Net metering is already a battery. You export surplus solar by day and draw it back at night. A physical battery does the same job at home, minus the wires, plus the cost.
A battery wins when the gap between the import tariff and the export credit is wide. If the grid pays you nearly the same for exports as it charges for imports, there is very little left for storage to capture.
Outages are a separate benefit. This calculator compares electricity costs. It does not add the value of keeping the lights on — that belongs in the battery backup and ROI tools.
What the calculator compares
Both scenarios use the same solar array, the same household load and the same tariff. The only difference is where the evening electricity comes from:
- Solar + grid (net metering): daytime surplus is exported; night load is imported. The bill is the net of the two flows.
- Solar + battery: some daytime surplus charges a battery; the battery serves night load; any remaining surplus is exported or curtailed. Less is imported, but less is exported too.
That last point is the honest trade-off most sales decks omit. Every kilowatt-hour shifted into a battery is a kilowatt-hour you did not sell. The battery’s real value is the difference between the import price you avoid and the export credit you give up, minus round-trip losses.
The inputs that decide the answer
Your electricity use and tariff
Enter either the monthly bill or the monthly kWh. The bill route divides by your unit rate to estimate usage, which is less accurate because bills contain fixed charges. If you can read the kWh figure, use it. The tariff is the price you pay per imported unit; every avoided import is valued at that rate.
Daylight share
The slider (roughly 25%, 40%, 60% or 75% daytime) is the single most important behavioural input. A battery can only shift surplus solar into the hours when the house is actually using power. A household that is empty all day and uses most of its electricity in the evening has a large storage opportunity. A household that runs air conditioners at noon already self-consumes the solar directly; a battery has less to do.
Solar system size and real production
If you have a system already, enter its kWp. If you have a meter reading for monthly generation, enter that instead — measured production beats a size-based estimate. The model uses the same climate data as the yield calculator when coordinates are available; without coordinates it falls back to a latitude-band seasonal estimate and says so.
Battery capacity, chemistry and power rating
Capacity is the nominal kWh you are buying. Usable energy is lower:
Lithium LFP preset: 95% round-trip efficiency, 90% usable DoD
Tubular lead-acid preset: 80% round-trip efficiency, 50% usable DoD
The power rating caps how fast energy can move in or out. The model uses a five-hour full-rated window as a simplification: a 5 kW battery can deliver roughly 25 kWh in a day, which is more than most homes need. The cap only binds for small batteries or very large evening loads. It is a modelling convention, not a physical limit printed on the datasheet.
Export credit and tariff escalation
If your utility pays for exports, enter the rate. If it pays nothing, leave it at zero and the surplus is treated as curtailed. The price-rise assumption (5%, 8% or 12% per year) applies to the import tariff, not to the export credit. That asymmetry is deliberate: buyback rates rarely rise in step with retail prices, and assuming they do would flatter the battery.
The daily simulation
The calculator runs 365 days, carrying the battery’s state of charge from one day to the next. That matters in a week of overcast weather: a battery charged on Sunday can still help on Tuesday.
1. Solar generation is split into direct daytime use and surplus.
2. Surplus charges the battery, capped by usable capacity and power rating × 5 h.
3. The battery discharges into the night load, after round-trip and inverter losses.
4. Remaining night load is imported at the tariff.
5. Remaining surplus is exported at the export credit, or curtailed.
6. The state of charge carries into the next day.
Monthly results are rolled up from the daily simulation. The headline 10-year net benefit is:
Two payback figures are shown: a simple payback at today’s tariff, and a realistic payback that includes price rises, capacity fade and replacements. The second is the one to use for a purchase decision.
A worked example
Take a household using 600 kWh/month at $0.15/kWh, with 40% of consumption in daylight, a 5 kWp array and a 10 kWh LFP battery costing $5,000. Assume the array produces about 20 kWh/day and the export credit is $0.05/kWh.
Daytime load ≈ 40% ≈ 7.9 kWh; night load ≈ 11.8 kWh
Usable battery ≈ 10 × 0.90 = 9 kWh
Battery delivers ≈ 9 × 0.95 ≈ 8.6 kWh to the house
Avoided import ≈ 8.6 × $0.15 = $1.29/day
Export credit given up ≈ 8.6 × $0.05 = $0.43/day
Net daily benefit ≈ $0.86 → about $314/year
At $5,000 installed, simple payback is roughly 16 years. With 8% annual tariff rises it shortens, but the battery may still be marginal on electricity savings alone. If the utility pays nothing for exports, the same battery avoids $1.29/day instead of $0.86 and payback falls to roughly 11 years. If outages are frequent and the battery is wired for backup, the non-financial value can justify it even when the arithmetic does not.
Reading the sensitivity table
The calculator re-runs the economics at 2, 5, 10, 15 and 20 kWh using the same prices and chemistry. The last column shows what each step up in size adds. This is often more useful than the headline result:
- If the first 5 kWh captures most of the bill saving, a 10 kWh battery is buying backup, not arbitrage.
- If the 15 kWh and 20 kWh rows add almost nothing, the extra capacity is idle for most of the year.
- If a larger battery improves payback, the constraint was capacity, not price — but check the cycle count and warranty before believing it.
Where the model stops
Use this tool to decide whether storage is worth investigating, then use the battery ROI calculator for degradation, replacements and levelised cost, and the battery backup calculator to size the bank around the loads you need during an outage. If you have not sized the array yet, start with the system size calculator.
Frequently asked questions
Is a battery worth it if I have net metering?
Often not on electricity savings alone. Net metering already lets you bank daytime exports and draw them back at night, usually at little or no cost. A battery only adds value on top of that if the export credit is much lower than the import tariff, if exports are capped, or if you need backup power. Run both scenarios with your real tariff and export rate.
How much can a battery reduce my monthly bill?
It depends on how many evening kWh it can shift and the gap between import and export prices. A 10 kWh LFP battery might deliver 8–9 kWh per day after depth-of-discharge and efficiency losses. At a $0.10/kWh price gap, that is roughly $25–27/month before accounting for the export credit you give up. At a $0.02 gap, it is only about $5/month.
What size battery should I choose?
Start with your night load, not your solar production. A battery larger than the energy you can consistently use at night sits idle. The sensitivity table shows the marginal benefit of each size step; choose the smallest size that captures most of the saving, unless backup requirements justify more. The backup calculator sizes around specific appliances and outage hours.
Does the calculator include power outages?
No. It compares electricity costs only. Backup value depends on how often the grid fails, how long outages last, which circuits are backed up and what an hour without power is worth to you. The battery ROI calculator has an optional outage value field; this one deliberately leaves it out to keep the financial comparison clean.
Why does the result change so much with the price-rise assumption?
Because a battery saves a fixed number of kWh each year, and the value of those kWh grows with the tariff. At 5% annual escalation the 10-year value is much lower than at 12%. Neither number is a forecast. Run the low and high cases and ask whether you would still buy the battery if the low case is what happens.
Can I add a battery to an existing solar system?
Usually yes, but the inverter matters. A hybrid or battery-ready inverter can accept a battery directly; a standard string inverter may need a separate battery inverter or replacement. The calculator models the energy and cost, not the wiring compatibility. Confirm the equipment list with your installer before relying on the payback figure.