How to use the generator size calculator
- Edit each load’s running and starting watts from the nameplate or a chart you trust. The form starts with example figures from a Champion Power Equipment wattage chart (refrigerator 400 / 2,200 W, LED TV 80 W, microwave 800 / 2,000 W).
- The result is the sum of running watts plus the largest starting increment (that load’s starting watts minus its running watts).
- Optionally add 20% planning headroom. That percentage is a CalcTheJob factor, not a manufacturer or electrical-code requirement.
Circuit length and conductor size are a different problem — see the voltage drop calculator if you are running a feeder. Job-site running cost of tools and temporary power is on the job-site electricity cost calculator.
Running watts, starting increment, and chart methods
Motor loads need a short surge above their running watts. This page adds only the largest of those increments, which is the physically required peak if one motor starts while everything else is already running.
Champion’s chart instead tells you to add the full starting watt of the largest motor on top of total running watts, which counts that load’s running watts twice and is more conservative by that amount. The article states both methods so you can compare.
A worked example: refrigerator, TV and microwave
Using the example chart figures: 400 + 80 + 800 = 1,280 W running. The starting increments are 1,800 W (refrigerator) and 1,200 W (microwave), so the largest increment is 1,800 W. Required size is 3,080 W. With 20% planning headroom that is 3,696 W. Champion’s more conservative add-the-full-starting-watt method on the same loads would be 1,280 + 2,200 = 3,480 W before headroom.
Sources & assumptions
How this calculator gets its numbers. Full method: our methodology.
- Required watts = sum of running watts + the single largest starting increment (that load’s starting watts minus its running watts). Starting watts are never taken below running watts.
- The default refrigerator (400 / 2,200 W), LED TV (80 W) and microwave (800 / 2,000 W) figures are example values from a Champion Power Equipment wattage chart, for you to edit. The nameplate on the appliance wins.
- Optional 20% headroom is a CalcTheJob planning factor, not a manufacturer or electrical-code requirement.
- Some manufacturer charts add the full starting watt of the largest motor on top of total running watts, which counts that load’s running watts twice and is more conservative. This page uses the increment and states the difference.
- This is a planning check, not a shopping guide and not a transfer-switch design.
Sources
- Champion Power Equipment, Choosing a Generator / Quick Reference Wattage (wattage chart flyer). Example running / starting watts used as editable form defaults: refrigerator 400 / 2,200 W, LED television 80 W, microwave 800 / 2,000 W. The flyer’s own method adds the highest single starting watt to total running watts; this page uses the starting increment instead and discloses that difference.
Last reviewed . The nameplate watts and an electrician’s design win.
Frequently asked questions
How do I size a generator for a fridge and a microwave?
Add every running watt, then add the single largest starting increment. With the form’s example figures that is 1,280 + 1,800 = 3,080 W, or 3,696 W with 20% planning headroom. Use the watts on your nameplates when you have them.
Why don’t you add the full starting watts?
Starting watts already include the running watts of that load. Adding the increment avoids counting those running watts twice. Some manufacturer charts add the full starting watt anyway, which is more conservative.
Is the 20% headroom required?
No. It is an optional CalcTheJob planning factor. Turn it off if you want the raw running-plus-increment total.
Is this a recommendation to buy a particular generator?
No. It is a planning watt check, not a shopping guide, and not a transfer-switch or inlet design.