How to Calculate Solar Battery Storage
What is Solar Battery Storage?
The Home Battery Storage Calculator helps you size a home battery system (like Tesla Powerwall or Enphase IQ) based on your backup power needs, solar production, and electricity usage patterns for self-consumption or backup protection.
Formula
Battery Size (kWh) = Daily Energy Shortfall (kWh) x Days of Backup x 1 / Depth of Discharge
- Ed
- Daily Energy Shortfall (kWh) — Difference between daily consumption and solar generation
- DoD
- Depth of Discharge (%) — Usable percentage of battery capacity (typically 80-100%)
- Db
- Backup Days (days) — Number of days of backup power desired
- RTE
- Round-Trip Efficiency (%) — Percentage of stored energy recoverable, typically 85-90%
Step-by-Step Guide
- 1Determine your daily energy consumption and solar production to find the shortfall
- 2Decide backup duration: overnight bridging (1 day) or full outage protection (2-3 days)
- 3Account for battery depth of discharge (typically 80-100% for lithium-ion)
- 4The calculator recommends the number of battery units needed
Worked Examples
Input
Daily use: 30 kWh, solar produces 25 kWh, 1-day backup, 90% DoD
Result
Shortfall: 5 kWh/day. Battery needed: 5 / 0.90 = 5.6 kWh. One 13.5 kWh Powerwall provides ample backup.
Input
Essential loads only: 10 kWh/day, 2-day outage backup, 90% DoD
Result
Battery: 10 x 2 / 0.90 = 22.2 kWh. Need 2 Powerwalls (27 kWh total) for 2-day essential backup.
Common Mistakes to Avoid
- ✕Sizing battery for total home load instead of essential circuits only (refrigerator, lights, internet, medical equipment)
- ✕Forgetting that battery round-trip efficiency is 85-90%, meaning you lose 10-15% of stored energy
- ✕Not considering that batteries cannot power high-draw appliances like central AC or electric dryers without multiple units
Frequently Asked Questions
How many Powerwalls do I need to power my whole house?
Most homes need 2-3 Powerwalls (27-40.5 kWh) to power the entire home for a day. For essential loads only (refrigerator, lights, internet), 1 Powerwall (13.5 kWh) is usually sufficient.
Is a home battery worth it without solar?
It can be, if you have time-of-use electricity rates with a large peak/off-peak spread (>$0.15/kWh difference). Otherwise, batteries are most cost-effective paired with solar panels.
Ready to calculate? Try the free Solar Battery Storage Calculator
Try it yourself →