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Solar Sump Pump Calculator

Size a solar backup system for your sump pump. Enter pump HP, daily runtime, and backup days — get battery capacity, panel count, inverter surge rating, and emergency runtime.

Sump pump system details
hrs/day
days
Solar sump pump system sizing
1 × 400W panels — 3.6 kWh battery — 1200W inverter
Pump running watts400 W
Motor surge watts (inverter min)1,200 W surge — inverter must handle this
Daily energy consumption1200 Wh/day
Battery bank (24V LiFePO4)3.6 kWh — 188 Ah @ 24V
Solar panels (recharge daily)1 × 400W panels (300W needed)
Charge controller19A MPPT
Inverter (pure sine wave)1,200W minimum (surge rated)
Emergency runtime at full battery7 hours continuous pumping
Est. total system cost$5,020
vs. grid extension (rural)$8,000 (estimated rural grid ext.)
Flood risk levelMODERATE
Motor surge: Always use a pure sine wave inverter rated for 1,200W surge minimum. Modified sine wave inverters can damage pump motors. Battery sizing is conservative by design — a flooded basement costs $20,000-50,000 in damage.
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How to Use This Calculator

Select your pump HP and daily runtime

Start with your sump pump's horsepower rating — this is stamped on the motor nameplate or listed in the manual. Running watts are the steady-state draw; the calculator automatically computes surge watts (2.5-3x running) required for motor startup. Daily runtime depends on your water table and soil drainage — ask your plumber or measure how often your float switch triggers. In storm-prone areas, use your worst-case storm season runtime, not the annual average.

Set battery backup days

This is the most critical input. Sump pump failure equals flooded basement. The minimum recommended backup is 3 days, but 5-7 days is strongly advised for high water table situations or areas prone to multi-day storms. A flooded basement causes $20,000-50,000 in damage — the battery cost is insurance, not a luxury. The calculator sizes conservatively: battery capacity assumes the pump runs its full daily runtime every day of the backup period, with no solar recharge on cloudy days.

Read the inverter surge warning

The most common solar sump pump installation mistake is undersizing the inverter. Sump pump motors draw 2.5-3x their running watts for 1-3 seconds at startup (locked rotor current). An inverter rated for 550W running cannot start a 1/2 HP pump — it requires a 1,375W surge rating minimum. Always use a pure sine wave inverter rated at or above the surge wattage shown in the results. Modified sine wave inverters can damage pump motor windings over time.

The Formula

Daily Wh = Pump Watts × Daily Runtime Hours Battery kWh = Daily Wh × Backup Days ÷ 1000 Battery Ah (24V) = Battery kWh × 1000 ÷ (24V × 0.80 DoD) Solar Watts = Daily Wh ÷ Peak Sun Hours ÷ 0.80 efficiency Panels = Solar Watts ÷ 400W (round up) Surge Watts = Running Watts × 2.5-3.0 (motor type) Inverter Rating = Surge Watts minimum (pure sine wave) Emergency Runtime = Battery kWh × 1000 × 0.80 ÷ Pump Watts

The 24V battery system is used for better efficiency at higher power draws — 24V allows thinner wiring and better inverter efficiency than 12V. The LiFePO4 depth of discharge of 80% is used. Conservative battery sizing (full daily runtime per backup day, no solar recharge) is intentional: during extended storm events, multiple days of reduced solar irradiance coincide with increased pump demand. This overlap is the scenario that causes flooded basements.

Example

Mark — 1/2 HP pump, moderate water table, 3 days backup in Atlanta

Mark's Atlanta home has a 1/2 HP submersible sump pump (550W running). His water table is moderate — the pump runs about 3 hours per day in spring. He wants 3 days of backup in case of a severe storm knocking out grid power.

Pump1/2 HP (550W running / 1,375W surge)
Daily runtime3 hours/day
Backup needed3 days
LocationAtlanta, GA (5.0 PSH)

Result

Daily energy1,650 Wh/day
Battery (24V LiFePO4)~257 Ah (4.95 kWh)
Inverter required1,375W pure sine wave (surge rated)
Solar panels2 × 400W panels
Charge controller24A MPPT
Emergency runtime~7.2 hours continuous pumping
Est. system cost~$6,500

Mark's $6,500 system provides 3 days of reliable pump operation during grid outages, protecting his basement against flooding damage. The 2 solar panels recharge the battery in about 2 days of normal sunshine — keeping the battery topped up during extended storm recovery. The 1,375W surge-rated inverter correctly handles the motor start current that would trip a standard 550W inverter.

FAQ

Sump pumps are induction motors, and all induction motors have a "locked rotor current" — the surge of electricity needed to overcome the motor's inertia at startup. For submersible sump pumps, this surge is typically 2.5-3x the running watts and lasts 1-3 seconds. A 1/2 HP pump running at 550W needs 1,375W of surge capacity on startup. If your inverter can't deliver that surge, it will either trip its overload protection, shut down, or — with cheaper inverters — fail permanently. Always buy an inverter whose continuous AND surge rating exceeds your pump's startup requirement. For sump pumps specifically, size the inverter to the surge watts, not the running watts.
No — not safely for a basement sump pump. Without a battery, the pump would only run when the sun is shining (roughly 6-8 hours on a clear day), leaving your basement unprotected at night and during cloudy conditions. Since severe rain events typically occur during storms (cloudy, at night, or both), a battery-less solar sump pump system would fail precisely when you need it most. The battery is non-negotiable for a sump pump system. The solar panels are there to recharge the battery during fair weather, ensuring it's ready for the next storm event.
DC brushless pump: More efficient (no inverter loss), quieter, longer lifespan, compatible with 12V/24V battery systems. Ideal if you're building a solar-specific sump system from scratch. Many DC sump pumps are designed for solar backup use and include smart float switches. AC pump (standard): Cheaper, widely available, easy to replace. Requires a pure sine wave inverter, which adds cost and conversion losses (~15%). If you already have an AC sump pump and just want solar backup, the AC + inverter approach is the most cost-effective retrofit. Don't use a modified sine wave inverter with AC pumps — it can damage the motor.
For a typical 1/3-1/2 HP sump pump running 2-3 hours per day in a 4-5 PSH location, 1-2 × 400W solar panels are sufficient to recharge the battery daily. The key is recharge time, not instantaneous power — the pump might draw 550W for 3 hours (1,650 Wh), and your panels need to replace that within a reasonable number of sun hours. In cloudy regions (Seattle: 3.6 PSH), add 1 extra panel to account for reduced solar days. More panels speed recharge time after back-to-back storm days. For high water table situations (6+ hrs/day runtime), 2-3 panels are typically needed.
It depends on the situation. Dedicated battery backup sump pumps (like Basement Watchdog, Zoeller, etc.) cost $300-600 and run 8-24 hours on a pre-charged sealed battery. They are the simplest solution for occasional power outages. Solar + LiFePO4 battery systems cost $3,000-8,000 but provide indefinite runtime as long as the sun shines, never need replacement batteries, and can be expanded to power other emergency loads. Solar sump systems are worth the investment if: (1) You have frequent extended outages. (2) Your water table is high and requires multi-day runtime. (3) You're in a storm-prone area where 3-7 day outages are possible. (4) You want the system to self-sustain indefinitely without manual intervention.

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