Match your generator to your pump's starting power, not its running power
A sump pump needs a generator large enough to handle its starting surge, not just the steady power it uses while running. Most sump pumps draw 750 to 2,000 watts while operating, but they can pull 2 to 3 times that amount in the first second they turn on. If your generator can't deliver that initial spike, the pump won't start at all—even if the generator has enough power to run it once it's going.
The size you actually need depends on your pump's horsepower, whether it's a submersible or pedestal model, and what else you might run at the same time. A 5,500-watt generator handles most single-pump setups. If you have a larger pump, two pumps, or plan to run other devices during an outage, you'll need 7,000 watts or more.
Key Takeaways
- Check your pump's nameplate for its horsepower and wattage rating—this is the only reliable way to size a generator correctly.
- A 5,500-watt generator is sufficient for most household sump pumps, but larger pumps or dual-pump systems need 7,000 to 10,000 watts.
- The pump's starting wattage (inrush) is what matters most; running wattage alone will cause the pump to fail to start.
- Portable generators work for temporary backup, but a permanently installed standby generator is more reliable during extended outages.
How to find your pump's actual power requirements
The most accurate way to size a generator is to look at the nameplate on your pump itself. If the pump is already installed, you can usually see it on the motor housing. The nameplate lists horsepower (HP) and sometimes running watts. If you don't have the pump yet, check the product manual or the manufacturer's website.
A typical 1/3 HP submersible pump draws about 800 to 1,000 running watts and 2,400 to 3,000 starting watts. A 1/2 HP pump uses roughly 1,000 to 1,200 running watts and 3,000 to 3,600 starting watts. A 3/4 HP pump pulls 1,500 to 1,800 running watts and 4,500 to 5,400 starting watts. These ranges vary by manufacturer and motor type, so the nameplate is always more accurate than a general estimate.
If you can't find the nameplate and don't know the pump's horsepower, call the installer or the previous homeowner. Many sump pump companies also have online tools where you can enter the model number and get the specifications.
Generator sizes for common pump configurations
The table below shows the relationship between pump size, power draw, and the minimum generator capacity needed. These recommendations include a safety margin so the generator isn't running at its absolute maximum. Generators perform best when loaded to 50 to 80 percent of their rated capacity, so a 5,500-watt generator should handle a 1/3 or 1/2 HP pump comfortably.
| Pump Size | Running Watts | Starting Watts | Minimum Generator Size |
|---|---|---|---|
| 1/3 HP | 800–1,000 | 2,400–3,000 | 5,500 W |
| 1/2 HP | 1,000–1,200 | 3,000–3,600 | 5,500 W |
| 3/4 HP | 1,500–1,800 | 4,500–5,400 | 7,000 W |
| 1 HP | 1,800–2,200 | 5,400–6,600 | 10,000 W |
| Two 1/2 HP pumps | 2,000–2,400 | 6,000–7,200 | 10,000 W |
If you're at the edge of a size category, go up—an undersized generator will overheat and shut down during an outage when you need it most. A generator that's slightly larger than necessary costs more upfront but runs cooler and lasts longer.
Portable versus standby generators for sump pump backup
A portable generator is cheaper upfront and works during short outages. You connect it to the pump using a heavy-duty extension cord and a GFCI-protected outlet. Portable units are loud, produce exhaust, and need to be stored and refueled. They're practical if outages in your area are rare and brief, but they require you to be home and awake to start them.
A standby generator is permanently installed outside your home, connected to your electrical panel through a transfer switch. It runs on natural gas or propane and starts automatically when power fails. Standby units are quieter, more reliable, and require no action from you—the pump keeps running without interruption. They cost significantly more to buy and install, but they're the standard choice for homes in flood-prone areas or places with frequent outages.
If you choose a portable generator, make sure it has a low-oil shutoff feature so it won't run dry and damage the engine. Also check that it has a 120/240-volt outlet if your pump requires 240 volts (most household pumps use 120 volts, but some larger models don't).
Connecting a portable generator safely to your sump pump
Never plug a generator directly into a wall outlet or run it indoors—this creates a fire and carbon monoxide hazard. Instead, run a heavy-duty extension cord (12 or 10 gauge for most pumps) from the generator to a GFCI outlet outside or in a garage, then plug the pump's cord into that outlet. The GFCI protects you from electrical shock if the cord gets wet.
Keep the generator at least 20 feet away from windows and doors so exhaust doesn't enter the house. Place it on a dry, level surface and shield it from rain with a canopy or cover—but never enclose it completely, as it needs air circulation to cool the engine.
Test the setup before you need it. Run the pump for a few minutes to confirm it starts and operates normally. Check that the generator's fuel tank is full and that the extension cord has no cuts or exposed wires. During an actual outage, refuel the generator outside and away from the house to avoid spilling gasoline near your foundation.
What to do if your generator is too small
If you already own a generator that's smaller than your pump's starting wattage, you have a few options. The simplest is to upgrade to a larger unit—a 5,500-watt portable generator costs $500 to $1,200 depending on brand and features. A standby generator costs $3,000 to $6,000 installed, but lasts 15 to 20 years and covers your whole house.
Some people use a soft starter or variable frequency drive to reduce the pump's starting surge. These devices gradually ramp up the motor instead of slamming it with full power. They're effective but add $300 to $800 to the cost and require an electrician to install. They're most practical if you already have a generator you like and want to avoid replacing it.
Another option is to run the pump on battery backup instead of a generator. A battery backup sump pump is a separate, smaller pump powered by a rechargeable battery. It won't run as long as a generator, but it requires no fuel, no extension cords, and no setup—it activates automatically if the main pump fails or power goes out. Many homes use both: a battery backup for short outages and a generator for longer ones.
Frequently Asked Questions
Can I use a smaller generator if I only run the sump pump and nothing else?
No. The generator must handle the pump's starting wattage, which is determined by the motor's design, not by what else is plugged in. A 3,000-watt generator cannot start a 1/2 HP pump even if nothing else is running, because the pump's starting surge exceeds 3,000 watts. You need the full size the pump requires.
What if my pump is 240 volts instead of 120 volts?
Most household sump pumps are 120 volts, but some larger models use 240 volts. Check your pump's nameplate. If it's 240 volts, your generator must have a 240-volt outlet. Many portable generators have both 120 and 240-volt outlets, but confirm before you buy. A standby generator always provides both.
How long can a portable generator run a sump pump?
A typical portable generator with a 5-gallon fuel tank runs for 8 to 12 hours at half load. A sump pump uses only a fraction of the generator's capacity, so you'll get closer to the full runtime. Refuel before the tank is empty to avoid running out during an outage. For outages longer than 12 hours, a standby generator or battery backup is more practical.
Do I need a generator if I have a battery backup pump?
A battery backup pump is valuable insurance for short outages, but it typically runs for 6 to 24 hours depending on how often the pump cycles. If outages in your area last longer than that, or if you live in a flood-prone zone where the pump might run continuously, a generator or standby unit is worth the investment.
Should I have an electrician install a standby generator?
Yes. A standby generator must be wired to your home's electrical panel through a transfer switch, which requires a licensed electrician. The transfer switch prevents backfeed—electricity flowing back into the utility lines—which can injure utility workers. DIY installation is dangerous and usually violates local electrical code.