Sump Pump Battery Backup Calculator
Calculate the battery size, inverter capacity, and backup runtime needed to keep your sump pump running during a power outage. Prevent basement flooding with properly sized emergency backup power.
Appliance Loads
Backup Settings
How long you need backup power during the outage.
12V for small portable setups, 24V for mid-size, 48V for whole-home.
LiFePO4 recommended: safest, longest life, deepest discharge.
Auto-set by chemistry. LiFePO4: 80%, Lead-Acid: 50%.
Modern pure sine wave inverters: 90–95%. Use 90% for conservative sizing.
Extra capacity buffer for temperature, aging, and unexpected loads.
Total wattage of solar panels connected to charge controller.
Average daily peak sun hours for your location.
MPPT controllers: 95–98%. PWM controllers: 80–90%.
Backup System Results
Suggested Battery Configurations
Solar Recharge Estimate
Planning Notes
A sump pump has been pre-loaded above. Add other critical loads and adjust backup duration.
Mathematical Formulas
Note: Sump pumps cycle intermittently — actual runtime may be 2–4× longer than continuous running estimates suggest.
Formulas & Worked Examples
Example 1: 1/3 HP Sump Pump (Storm Backup)
Keep a 1/3 HP sump pump running during a 4-hour severe storm power outage with frequent cycling.
Given Values
- 1/3 HP Sump Pump: 800W running / 2,000W surge
- Backup Duration: 4 hours
- Cycle Pattern: 30 sec on / 8 min off
Step 1: Effective duty cycle
= ~6.3% duty cycle
Step 2: Average draw
= ~50W average
Step 3: Energy for 4 hours
= 200 Wh
Step 4: After inverter efficiency (90%)
= 222 Wh
Step 5: With 20% margin
= 267 Wh
Step 6: Battery (24V LFP, 80% DoD)
= 13.9 Ah → 12V 20Ah LFP
Final Answer
A 12V 20Ah LFP battery handles a 1/3 HP sump pump for 4+ hours of intermittent storm cycling.
Sump pumps have very low duty cycles during moderate rain — battery lasts far longer than continuous running estimates suggest.
Example 2: 1/2 HP Sump Pump (Heavy Rain)
Power a larger 1/2 HP sump pump during heavy rain causing frequent cycling for 6 hours.
Given Values
- 1/2 HP Sump Pump: 1,000W running / 2,500W surge
- Backup Duration: 6 hours
- Cycle Pattern: 45 sec on / 3 min off
Step 1: Effective duty cycle
= 20% duty cycle
Step 2: Average draw
= 200W average
Step 3: Energy for 6 hours
= 1,200 Wh
Step 4: After inverter efficiency (90%)
= 1,333 Wh
Step 5: With 20% margin
= 1,600 Wh
Step 6: Battery (24V LFP, 80% DoD)
= 83.3 Ah → 24V 100Ah LFP
Final Answer
A 24V 100Ah LFP battery (2,560 Wh) handles a 1/2 HP sump pump for 6 hours of heavy-rain cycling.
Heavy rain increases pump cycling frequency — plan for 20–30% duty cycle during severe storms.
Example 3: Sump Pump + Water Alarm (Combo)
Power a sump pump plus a battery-powered water alarm sensor during an overnight storm outage (10 hours).
Given Values
- Sump Pump (1/3 HP): 800W running / 2,000W surge
- Water Alarm Sensor: 5W continuous
- Backup Duration: 10 hours
- Pump Cycle Pattern: 30 sec / 10 min
Step 1: Pump average draw (5% duty)
= 40W average
Step 2: Combined average draw
= 45W
Step 3: Energy for 10 hours
= 450 Wh
Step 4: After inverter efficiency (90%)
= 500 Wh
Step 5: With 20% margin
= 600 Wh
Step 6: Battery (24V LFP, 80% DoD)
= 31.3 Ah → 12V 35Ah LFP
Final Answer
A 12V 35Ah LFP battery runs a sump pump plus water alarm for 10 hours of overnight storm protection.
Adding a water alarm draws negligible power but provides critical early warning — always include one in your backup plan.
Sump Pump Backup Power Flow
Battery power flows through a high-surge inverter to handle the sump pump motor startup spike.
Sump Pump Backup Requirements by Size
| Pump Size | Running W | Surge W | Min. Inverter | Battery (4h backup) | Battery (8h backup) |
|---|---|---|---|---|---|
| 1/4 HP | 600 W | 1,500 W | 1,500W / 3,000W surge | 12V 30Ah (384 Wh) | 12V 60Ah (768 Wh) |
| 1/3 HP | 800 W | 2,000 W | 2,000W / 4,000W surge | 12V 40Ah (512 Wh) | 24V 50Ah (1,280 Wh) |
| 1/2 HP | 1,000 W | 2,500 W | 2,500W / 5,000W surge | 24V 60Ah (1,536 Wh) | 24V 100Ah (2,560 Wh) |
| 3/4 HP | 1,500 W | 3,500 W | 3,500W / 7,000W surge | 24V 100Ah (2,560 Wh) | 48V 100Ah (5,120 Wh) |
| 1 HP | 2,000 W | 5,000 W | 5,000W / 10,000W surge | 48V 100Ah (5,120 Wh) | 48V 200Ah (10,240 Wh) |
| + Water Alarm | +5 W | — | No additional | +6 Wh | +12 Wh |
Battery sizes assume intermittent cycling (5–20% duty). Continuous running requires 3–5× the listed capacity. Always verify surge rating matches your specific pump.
Frequently Asked Questions
What size battery for a sump pump?
A 1/3 HP sump pump (~800W running) needs a 24V 100Ah LiFePO4 battery (2,560 Wh) for 4–6 hours of intermittent pumping. The high motor surge (2,000–3,000W) requires a high-surge inverter — at least 2,000W continuous with 4,000W+ peak.
How long can a battery run a sump pump?
Runtime depends on how often the pump cycles. During steady rain, a sump pump may run 30 seconds every 5–10 minutes. A 24V 100Ah battery can support this cycling pattern for 6–12 hours. Continuous running depletes it in 1–2 hours.
Can a portable power station run a sump pump?
Only high-capacity units (2,000W+ output, 2,000+ Wh capacity) can handle a sump pump's motor surge. Most 1,000W stations cannot handle the 2,000–3,000W startup spike. Always check the surge rating, not just continuous output.
Why does a sump pump need so much power?
Sump pumps use induction motors that draw 3–5× their running wattage during startup (1–3 seconds). A 1/3 HP pump draws ~800W running but 2,500W+ during startup. This surge is the limiting factor for battery backup sizing.
What Is Sump Pump Battery Backup?
Why This Calculation Matters
→ Sump pumps draw 800–1,200W running and 2,000–3,000W surge during motor startup — one of the highest-draw residential appliances to back up.
→ Power outages during storms are exactly when you need your sump pump most — without backup power, basements flood within hours.
→ A 1/3 HP sump pump moves 7–10 gallons per minute. A flooded basement causes $5,000–$15,000 in average damage.
→ Battery backup sump pump systems are a separate product, but a whole-home battery provides more flexibility and higher surge capacity.
→ Sump pump motors have very high startup surge (3–5× running wattage) — most cheap inverters cannot handle the load.
Practical Applications
Storm Flood Prevention
Keep your sump pump running when power goes out during heavy rain — the exact time flooding risk is highest.
Basement Protection
Prevent thousands in water damage by ensuring your primary sump pump has power during grid outages.
Whole-Home Battery Sizing
Factor your sump pump's high surge into your whole-home battery backup system sizing.
Construction & Renovation Sites
Temporary dewatering during construction projects often requires battery-powered pump operation.
Common Mistakes to Avoid
✗ Ignoring motor startup surge — a 1/3 HP sump pump draws 2,000–3,000W during startup (3–5× running wattage), and most inverters cannot handle this peak.
✗ Sizing battery for continuous running instead of intermittent cycling — sump pumps cycle 30 seconds every 5–10 minutes, so actual consumption is much lower.
✗ Using a low-surge inverter — sump pump motors require at least 4,000W surge capacity; a 2,000W continuous inverter without sufficient surge will trip.
✗ Forgetting that power outages during storms are when sump pump backup is most needed — undersizing leaves you flooding exactly when risk is highest.
✗ Relying solely on a battery backup sump pump without whole-home battery support — dedicated backup pumps have limited capacity and runtime.
✗ Not testing the backup system before storm season — a dead or degraded battery discovered during a flood event is useless.
✗ Using automotive batteries for sump pump backup — car batteries are not designed for deep cycling and will fail rapidly at the discharge rates required.
✗ Ignoring wiring voltage drop between battery and pump — a 12V system at high current can lose significant voltage over long cable runs.
Why Trust These Calculations?
This calculator uses standard electrical engineering formulas for battery system sizing. Sump pump surge varies significantly by model — always verify with your specific device.
View our methodology →Home Power Outage Battery Backup Calculator
Full backup power calculator for all household appliances.
Fridge Battery Backup Calculator
Battery sizing specifically for refrigerator backup during outages.
CPAP Battery Backup Calculator
Size batteries for overnight CPAP machine power.
Portable Power Station Runtime Calculator
See how long a portable power station runs your loads.
Solar Generator Sizing Calculator
Size a complete solar backup system with panels and battery.
Battery Runtime Calculator
Estimate discharge time from capacity and load.
Battery Sizing Calculator
Full battery bank sizing with temperature derating.
Solar Battery Sizing Calculator
Size batteries paired with solar panel arrays.
Energy Conversion Calculator
Convert between Ah, Wh, and kWh at any voltage.
Charging Time Calculator
Estimate charge time from solar or grid.
Voltage Drop Calculator
Size wire runs to minimize voltage drop.
DC Cable Loss Calculator
Calculate wiring losses in DC connections.
Was this calculator helpful?