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Solar Battery Size Calculator
Work out how many home batteries you need for backup, and what they cost.
Your result
Enter your numbers and press the button. Your result appears here.
One Tesla Powerwall 3 stores 13.5 kWh and delivers up to 11.5 kW continuously, according to Tesla's specifications.
How it works
Energy = average load × hours. Batteries = the larger of energy ÷ usable kWh per battery and peak load ÷ kW per battery, rounded up.
Example: 14.4 kWh of backup with a 5 kW peak needs 2 × 13.5 kWh batteries, about $31,294 installed.
At a glance
Same settings as the calculator, changing only the hours.
| Hours | Energy | Batteries | Cost |
|---|---|---|---|
| 4 h | 4.8 kWh | 1 | $15,647 |
| 8 h | 9.6 kWh | 1 | $15,647 |
| 12 h | 14.4 kWh | 2 | $31,294 |
| 24 h | 28.8 kWh | 3 | $46,941 |
| 48 h | 57.6 kWh | 5 | $78,235 |
Tesla Powerwall 3 at a glance
| Usable energy | 13.5 kWh |
|---|---|
| Continuous power | 11.5 kW |
| Off-grid continuous power with solar | up to 15.4 kW |
| Charging power | 5 kW |
| Average installed cost (13.5 kWh battery, EnergySage, H1 2026) | $15,647 |
All batteries calculators
Common questions
How many batteries do I need for a whole house?
The average home uses about 28 kWh a day. Two 13.5 kWh batteries cover a full day only if you trim heavy loads; essentials-only backup often needs one.
How much does a home battery cost?
EnergySage puts a 13.5 kWh battery at $15,647 installed on average in the first half of 2026, before incentives.
Is there a tax credit for batteries?
The 30% Residential Clean Energy Credit also covered batteries, but it ended for installations after December 31, 2025.
Sources
- Tesla, Powerwall 3 specifications: https://service.tesla.com/docs/Public/Energy/Powerwall/Powerwall-3-with-Gateway-3-Installation-Manual-NA-EN/GUID-EC527BC7-4750-4425-BBC4-DB8C000339B3.html
- EnergySage, Solar battery backup cost (H1 2026): https://www.energysage.com/energy-storage/solar-battery-backup/
- U.S. EIA, Today in Energy: average residential use 865 kWh a month in 2024: https://www.eia.gov/todayinenergy/detail.php?id=65244