What Size Generator Do You Need for a Garage or Workshop?
By the Generator Size Calculator Team · Updated · 7 min read
Quick answer: Motor tools draw several times their running watts for a moment at start-up, so size for the starting watts of your biggest tool. Honda lists a 1 HP air compressor at 1,500 running and 4,500 starting watts, a 1.7 HP table saw at 1,500 and 3,000 and a 100-amp welder at 3,600 watts. A 3,500 to 4,500-watt generator runs one big tool at a time, and a 2,000 to 2,500-watt unit handles drills, small saws and lights.
Watts for common tools
| Tool | Running watts | Starting watts |
|---|---|---|
| Hand drill, 1/2 inch | 600 | 600 |
| Circular saw, 7 1/4 inch | 1,400 | 2,300 |
| Table saw, 1.7 HP | 1,500 | 3,000 |
| Air compressor, 1 HP | 1,500 | 4,500 |
| Air compressor, 2 HP | 2,800 | 7,700 |
| Bench grinder, 8 inch | 1,400 | 2,500 |
| Electric welder, 100 amp | 3,600 | 3,600 |
These are Honda's typical figures. Honda's guide notes that motor tools can draw about three times their running amps at start-up, so a tool label's amps are the safest starting point. Tools that are cutting or under load use more than idling.
Which generator size to choose
| Tool | Generator for that tool alone |
|---|---|
| 1/2 inch drill | 2,000 W |
| 7 1/4 inch circular saw | 2,500 W |
| 1.7 HP table saw | 3,000 W |
| 1 HP air compressor | 4,000 W |
| 100 amp welder | 4,500 W |
The sizes use the 80% running-load rule and a starting capacity of about 125% of rating. A 2 HP compressor needs about 7,700 starting watts, which means a generator of 6,000 to 7,500 watts. Compare in what a 5000-watt generator can run and what a 7500-watt generator can run.
Rules for sizing around tools
- Size for the biggest tool, then add lights and a radio. Honda notes that all tools are seldom running at the same time.
- Start the largest motor first, with nothing else running, then add smaller loads.
- Use a compressor tank. It cycles on and off, so the generator only carries the motor while it fills.
- Watch voltage. A long, thin cord can cause voltage drop that makes motors overheat. Use the extension cord calculator or the voltage drop calculator.
Add the tools you run at the same time and see the generator size.
Cords and lights
Honda lists mercury halogen floodlights at 1,000 watts each, so LED work lights are a smart upgrade for a generator-powered job site. Use heavy-duty, outdoor-rated cords and keep the generator outdoors. For the engine, see the ventilation guide.
Run the generator outdoors, at least 20 feet from doors, windows and vents. Never in a garage or shed, even with the door open. Never run a generator inside a workshop, even with the door open. See how far a generator should be from your house.
Frequently asked questions
What size generator do I need to run a table saw?
Honda lists a 1.7 HP table saw at about 1,500 running and 3,000 starting watts, so a 3,000-watt generator is a realistic minimum and 3,500 watts is more comfortable.
Can a 3500-watt generator run an air compressor?
A 1/2 HP compressor (1,000 running, 2,000 starting watts), yes. A 1 HP unit starts at about 4,500 watts, which needs a larger generator.
Can a generator run a welder?
Yes. Honda lists a 100-amp welder at about 3,600 watts, so plan on a 4,500-watt generator. Check the welder label and your generator's duty rating.
Why do power tools need so many starting watts?
Electric motors draw a large current for a moment as they spin up, often two to three times their running watts.
Can I run more than one tool at a time?
Yes, if the generator covers the total running watts plus the largest start-up surge. Start the biggest tool first.
Related calculators
Sources: Appliance and tool running and starting watts from the Honda generator wattage guide (contractor tools); Honda sizing notes on reactive loads. Generator sizes in the table apply the 80% running rule and an assumed starting capacity about 25% above the running rating, so confirm your own generator and tool labels.