during an outage
Essential loads: fridge 180 W running and 1,000 W starting, well pump 1,500 W running and 4,500 W starting, furnace fan 800 W running, and lights plus router and TV 500 W running.
- •Simultaneous running watts: 180 + 1,500 + 800 + 500 = 2,980 W
- •Largest motor extra surge, well pump: 4,500 - 1,500 = 3,000 W extra at startup
- •Peak watts: 2,980 + 3,000 = 5,980 W
- •With about 25% headroom on the peak: 5,980 x 1.25 = 7,475 W
- •Check both ratings: running rating must cover 2,980 W, and peak rating must cover 5,980 W
Common Mistakes to Avoid
- ✕Buying by the headline wattage, which is usually the peak rating, not the running rating.
- ✕Adding the starting watts of every motor together, which oversizes the generator.
- ✕Forgetting the well pump or furnace fan surge, then wondering why the generator stalls at startup.
- ✕Backfeeding through an outlet instead of using a transfer switch or listed interlock.
- ✕Running a generator in a garage or near a window, which is a carbon monoxide danger.
Frequently Asked Questions
What size generator do I need for a house with a well pump?
It depends on the pump and the other loads running at the same time. As a planning example, a fridge, well pump, furnace fan and lights total 2,980 W running and 5,980 W at well-pump startup, so about a 7,500 W running and 9,500 W peak unit fits that example with headroom.
Can I run my whole house on a portable generator?
Usually not with a small portable unit. Pick essential loads, size from running watts plus the largest motor surge, and use a transfer switch or interlock. Whole-house standby sizing needs a load calculation and a licensed installer.