Sump Pump Battery Backup: Power Station Sizing for Flood Protection

Your sump pump needs electricity at the exact moment a storm is most likely to take it away. Heavy rain saturates the ground, water table rises, the pit fills, and the pump kicks on to push water out through the discharge line. If the grid goes down during that sequence, the pump goes silent. Water keeps coming. Within hours, you have inches of standing water in your basement.

According to insurance industry data, a single basement flood event costs homeowners between $10,000 and $50,000 or more in damage. Drywall, flooring, furniture, appliances, stored belongings, and mold remediation add up fast. A power station that keeps your sump pump running through a 6 to 12 hour outage costs a fraction of one flood cleanup. The math on this purchase is simple even before you factor in the stress and disruption of a flooded home.

But sizing a power station for a sump pump is not as simple as matching wattage numbers. Sump pumps are motor-driven devices with high startup surge. Every time the float switch trips and the pump kicks on, it draws two to three times its running wattage for a fraction of a second. If your power station cannot handle that surge, its inverter shuts down to protect itself. Your pump never starts. Your basement floods. The key to sizing is not running watts. It is surge watts.

60-second sizing

1. Check your pump voltage. Most residential sump pumps are 120V. If yours is 240V, you need a dual-voltage station.
2. Find running amps on the nameplate. Multiply by 115V to get running watts. If only horsepower is listed, assume 920 to 1,081W for a 1/2 HP pump.
3. Estimate startup surge. Multiply running watts by 3× (NEC standard for motors without published LRA). Your power station’s surge rating must exceed this number with a 1.15 safety buffer.
4. Size the battery for your storm. At 50% duty cycle during heavy rain, a 1/2 HP pump averages about 460 to 541W. A 2,000 Wh station lasts roughly 2.6 hours. A 4,000 Wh station lasts about 5.3 hours.

Jump to soft-start options if your station’s surge rating falls short.

Sump Pump Power Requirements

A residential sump pump is a submersible electric motor connected to an impeller that moves water. The motor draws a steady current while running, but at the moment of startup, the rotor is stationary and the motor windings act almost like a short circuit, pulling a large inrush of current for a fraction of a second before the rotor gets up to speed.

This startup inrush, measured as Locked Rotor Amps (LRA), is the single most important specification for power station sizing. Here are verified figures from two of the most widely installed 1/2 HP sump pumps in the United States:

Zoeller M98 Flow-Mate (model 98-0001): 9.4 amps running at 115V (1,081W). Startup inrush: estimated at 3,243W (3× running watts per NEC standard; no OEM locked rotor amps published for this model). Permanent split capacitor motor, 1,725 RPM, 1/2 HP. Surge-to-running ratio: 3.0×.

Liberty Pumps 287 (280-series): 8.0 amps running at 115V (920W). Startup inrush: 23 amps at 115V (2,645W). 3,450 RPM motor, 1/2 HP. Surge-to-running ratio: 2.88×. The Liberty 287 draws less power both running and starting, primarily because its higher-RPM motor design achieves the same 1/2 HP output with a smaller, more efficient stator. The Liberty 287 is the only pump in this comparison with OEM-published locked rotor amps.

The cycling pattern

A sump pump does not run continuously. It cycles: the water level rises, the float switch activates, the pump runs for 30 to 90 seconds to lower the water level, then shuts off and waits. During moderate rain, the pump might cycle a few times per hour. During heavy rain or rapid snowmelt, it can cycle every few minutes.

Each cycle begins with a full surge event. If your pump runs 30 times during a 6-hour storm, that is 30 separate startup surges your power station must handle. This is why surge capacity is not a one-time concern. It is a repeated stress test.

During heavy rain, a typical duty cycle is approximately 50 percent: the pump runs about half the time and rests about half the time. Using the Zoeller M98 as our worst-case reference, the average power draw during heavy rain works out to roughly 1,081W × 0.50 = 541W.

What Size Power Station Do You Need?

The answer has two parts. First, the power station must handle the startup surge without its inverter tripping. Second, it must have enough battery capacity to sustain the pump through the duration of the outage.

Part 1: Surge capacity (the gating factor)

We apply a 1.15 safety buffer to the raw surge requirement. This accounts for voltage sag under load, minor variations in motor condition, and the fact that a cold motor or a pump working against high head pressure can draw slightly more than its rated inrush.

For the Zoeller M98: 3,243W × 1.15 = 3,729W minimum surge.

For the Liberty 287: 2,645W × 1.15 = 3,042W minimum surge.

Part 2: Battery capacity (runtime during the storm)

Once you have confirmed your power station can handle the surge, the next question is how long it lasts. We use the same 0.70 derate factor applied throughout our sizing guides, accounting for inverter losses, voltage sag, and efficiency at partial loads.

Using the Zoeller M98 at 50% duty cycle (541W average draw):

EcoFlow DELTA 2 Max (2,048 Wh): 2,048 × 0.70 / 541 = 2.6 hours

Jackery Explorer 2000 Plus (2,042 Wh): 2,042 × 0.70 / 541 = 2.6 hours

EcoFlow DELTA Pro 3 (4,096 Wh): 4,096 × 0.70 / 541 = 5.3 hours

These numbers tell an important story. Even a 4,096 Wh power station provides only about 5 hours of sump pump operation during heavy rain. If you expect outages lasting 8 to 12 hours, you need either an expandable system, solar recharging, or a generator backup plan.

The Liberty 287 at 50% duty cycle averages 460W, giving you longer runtimes: the EcoFlow DELTA Pro 3 would last approximately 6.2 hours (4,096 × 0.70 / 460).

For moderate rain with lower duty cycles (25% or so), runtimes roughly double. A 50% duty cycle represents the worst case during sustained heavy rainfall.

Extended runtime options

The Jackery Explorer 2000 Plus accepts up to five expansion battery packs, bringing total capacity to approximately 12,000 Wh. At 541W average draw, that provides about 15.5 hours of heavy-rain sump pump operation.

The EcoFlow DELTA Pro 3 at 4,096 Wh is a single-unit option with enough surge headroom (8,000W) to never worry about motor startup. If you pair it with 400W of solar panels, you can offset some of the drain during daylight hours. In partly cloudy storm conditions, expect 30 to 50 percent of rated panel output, so a 400W array would contribute roughly 120 to 200 watts on average. That extends your runtime meaningfully but does not eliminate the need for adequate battery capacity.

The Soft-Start Option

If you already own a power station with insufficient surge capacity, or you want to buy a smaller, less expensive unit, a soft-start device can solve the surge problem for roughly $50 to $100.

A soft-start module wires inline between the power station’s outlet and the sump pump’s plug. It limits the initial inrush current by gradually ramping voltage to the motor over a fraction of a second. This typically reduces the effective surge to about 45% of the raw startup watts.

For the Zoeller M98: 3,243W × 0.45 = 1,459W reduced surge.

For the Liberty 287: 2,645W × 0.45 = 1,190W reduced surge.

With a soft-start module installed, even the Anker SOLIX C1000 (2,400W surge capacity) can handle either pump with significant margin. This opens up the entire market of power stations rated at 1,500W surge or above.

Soft-start compatibility varies by motor type and controller design. Not every sump pump responds well to reduced-voltage starting. Verify compatibility with your pump manufacturer or a licensed electrician before relying on a soft-start module for flood protection.

The soft-start approach makes the most financial sense when you already own a power station for other purposes (camping, general backup) and want to extend its usefulness to sump pump protection without buying a larger unit.

Installation and Testing

A sump pump battery backup only works if it actually works when you need it. The most common failure mode is not a dead battery or an undersized inverter. It is a setup that was never tested.

Plug the pump directly into the power station. Never backfeed a home outlet by plugging a power station into a wall socket to power your house wiring in reverse. Backfeeding is illegal in most jurisdictions, creates a lethal electrocution hazard for utility line workers, and can damage your power station. If you need to connect a power station to your home electrical panel, hire a licensed electrician to install a proper transfer switch or power inlet box.

Position the power station near the sump pit. Keep it on a shelf or elevated surface, never on the basement floor where rising water could reach it. Route the power cable from the station to the pump outlet. Use the shortest extension cord possible (10 feet or less) with a gauge rating appropriate for the pump’s amperage (12 AWG for a 1/2 HP pump).

Test monthly. Unplug your sump pump from the wall outlet and plug it into the fully charged power station. Wait for the pump to cycle naturally, or manually lift the float switch to trigger a startup. Confirm three things: the power station handles the surge without tripping, the pump runs normally, and the power station displays a reasonable wattage reading that matches expected draw.

Run a full duration test at least once per year. Let the power station run the pump through multiple cycles until the battery dies. Record the actual runtime. Compare it to your calculated estimate. If actual runtime is more than 20 percent below the calculated figure, the battery may be degraded.

Consider a UPS-capable station for automatic switchover. If you are not home when the power goes out, a power station with UPS functionality can switch to battery power automatically. The station stays plugged into the wall with the sump pump plugged into the station. The station passes grid power through to the pump under normal conditions while keeping its battery fully charged. When the grid drops, it switches to battery power with minimal interruption.

The Zendure SuperBase V4600 (4,608 Wh) offers 1ms switchover time, effectively instantaneous. At 4,608 Wh, it provides roughly 6.0 hours of heavy-rain runtime (4,608 × 0.70 / 541). However, its surge capacity is 3,800W, which leaves only 71W of margin above the Zoeller M98’s buffered surge requirement (3,729W). That is too tight for comfort. For the Liberty 287 (3,042W buffered), the Zendure passes with 758W of margin. If you own a Zoeller or similar high-surge pump, pair the Zendure with a soft-start module to bring the startup demand well within its capacity.

The Cost Equation

A basement flood from a failed sump pump during a 6-hour power outage can cost $10,000 to $50,000 or more. A power station capable of running a 1/2 HP sump pump through that same outage costs between $1,500 and $3,700 depending on the model and battery capacity.

That is a one-time purchase (with a 10-year or longer lifespan for LFP battery chemistry) versus a potential recurring loss every time a severe storm coincides with a grid failure. The power station also serves double duty as backup power for other household needs during outages.

If you live in an area prone to heavy rainfall, rapid snowmelt, or a high water table, sump pump battery backup is not a convenience. It is property insurance that you control.

Next Steps

The Surge Watts Explained guide covers the physics of motor startup surge and why it matters for every motor-driven device. If you are considering the soft-start route, the Soft-Start Devices Guide walks through product options, installation, and compatibility testing. And the How to Size a Portable Power Station guide covers the general framework that applies to any device, including the 0.70 derate factor and duty cycle calculations used throughout this article.

Sources: Device wattage from OEM manuals and spec sheets in the GeneratorChecker device database. Zoeller M98 (model 98-0001) running amps from Zoeller technical data sheet FM2779; surge estimated at 3× running watts per NEC standard (no OEM LRA published). Liberty Pumps 287 specifications including 23A LRA from Liberty Pumps 280-series spec sheet R04/2025. Bluetti Power Lifting behavior documented on Bluetti AC200L product page. Methodology: How we source and verify device data.