What Size Generator Do You Need for a Sump Pump?
By the Generator Size Calculator Team · Updated · 7 min read
Quick answer: Honda's wattage guide lists a 1/3 HP sump pump at 750 running and 1,500 starting watts, a 1/2 HP pump at 1,000 and 2,100, and a 1 HP pump at 2,300 and 4,500. In practice, a 2,000-watt generator can handle a 1/3 HP pump alone, a 2,500-watt unit suits a 1/2 HP pump, and a 1 HP pump needs about 5,000 watts. Add a fridge and lights and you will usually want a 3,500-watt unit for the smaller pumps.
Sump pump watts by horsepower
A sump pump has an electric motor, so it draws extra power for a moment when it starts. The table below uses Honda's generator wattage guide.
| Sump pump | Running watts | Starting watts |
|---|---|---|
| 1/3 HP | 750 | 1,500 |
| 1/2 HP | 1,000 | 2,100 |
| 1 HP | 2,300 | 4,500 |
These are typical figures. Your pump's label gives the real amps and horsepower. If you also have a well, see our guide to what size generator a well pump needs.
Which generator size to choose
Two rules help. First, keep the running load under about 80% of the generator's running rating, so it isn't maxed out for hours. Second, make sure the generator's starting capacity covers the pump's starting watts. These sizes assume typical ratings, with starting capacity about 25% above running.
| Pump | Pump alone | Pump plus fridge, TV and lights |
|---|---|---|
| 1/3 HP (750 / 1,500 W) | 2,000 W | 3,500 W |
| 1/2 HP (1,000 / 2,100 W) | 2,500 W | 3,500 W |
| 1 HP (2,300 / 4,500 W) | 5,000 W | 5,000 W |
Ratings vary between models. Always confirm the running and starting watts on the generator you plan to buy. Our sump pump generator calculator does the math from your own numbers.
Example: a pump plus the essentials
Say you have a 1/2 HP sump pump and want to run a fridge and a TV, lights and Wi-Fi at the same time.
| Load | Running watts |
|---|---|
| Sump pump, 1/2 HP | 1,000 |
| Refrigerator | 700 |
| TV, lights, Wi-Fi | 475 |
| Running total | 2,175 |
The surge adds about 1,500 watts if the fridge starts while everything else is running, so the peak is about 3,675 watts. A 3,500-watt generator carries 2,175 running watts under its 80% mark of 2,800 watts, and its approximate 4,400-watt starting capacity covers the peak. A 2,500-watt generator would run out of headroom, which is why the table above suggests 3,500 watts.
Enter your pump and appliances to see the generator size that fits.
Find your pump's horsepower
The label on the pump motor lists horsepower, volts and amps. Many home pumps are 1/3 HP or 1/2 HP, but check yours. If it is a plug-in pump, you can also use the amps on the label: amps multiplied by volts gives watts. A watt meter on the cord shows the real draw. If you can't find a label, size for the larger option.
How to connect the pump safely
- Plug-in pumps can run from the generator through a heavy-duty, outdoor-rated extension cord. Use the extension cord calculator for gauge and length.
- Hardwired pumps need a transfer switch or interlock installed by a licensed electrician. Never backfeed an outlet. Read our guide to connecting a generator to your house.
- Use a GFCI-protected outlet or cord, since sump pits are damp. See grounding and GFCI basics.
Keep the generator outdoors, at least 20 feet from doors, windows and vents. A basement or garage is never a safe place, even when the pump pit is down there. Run the cord out, not the generator in.
Tips for storm season
- Test the pump now. Pour water into the pit and confirm the float switch starts the motor.
- Test the generator too, and refresh stored fuel before the season starts.
- Plan for long runs. A pump may cycle on and off for many hours, so look at how long a tank lasts in our fuel runtime guide.
- Think about a battery backup pump for short outages. It runs with no engine or exhaust.
Frequently asked questions
What size generator do I need for a 1/2 HP sump pump?
A 1/2 HP pump is typically 1,000 running and 2,100 starting watts. A 2,500-watt generator can run it alone, and a 3,500-watt generator leaves room for a fridge and lights.
Can a 2000-watt generator run a sump pump?
Usually a 1/3 HP pump, yes: about 750 running and 1,500 starting watts. A 1/2 HP pump at 1,000 and 2,100 watts is a tight fit on a typical 2,000-watt inverter, so check its peak rating.
What size generator for a 1 HP sump pump?
A 1 HP pump is typically 2,300 running and 4,500 starting watts, so plan on about a 5,000-watt generator.
Can I run a sump pump and a refrigerator on one generator?
Yes. A 1/2 HP pump, a fridge and a TV, lights and router total about 2,175 running watts and a peak near 3,675 watts, which fits a 3,500-watt generator.
Should I run a sump pump on a generator in the basement?
No. Put the generator outdoors at least 20 feet from doors, windows and vents and run a cord to the pump, because carbon monoxide builds up indoors even when a door is open.
Related calculators
Sources: Sump pump running and starting watts from the Honda generator wattage guide; outdoor placement, GFCI and carbon monoxide guidance from the U.S. Consumer Product Safety Commission. The generator sizes in the table apply the 80% running-load rule and an assumed starting capacity about 25% above the running rating; confirm your own generator and pump labels.