What Size Generator for a Sump Pump?

How many watts does a sump pump use?

Running watts are what the appliance draws continuously. Starting watts (also called surge watts) are the brief spike a motor needs for a second or two when it starts. Needs alone is what that appliance requires by itself. With basics adds a refrigerator, a chest freezer, 20 LED bulbs, wifi, and phone chargers, which together draw 720 running watts and 1,620 peak.

Both need figures are the peak load with a 20% safety margin, rounded up to the nearest 100W. The recommended class is the smallest generator whose continuous rating covers the with-basics figure.

Why are storms the worst case for a sump pump?

The reason a sump pump matters so much in an outage is that the events line up. Heavy rain is what makes the pump run constantly, and heavy rain is also what takes the power out. You need the pump most at the exact moment the grid is most likely to fail.

A sump pump does not run continuously. In dry weather it might cycle a few times a day. In heavy rain it can cycle several times an hour, running 15 to 60 seconds each time. That pattern is good news for fuel: you are powering a pump that is mostly resting, so a tank lasts much longer than the math below suggests if the pump only runs part of the time.

Most sump pumps are 120 volts, which is one useful difference from well pumps. A standard portable generator with ordinary household outlets can run one.

How is this calculated?

Two rules decide everything on this page:

1. Total running watts must stay under the generator's continuous rating.
2. Total running watts plus the single largest surge must stay under the generator's peak rating.

For the second rule you add only the largest surge, not every surge, because appliances rarely start at the same instant. We then add a 20% margin so the generator is not running flat out.

Appliance wattages are typical values for common residential models. Your specific appliance may differ. The nameplate on the appliance itself is always the better number.

What else should you know before you buy?

• Size on the surge, not just the running watts. A 1/2 HP sump pump draws 800 watts running but 2,150 watts to start. A 1 HP pump goes from 1,500 running to 3,900 starting. The starting number is what trips a generator that looked big enough.
• A pump that cycles often is the real load. One cycle is trivial. Ten cycles an hour means the motor surges ten times, and repeated startups are harder on a generator than a steady draw. If your pump runs that often, leave headroom.
• Buy for the water, not the average. A pump that keeps up in a normal rain can fall behind in a storm. If your basement has flooded before, size up rather than trying to match the minimum.
• A battery backup covers the seconds, a generator covers the hours. Many homes use both. The battery handles short outages automatically, and the generator takes over when the outage runs long.

Frequently asked questions

Related

• Generator size calculator (add up your own appliances)
• Appliance wattage chart (running and starting watts)
• What will a 7,500 watt generator run? (a full worked example)
• Portable vs standby generator (which type fits your situation)