A flooded basement costs thousands of dollars in repairs. When the power grid fails during a heavy storm, your primary electric sump pump stops working exactly when you need it most. A power inverter system is a reliable solution to keep that pump running using a battery bank.
However, you cannot simply match the inverter wattage to the running wattage of the pump. Sump pumps are inductive loads. They require a massive surge of power to start spinning. If your inverter is too small, the pump will not start, and your basement will flood.
To run a standard 1/3 HP or 1/2 HP sump pump, you generally need a power inverter rated for at least 2,000 to 3,000 watts continuous output with a high surge capability. The specific size depends on the motor’s startup requirements.
- Surge Power is Critical: Motors need 3x to 5x their running watts to start.
- Pure Sine Wave: Always use Pure Sine Wave inverters for AC motors to prevent overheating.
- Minimum Sizing: A 2000W inverter is the safe minimum for most 1/3 HP pumps.
- Battery Bank: The inverter needs enough battery capacity (Amp Hours) to last through the storm.
Understanding Surge vs. Running Watts
Electric motors work differently than light bulbs or heaters. A light bulb draws a steady amount of power. A sump pump motor is an inductive load. This means it uses magnetic fields to create motion.
When a pump is sitting still, it takes a lot of energy to get the heavy impeller moving against the weight of the water in the pit. This initial burst of energy is called Locked Rotor Amps (LRA) or surge wattage. This surge lasts only for a fraction of a second, but if the inverter cannot supply it, the system trips.
The Multiplier Rule
As a general rule in our testing, you must multiply the pump’s running wattage by at least 3 to find the starting wattage. Some older or less efficient pumps may need up to 5 times the running watts.
How Big Power Inverter to Run Sump Pump
Determining exactly how big power inverter to run sump pump hardware requires looking at the pump’s data plate. You will usually see the power listed in Horsepower (HP) or Amps.
Here is the breakdown of the math we use in the field:
- Find the Amps: Look at the sticker on the pump. A typical 1/3 HP pump draws roughly 9 to 10 Amps at 120 Volts.
- Calculate Running Watts: Multiply Volts x Amps. (120V x 10A = 1200 Watts).
- Calculate Surge Watts: Multiply Running Watts x 3. (1200W x 3 = 3600 Watts).
Wait, 3600 Watts? Yes. While a 1/3 HP pump might only use 800-1000 watts while running, that split-second startup demand is huge. If you buy a 1500W inverter, it will likely fail immediately.
Inverter Sizing Matrix
- If you have a 1/4 HP Pump: Use a 1500W – 2000W Pure Sine Inverter.
- If you have a 1/3 HP Pump: Use a 2000W – 3000W Pure Sine Inverter.
- If you have a 1/2 HP Pump: Use a 3000W – 4000W Pure Sine Inverter.
- If you have a 3/4 HP+ Pump: Consider a standby generator. Inverters of this size are expensive and drain batteries rapidly.
From the Shop
We recently had a customer, Dave, who tried to save money by buying a 1000W inverter for his 1/3 HP Zoeller pump. He tested it dry (without water), and it spun. He thought he was safe. During the next storm, the pit filled with water. The weight of the water added resistance. When the pump tried to start under load, the 1000W inverter overloaded instantly. His basement flooded. We swapped him to a 3000W unit, and it has handled three storms since without a hiccup. Always size for the loaded start, not a dry run.
Why Waveform Matters (Pure vs. Modified)
You will see two main types of inverters on the shelf: Modified Sine Wave and Pure Sine Wave.
Modified Sine Wave inverters are cheap. They produce a blocky, stair-step electrical signal. Simple electronics like toaster ovens or incandescent lights handle this fine. AC motors, like those in sump pumps, hate it.
Running a motor on a modified sine wave causes:
- Excess Heat: The motor runs about 20% hotter.
- Buzzing Noise: You will hear a loud hum from the pump.
- Reduced Efficiency: The pump pumps less water per minute.
- Early Failure: The electronics inside the pump switch can burn out.
Pure Sine Wave inverters produce smooth electricity, identical to what comes out of your wall outlet. For any pump with a motor or electronic float switch, a Pure Sine Wave inverter is mandatory.
For more technical details on how motors react to voltage irregularities, you can review this guide on inverter selection from the Department of Energy.
Battery Bank Requirements
The inverter is just the gateway. The batteries are the fuel tank. A 2000W inverter connected to a small car battery will run your pump for perhaps 10 minutes before dying.
Sump pumps cycle on and off. We measure battery needs based on the “duty cycle” (how many minutes per hour the pump runs).
The Calculation
If your pump draws 1000W while running, that is roughly 100 Amps from a 12V battery (1000W / 10V = 100A, accounting for efficiency losses).
- Scenario: The pump runs for 10 seconds every minute (10 minutes per hour).
- Consumption: 100 Amps / 6 = 16.6 Amp-Hours (Ah) removed from the battery every hour.
- Goal: You want 10 hours of runtime.
- Math: 16.6 Ah x 10 hours = 166 Ah needed.
Since you should not drain Lead-Acid batteries below 50%, you would need a battery bank with a capacity of roughly 330 Ah. This usually means connecting two or three large deep-cycle marine batteries in parallel.
Safety and Setup
Installing a high-wattage inverter requires respect for electricity. Low voltage (12V) does not mean low danger. The amperage moving through the cables is massive.
Cable Sizing
Do not use the thin cables that come with cheap inverters. For a 2000W or 3000W inverter, you must use thick gauge wire to connect the inverter to the battery.
- Distance 0-6 feet: Use 2/0 AWG (00 Gauge) wire.
- Distance 6-10 feet: Use 4/0 AWG wire.
If the wires are too thin, they will get hot and could start a fire. They will also cause a voltage drop, which makes the inverter shut down early.
Fusing
Always place a catastrophic fuse (ANL or Class T fuse) on the positive cable, as close to the battery terminal as possible. If the cable shorts out, the fuse blows instead of the battery exploding.
Grounding
Ensure the inverter case is grounded to your home’s earth ground system. This protects you from shock if a wire comes loose inside the unit.
Refer to NFPA 70 (National Electrical Code) guidelines or consult a licensed electrician when setting up permanent backup power systems.
How We Evaluated This
To provide these recommendations, we analyzed the startup curves of common submersible pumps found in residential homes (Zoeller M53, Liberty 257). We compared these against the surge duration capabilities of standard consumer inverters. Our sizing recommendations are based on ensuring a safety margin of 20% above the Locked Rotor Amp rating to account for battery voltage sag under load.
Frequently Asked Questions
Can I use a car battery for my sump pump inverter?
You can in an emergency, but it is not recommended. Car batteries are designed for short bursts of power (starting a car), not deep discharge. Using a car battery will damage it quickly. You should use Deep Cycle Marine or AGM batteries designed for continuous power delivery.
How long will a 12V battery run a sump pump?
It depends on how often the pump cycles. A standard 100Ah deep cycle battery will typically run a 1/3 HP sump pump for 4 to 6 hours of continuous pumping, or 12 to 24 hours if the pump only cycles on occasionally. For longer runtimes, add more batteries in parallel.
Can I plug the inverter into a wall outlet to charge the batteries?
No, a standard inverter only takes power from batteries. To charge the batteries from the wall when power is on, you need an “Inverter/Charger” combo unit. These units automatically switch to battery power when the grid fails and recharge the batteries when the grid returns.
Why does my inverter beep when the pump starts?
The beep usually indicates a “Low Voltage Warning.” When the pump demands the huge startup surge, it pulls so much power that the battery voltage drops temporarily. If your cables are too thin or your battery is too small, the voltage drops below the threshold (usually 10.5V or 11V), causing the alarm.
Is a generator better than an inverter for a sump pump?
A generator offers unlimited runtime as long as you have fuel, but it requires manual setup and must run outside. An inverter/battery system is silent, automatic (if using a charger/transfer switch), and works immediately. For outages lasting less than 24 hours, an inverter is often more convenient. For multi-day outages, a generator is better.
What happens if I use a Modified Sine Wave inverter?
Your pump will run hotter and make a buzzing sound. Over time, this heat can damage the motor windings or the capacitor, shortening the life of the pump. While it might work in a pinch, we strongly recommend Pure Sine Wave inverters for peace of mind.
