Solar Battery Storage Calculator

How much battery storage does your home solar system need? Get recommended kWh, self-consumption rate, and payback period.

Solar system & consumption

Solar system size?

Daily solar production?

kWh/day

Daily home consumption?

kWh/day

System type?

Electricity rate?

$/kWh

TOU peak premium?

$/kWh

Recommended battery storage

18.8 kWh

Self-consumption without battery94%

Self-consumption with battery100%

Annual savings from battery$256/yr

Est. battery cost$9,375

Payback period36.7 yrs

Daily excess solar2.0 kWh

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How to Use This Calculator

Enter your solar production and consumption

Input your daily solar production (kWh your panels produce on an average day) and your daily home consumption (kWh you use). The difference — excess solar — is the energy a battery can capture. If your panels produce 32 kWh/day and you use 30 kWh, there's 2 kWh of excess to store. But even without excess, a battery lets you shift daytime production to nighttime use.

Grid-tied vs off-grid

The system type fundamentally changes how battery sizing works. Grid-tied systems add storage primarily to increase self-consumption and provide backup — the grid is your safety net. Off-grid systems must cover all nighttime and cloudy-day consumption from the battery alone, requiring significantly more storage.

Time-of-use (TOU) rates

For grid-tied systems, the TOU peak premium is the extra rate you pay during peak hours (typically 4–9pm). Utilities like PG&E, ConEd, and Duke are increasingly moving customers to TOU pricing where peak power costs 2–3× off-peak. A battery charged by cheap solar can discharge during expensive peak hours, and the premium field captures this arbitrage value.

Self-consumption percentage

Self-consumption measures what fraction of your solar production you consume directly (or via battery) rather than export to the grid. Without battery storage, a system producing 32 kWh for a 30 kWh/day home might only achieve 60–70% self-consumption if most of the production happens while nobody's home. A battery can push self-consumption to 90%+.

The Formula

Excess solar = Daily production - Daily consumption (if positive) Night demand = Consumption not covered by direct solar use Grid-tied battery sizing: Recommended kWh = max(excess solar, night demand) ÷ 0.80 DoD Off-grid battery sizing: Recommended kWh = (Night demand ÷ 0.80 DoD) × 1.2 safety margin Self-consumption without battery = Direct solar use ÷ Production × 100% Self-consumption with battery = (Direct use + Stored excess) ÷ Production × 100% Annual savings = (Extra night kWh × 365 × rate) + (Excess solar × 365 × TOU premium) Payback years = Battery cost ($500/kWh installed) ÷ Annual savings

Example

Average home with TOU pricing — San Jose, CA

An 8 kW solar system produces 32 kWh/day for a home using 30 kWh/day. PG&E charges $0.20/kWh off-peak and $0.35/kWh during 4–9pm peak (a $0.15 premium). The homeowner wants to maximize self-consumption and avoid peak rates.

Solar system size8 kW

Daily production32 kWh/day

Daily consumption30 kWh/day

Electricity rate$0.20/kWh

TOU peak premium$0.15/kWh

Result

Recommended battery10–13 kWh

Self-consumption without battery~65%

Self-consumption with battery~90%

Annual savings from battery~$800–$1,200/yr

Estimated battery cost~$6,500 installed

Payback period6–8 years

One Tesla Powerwall (13.5 kWh) or equivalent would serve this home well. The 30% federal ITC on the battery brings the net cost to ~$4,550, improving payback to 4–6 years.

FAQ

Is adding battery storage to existing solar worth it financially? ▼

It depends on your electricity rates and utility policies. In states with strong TOU pricing (California, New York, Massachusetts), batteries offer 7–10 year paybacks — reasonable for equipment lasting 15+ years. In states with flat rates and good net metering, paybacks stretch to 12–15 years and may not pencil out financially. Backup value (power during outages) is harder to quantify but is the primary driver for many homeowners in areas with frequent outages.

What is self-consumption rate and why does it matter? ▼

Self-consumption rate measures what percentage of your solar production you use yourself vs. exporting to the grid. It matters because utilities often pay less for exported solar (sometimes $0 under "zero export" rules) than you pay for grid power. A system with 60% self-consumption is effectively selling 40% of its production at a below-market rate. Batteries push self-consumption toward 90–95%, maximizing the value of every kWh your panels produce.

How does battery storage work with net metering? ▼

With traditional 1:1 net metering, batteries add less financial value because exported solar is credited at the full retail rate anyway. With NEM 3.0 in California (which reduced export credits by 75%), batteries became essential — you're better off storing and self-consuming than exporting at low rates. Check your state's net metering policy. States moving to "avoided cost" export rates make batteries significantly more valuable.

Can I add battery storage without solar panels? ▼

Yes — a "battery-only" or "standalone" storage system charges from the grid during cheap off-peak hours and discharges during expensive peak hours. This is called load shifting or demand charge management. In areas with extreme TOU pricing differences ($0.10/kWh off-peak vs $0.45/kWh peak), this can be quite profitable. However, the battery charging from the grid is not eligible for the 30% federal solar ITC — you need at least 90% solar charging for the ITC.

What size inverter do I need with a home battery? ▼

The inverter must handle your peak load, not your average load. A 10 kW hybrid inverter handles most homes — it covers a 5-ton AC unit (4–5 kW), refrigerator (0.3 kW), and normal lighting and electronics simultaneously. For whole-home backup, add up the starting (surge) wattage of your largest motors — AC compressors, well pumps, and sump pumps can surge to 3× their running watts. Size the inverter to your highest expected simultaneous load plus 25% headroom.