Home Battery Payback Calculator

Enter battery quotes and your current and proposed tariffs. Compare grid-charged battery savings, usable daily delivery and year-end payback without solar.

Results

Highest modeled final balance: Quote B: 2,806.28 currency units
Tariff switch without battery: annual savings: -273.75 currency units
Tariff switch without battery: final balance: -4,106.25 currency units

Quote comparison against existing flat tariff
Quote	First-year delivery kWh/day	Grid charge kWh/day	First-year total savings	Battery-only savings	Battery-only payback	Battery plus tariff-switch payback	Battery-only final balance	Combined final balance
Quote A	10	11.11	567.53 currency units	841.28 currency units	8 years	13 years	4,937.76 currency units	831.51 currency units
Quote B	12	13.04	752.50 currency units	1,026.25 currency units	8 years	11 years	6,912.53 currency units	2,806.28 currency units

One daily cycle and a 365-day year are assumed. Remaining outside-window load is bought at the new peak rate. Standby is an additional grid load valued at the peak rate for all 24 hours, a conservative convention.
Battery-only savings isolate the battery from the tariff switch and include maintenance. Total savings compare both changes against the old tariff. No solar, financing, rate escalation, replacements or residual value are modeled.
Each payback is the first nonnegative year-end balance after battery cost. Battery-only payback removes tariff-switch savings from every year; combined payback includes them. Capacity degrades annually; supplied charging power and round-trip efficiency must include inverter losses. Equivalent delivery hours are an energy cap, not a simulation of instantaneous household demand.

How it works

Queensland’s guide identifies efficiency, use patterns, capacity loss and tariffs as payback factors. This battery-only model compares one daily cycle against an existing flat tariff. Quote capacity means usable AC energy after any backup reserve.

Delivered kWh/day = min(outside-window load, degraded usable capacity, charge kW × window hours × efficiency, output kW × available delivery hours) Grid charging input = delivered ÷ round-trip efficiency New daily bill = night load × cheap rate + uncovered day load × peak rate + charging input × cheap rate + standby kWh × peak rate + standing-charge difference

Annual savings are 365 times the old-minus-new daily bill, less maintenance. Capacity declines once per year. Standby uses watts × 24 ÷ 1000 and is billed at peak rate as a separate grid load. Tariff-switch savings exclude the battery; battery-only savings are the additional benefit after that switch. This is an energy balance, not a household interval simulation. Both balances begin at minus battery cost. Battery-only payback accumulates annual savings minus tariff-only savings; battery plus tariff-switch payback accumulates total annual savings. The tariff-switch-without-battery comparison has no battery cost. Each payback is the first nonnegative year-end balance within the selected period, with scale-aware floating-point tolerance at zero.

All prefilled prices, loads and performance assumptions are hypothetical examples. Use one currency throughout. Calculations keep full precision internally; displayed energy and money are rounded, and year-end payback is shown in whole years where applicable.

Worked examples

Losses reduce arbitrage

For a quote costing 6,000 with 10 usable kWh, 3 kW charge/output power, 90% efficiency and zero standby, enter 10 outside-window kWh/day, zero night load, old and peak rates of 0.30, cheap rate 0.10, four charging and four delivery hours, zero maintenance, zero degradation and no standing-charge difference. Delivery is 10 kWh and grid input is 11.111111 kWh/day. Annual savings are 689.44 and payback is year 9.

Short charging window

For the same inputs but a one-hour charging window, delivery is limited to 2.7 kWh/day. Grid charging is 3 kWh/day and annual savings fall to 186.15. There is no payback within 15 years.

Frequently asked questions

Can a new tariff make me worse off?

Yes. Uncovered daytime use is billed at the new peak rate. The tariff-only result can be negative even before battery cost. Separate battery-only payback prevents tariff-switch savings from being mistaken for a return on the battery.

Can I compare installer quotes?

Yes. Enter up to 20 quote rows with each system’s net cost, usable AC capacity, charge/output power, efficiency and standby draw.

Does this include solar?

No. Charging energy is purchased from the grid. Foregone solar export revenue would require a different baseline.

Is quoted capacity always usable?

No. Use usable energy after reserve and inverter effects. Match the capacity, charge power and efficiency definitions before comparing quotes.

Is payback guaranteed within warranty?

No. Compare the modeled payback with your actual warranty and lifetime assumptions. No replacement cost or backup-power value is assigned.

Sources

Queensland Government — Battery Buyers Guide, sections 5 and 7 (checked 2026-11-14)

Last reviewed 2026-11-14. The method is written out above so you can check our math.