Quick answer: start with 250 W of solar
The energy is small; the reliability is everything. A wet day of pumping is about 520 Wh, roughly 250 W of solar, but the pump has to start on demand during the same storm that cuts your solar production, so the battery and the motor start matter more than the array.
Planning result
At 200 W per module, the array target is 2 panels. That count assumes four peak-sun-hours, 75% end-to-end solar yield, 90% conversion efficiency, and a 20% planning margin. The heaviest pumping comes with spring thaw and storms, the days with the least sun.
How a sump pump actually uses power
Planning case: a 1/3 HP submersible sump pump started by a float switch, drawing about 800 W while it runs, over about an hour of cycling on a wet day with the motor on 65% of that time. Little Giant rates its 1/3 HP 6-CBM at 720 W (9 A) running and 14.3 A starting, which this case covers with margin.
A float switch starts the pump when water in the pit reaches its trigger height and stops it when the pit is drawn down. On a dry day it may not run at all; in a storm it can cycle every few minutes. Each cycle is a short run at full motor power.
Load calculation from running watts and duty cycle
The appliance-class estimate is 800 W running for 1 h/day at a 65% duty cycle, an average of about 520 W across those hours:
Then losses and reserve raise the energy the system must collect to about 694 Wh/day.
Typical manufacturer nameplate/manual range for the sump pump class, triangulated with Little Giant (Franklin Electric): 6-CBM 1/3 HP sump pump (506257) and Zoeller: Mighty-Mate series technical data sheet FM2778, then reduced to a conservative planning midpoint; this is not a product specification. Verify the exact unit's nameplate and manual before sizing.
Count cycles and time one during a wet spell; watts times total run minutes gives the daily energy. A plug-in energy meter on the pump cord records both.
Solar and battery sizing table
| Design item | Planning value | Basis |
|---|---|---|
| Appliance energy | 520 Wh/day | Running W × hours × duty cycle |
| Solar collection target | 250 W | 4 peak-sun-hours, 75% yield, 20% margin |
| 200 W panel count | 2 | Rounded up; real modules must satisfy controller voltage/current limits |
| One-day battery allowance | 900 Wh | 90% conversion efficiency, 80% LiFePO4 usable fraction, 20% margin |
| 12.8 V LiFePO4 equivalent | 75 Ah | Nominal planning conversion |
| 25.6 V LiFePO4 equivalent | 40 Ah | Nominal planning conversion |
Plan storage for the storm, not the average: several wet, sunless days in a row is the design case. Many owners pair the main pump with a dedicated 12 V backup sump pump and its own battery.
Build the closest passing wiring-diagram preset
The Sump Pump Planning System preset is an engine-passing starting point whose array, battery bank, continuous inverter rating, and surge rating each meet or exceed this page's stated planning allowances. Adjust it only from measured load data and verified equipment limits.
Inverter and startup current
Use at least 1,000 W continuous with 2,700 W surge for this planning case. Each float-switch start is a full motor start; the 2,700 W surge allowance covers it, and it repeats every cycle.
The 2,400 W startup estimate is not a substitute for the manual's locked-rotor, compressor-start, or motor-start requirements, and an inverter's surge rating usually lasts seconds, not minutes. A battery-to-inverter cable that is too long or too small can pull the DC input below the inverter's low-voltage threshold during that start even when the battery has plenty of energy, so use the diagram machine's real run length.
The common mistake
Ways to cut the load
- Direct gutters and grading away from the house so less water reaches the pit.
- Use a larger pit or adjust the float so each cycle runs longer and starts less often.
- Add a battery-backup 12 V sump pump as a second line of defense.
- Test the pump and float before the wet season.
What to verify before buying equipment
- The exact running watts, startup current, voltage, frequency, and required circuit from the appliance label and manual.
- Actual watt-hours over a representative day; seasonal heating or cooling loads need a design-day measurement.
- Local peak-sun-hours for the worst operating season, plus shade and array orientation.
- Controller maximum input voltage at the coldest expected temperature and maximum output current.
- Battery discharge current, series/parallel permission, terminal torque, low-temperature controls, and overcurrent protection.
- Conductor ampacity, voltage drop, terminal temperature ratings, disconnecting means, grounding, local amendments, and inspection requirements.
For a whole-system load list, use the load and appliance calculator. For the system context around this appliance, continue to the Solar water and well-pump guide.
Frequently asked questions
How much solar do I need for a sump pump?
For the planning case on this page (a 1/3 HP sump pump on a wet day), allow about 250 watts of solar at four peak-sun-hours per day. That covers an estimated 520 Wh daily load plus conversion, temperature, and recharge margin. The battery and the backup plan matter more than the array for this load.
What size inverter should I use for a sump pump?
Plan on at least 1,000 W continuous and 2,700 W surge. Every start is a motor surge; size from the starting current in the pump manual and keep the inverter on whenever the pump may run.
What battery size keeps a sump pump running for one day?
The one-day planning allowance is about 900 Wh nominal usable design capacity, roughly 75 Ah at 12.8 V or 40 Ah at 25.6 V for LiFePO4. Plan for several consecutive storm days with little solar recharge.