Calculator · Electrical & power for tools
Generator wattage calculator
Add the running watts of everything you will use at once, then the largest additional starting load. Get a running and a starting (surge) estimate.
Calculator
and at least 3,300 W starting (surge). Check the generator's rated and starting watts, and derate for altitude and heat per its manual.
How the math works
- Running: add the running watts of everything that will be on at the same time, then add the headroom (20% by default) and round up to the next 100 W.
- Starting: add the single largest additional starting load to the total running watts, add 10%, and round up. Only one motor starts at a time.
Wattage reference table
Typical figures from two published generator charts. Your tool's nameplate is the better number.
| Tool | Running watts | Additional starting watts | Source |
|---|---|---|---|
| Circular saw, 7-1/4" | 1,400 | 2,300 | Lowe's |
| Circular saw | 1,500 | 2,000 | Champion |
| Miter saw, 10" | 1,800 | 3,300 | Lowe's |
| Table saw, 10" | 2,000 | 2,000 | Lowe's |
| Table saw | 1,800 | 2,500 | Champion |
| Hand drill, 1/2" | 600 | 900 | Lowe's |
| Hammer drill | 1,000 | 3,000 | Champion |
| Angle grinder | 800 | 2,400 | Champion |
| Bench grinder, 8" | 1,400 | 1,100 | Lowe's |
| Belt sander, 3" | 1,200 | 2,400 | Lowe's |
| Reciprocating saw | 960 | none listed | Lowe's |
| Air compressor, 1 HP | 1,400 | 3,600 | Lowe's |
| Air compressor, 1/2 HP | 1,000 | 2,000 | Lowe's |
| Airless paint sprayer, 1/3 HP | 600 | 1,200 | Lowe's |
| Quartz halogen work light | 1,000 | none listed | Lowe's |
Worked example
Lowe's sample load: refrigerator (700 W running, 2,200 W additional starting), sump pump (1,050 / 2,150), 3" belt sander (1,200 / 2,400) and a 1/2 HP air compressor (1,000 / 2,000). Total running = 3,950 W. The largest additional starting load is the sander's 2,400 W, so Lowe's minimum is 3,950 + 2,400 = 6,350 starting watts. With this calculator's margins: 4,800 W running and 7,000 W starting.
Questions people ask
What are additional starting watts?
The extra power a motor needs for a moment while it starts, on top of its running watts. Charts list it for tools with motors.
Do all tools start at once?
This calculator assumes they do not: only the largest starting load is added, the method the Lowe's and Champion charts use.
Sources
Change log: Nov 6, 2026: Rebuilt to spec: sourced reference data, formulas and worked examples; no unverified people or test claims.