Is a Battery Worth Adding to Your Solar?
You already pay for the panels. This calculator shows what a battery adds on top: yearly savings, payback period and lifetime ROI — using your actual surplus and rates, with no installer contact form in sight.
Built for existing solar owners Surplus from your meter or system size Multi-currency — switch in the header
1 Your solar system & battery
Your solar surplus
kWh/day
Find your surplus on your inverter app or export meter — the "sent to grid" number on a typical day.
Battery to add
kWh
USD
%
yrs
%
×
Electricity rate
/kWh
/kWh
US average residential ~18¢/kWh (Nov 2026); peak/off-peak are typical utility TOU values. Source: EIA, 2026-11.
Incentives
USD
The 30% federal credit for purchased systems ended Dec 31, 2025. See what still qualifies →
2 What the battery adds
—
Calculating… Adjust inputs to see results instantly.
Extra year-1 savings
—
beyond what solar already saves
Net cost after incentive
—
before incentives
Lifetime savings
—
over warranty period, degraded
Lifetime ROI
—
on the added battery
Battery capacity fade
linear model to warranty floorUsable capacity (kWh)
Cumulative savings vs. investment
breakeven = where the curve crosses the line Cumulative savings Net cost
Compare battery sizes for your surplus
price scaled linearly from your cost per kWh| Capacity | Est. installed cost | Year-1 savings | Payback | Lifetime net gain |
|---|
Bigger batteries store more surplus — until they outgrow your daily excess (see the capture note above).
How this calculator works (methodology & assumptions)
Stored each day = min(surplus, capacity × cycles)
Flat rate: saving = stored × efficiency × flat rate
Time-of-use: saving = stored × efficiency × peak rate
+ capacity × remaining cycles × (peak − off-peak) × efficiency
Degradation: capacity(y) declines linearly to the warranty floor over lifespan
Flat rate: saving = stored × efficiency × flat rate
Time-of-use: saving = stored × efficiency × peak rate
+ capacity × remaining cycles × (peak − off-peak) × efficiency
Degradation: capacity(y) declines linearly to the warranty floor over lifespan
- System-size mode estimates surplus as: production ≈ kWp × 4.5 h/day, of which ≈55% is exported without storage (typical split; your meter reading is more accurate).
- Surplus valuation: stored energy is valued at the peak rate under TOU (it offsets your most expensive evening energy first), or the flat rate otherwise.
- Remaining cycles after solar charging can still arbitrage the grid under TOU rates.
- Excluded: export tariffs, demand charges, financing, tax on savings.
- Same model as the full Battery ROI Calculator — this page just starts from your solar surplus.