Calculate home battery payback using solar export credits or off-peak grid charging, efficiency, backup reserve, capacity fade and local rates. Compare savings and NPV.

Battery energy you can actually use

Deliverable AC energy per full cycle

Energy delivered to household loads before the reserve setting. Use usable AC output, not nameplate DC capacity; apply inverter/window limits to this estimate.

Output energy held for backup

If your capacity estimate already excludes the backup reserve, use 0 here to avoid counting it twice.

AC-to-AC round-trip efficiency

Delivered electricity divided by charging electricity. Includes charging and discharging losses.

Available charging energy per active day

For solar, use surplus after direct household use. For grid charging, use energy available in the cheap window.

Household energy to replace in the peak window

Only consumption the battery can actually serve. Do not use total daily consumption.

Days per year this cycle is available

One equivalent cycle per active day. Reflect seasonal surplus, weekends and downtime.

Charging source and utility rates

Where does charging energy come from?

Avoided peak import price per kWh

All-in marginal import price. Fixed account charges are not avoided.

Solar export credit forgone per kWh

Charging uses energy that would otherwise be exported. Enter 0 only if that surplus has no export value.

Annual change in both energy rates

Both import and charging/export rates change by the same percentage in this simplified model.

Incremental battery project costs

Battery installed project cost

Battery, inverter, labor and electrical work. Existing solar cost is outside this comparison.

Confirmed battery incentive

Default is 0. Enter only an amount you qualify for; capped at project cost.

Annual incremental maintenance

Annual deliverable-capacity fade

A scenario assumption, not a manufacturer warranty curve.

Comparison horizon without replacement

Choose an appropriate useful-life horizon. No replacement or salvage value is modeled.

Annual nominal discount rate

Future savings are discounted; utility price growth is also nominal.

Does storage pay for itself?

Battery has $753.62 negative net present value over 15 years

First-year net bill savings

$776.67

Avoided imports minus charging/export cost and maintenance

Net installed battery cost

$10,000.00

Project cost minus confirmed incentive

First-year simple payback

12.9 years

Flat first-year savings; ignores fade and changing prices

Modeled sustained payback

Year 13

Undiscounted savings exceed cost and remain ahead through the horizon

Net present value

-$753.62

Discounted savings less initial cost; positive favors storage

Peak-rate operating threshold

$0.076/kWh

Covers first-year charging/export cost and maintenance, excluding installation

First-year energy and bill accounting

Delivered energy per active day

8.00 kWh

Charging energy per active day

8.89 kWh

More energy is needed to charge because of round-trip losses

Annual avoided peak imports

$960.00

Annual export revenue forgone

$133.33

Backup reserve scenario: 20%

A larger backup reserve may reduce bill savings. This tool assigns no dollar value to outage protection and does not estimate backup runtime.

Year-by-year incremental savings

Battery-only savings, charging opportunity cost and cumulative cash flow
Year	Delivered kWh	Avoided import cost	Charging/export cost	Net savings	Cumulative net	Discounted net
1	2,400	$960.00	$133.33	$776.67	-$9,223.33	-$9,245.95
2	2,352	$959.62	$133.28	$776.34	-$8,447.00	-$8,514.18
3	2,305	$959.23	$133.23	$776.01	-$7,670.99	-$7,804.03
4	2,259	$958.85	$133.17	$775.68	-$6,895.32	-$7,114.85
5	2,214	$958.46	$133.12	$775.34	-$6,119.97	-$6,446.03
6	2,169	$958.08	$133.07	$775.01	-$5,344.96	-$5,796.97
7	2,126	$957.70	$133.01	$774.68	-$4,570.27	-$5,167.08
8	2,084	$957.32	$132.96	$774.35	-$3,795.92	-$4,555.80
9	2,042	$956.93	$132.91	$774.03	-$3,021.89	-$3,962.57
10	2,001	$956.55	$132.85	$773.70	-$2,248.20	-$3,386.87
11	1,961	$956.17	$132.80	$773.37	-$1,474.83	-$2,828.17
12	1,922	$955.78	$132.75	$773.04	-$701.79	-$2,285.98
13	1,883	$955.40	$132.69	$772.71	$70.91	-$1,759.80
14	1,846	$955.02	$132.64	$772.38	$843.29	-$1,249.17
15	1,809	$954.64	$132.59	$772.05	$1,615.34	-$753.62

How this calculator works

Daily delivered energy is the least of available AC output after reserve and capacity fade, household peak-window demand, and available charging energy multiplied by AC-to-AC efficiency. Required charging energy is delivered energy divided by efficiency. Each active day represents one equivalent cycle. Grid mode values charging at the off-peak price; solar mode values it at the export credit forgone.

Annual net savings subtract charging opportunity cost and incremental maintenance from avoided peak imports. Both energy rates change at the same entered annual rate. Capacity fades geometrically, while maintenance remains flat. Net present value discounts annual savings at year-end and subtracts the net project cost once. Simple payback uses first-year savings; modeled payback includes the scenario’s fade and rate growth.

This is a battery-only daily-energy approximation for an existing solar system or permitted grid charging. It excludes new solar installation costs, hourly dispatch, demand charges, tariff tiers, battery exports, virtual power plant payments, standby losses not included in efficiency, financing, replacement and outage value. Actual tariffs and inverter power limits require a more detailed design.

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Home battery investment

Solar storage savings depend on the price spread, not free electricity

Adding a battery changes when energy is used. The value is the imports you avoid minus what it costs to charge, including solar export revenue you give up. Compare that incremental value with the battery project cost.

How to use this tool

Estimate deliverable peak energy
Use usable AC output and actual demand the inverter can serve in the expensive window.
Value charging correctly
Use the solar export credit or the all-in off-peak grid rate, with round-trip losses.
Use realistic cycling days
Solar surplus and demand vary through the year. One daily cycle is not guaranteed.
Include cost and capacity fade
Compare a confirmed quote with savings over an appropriate useful-life horizon.

Worked example: solar storage with export opportunity cost

The example can deliver 8 kWh per active day after a 20% reserve. At 90% efficiency it requires 8.89 kWh of charging energy. Over 300 days, avoided imports at $0.40/kWh are worth $960, while forgone exports at $0.05/kWh cost $133.33.

After $50 maintenance, first-year savings are $776.67. A $10,000 battery therefore has 12.9-year simple payback before capacity fade, rate changes and discounting. This is illustrative rather than a product quote.

Why one-to-one net metering changes the comparison

If surplus solar earns the same price as the electricity you avoid importing, storage losses can make bill savings negative. Backup protection may still be valuable to you, but it is a separate benefit this bill-payback calculation does not monetize.

Sources and further reading

Frequently asked questions

Is solar battery charging free?

Surplus energy may earn an export credit. Using it to charge a battery gives up that credit, and round-trip losses mean charging takes more energy than the battery later delivers.

Can this compare a battery without solar?

Yes. Choose grid charging and enter the off-peak price. Confirm that your utility and equipment permit the intended charging operation.

Does this include outage protection?

No dollar value or runtime estimate is assigned to backup power. The reserve setting only changes the energy available for bill savings.

Solar & Home Battery Payback Calculator