RuntimeCalculator

Battery Capacity Calculator

Tell it the watts and the hours. It returns the nameplate you need.

Advanced

Recommended default. Leaving a little capacity unused is easier on cycle life.

Real usable capacity still depends on the maker's BMS, age, and the temperature of the cells.

Template watts are examples. CPAP rows are power estimates only, not a medical clearance. Trolling-motor rows are estimates too: follow local water rules.

Editorial estimate

1,200 Wh nameplate, 100 Ah at 12 V

Rated amp-hours to buy

49.38

592.59 Wh rated

480 Wh usable

Suggested chemistry: LiFePO4. Recommended default. Leaving a little capacity unused is easier on cycle life.

60 W × 8 h = 480 Wh usable. Rated watt-hours = usable / (0.90 × 1.00 × 0.90) = 592.59 Wh, or 49.38 Ah at 12 V.

Round up to the next published size. This is a minimum nameplate, not a lab specification.

Depth of discharge
90%
Efficiency
90%
Temperature factor
1.00 at 77°F (25°C). Editorial estimate, not a lab curve.

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Affiliate recommendations

These are starting points, not lab-tested picks. Check current specs and prices on the merchant site.

  • 12 V LiFePO4 battery (example search)

    Example search in the same voltage class. Match amp-hours on the product page. Not hands-on tested. Estimated need: 49.38 Ah and 592.59 Wh rated.

    Not hands-on tested

    Check price on Amazon

  • Renogy batteries (example brand catalog)

    Example brand catalog for deep-cycle batteries. Compare listed amp-hours with your estimate. Not hands-on tested. Estimated need: 49.38 Ah and 592.59 Wh rated.

    Not hands-on tested

    View on Renogy

  • Dakota Lithium batteries (example brand catalog)

    Example LiFePO4 catalog. Check voltage, amp-hours, and continuous current. Not hands-on tested. Estimated need: 49.38 Ah and 592.59 Wh rated.

    Not hands-on tested

    View on Dakota Lithium

How it works

Wh rated = Ah × V
(Skip that step if you type watt-hours.)

usable Wh = Wh rated × depth of discharge × temperature factor × inverter efficiency
hours = usable Wh / average watts

average watts = sum of (watts × duty) for each enabled row
duty is the share of time the device is actually drawing power (0 to 1)

DC amps, no inverter:
usable Ah = Ah × depth of discharge × temperature factor
hours = usable Ah / amps

Size a battery:
usable Wh needed = watts × target hours
rated Wh = usable Wh / (depth of discharge × temperature factor × efficiency)
rated Ah = rated Wh / volts

Conservative planning hours = hours × 0.85
That 0.85 factor is an editorial margin, not a second physical model.

This page opens in sizing mode. You enter a load and a target runtime. The result is rated watt-hours and amp-hours, not hours of runtime. The default case is 60 W for 8 hours at 12 V, LiFePO4, 90% depth of discharge, 90% inverter, 77°F. That needs about 592.59 Wh rated, or about 49.38 Ah.

The steps are the runtime formula run backwards. Watts times hours is the usable energy you must have left after losses. Divide by depth of discharge, the temperature factor, and inverter efficiency to get the nameplate watt-hours. Divide by voltage to get amp-hours. A colder night lowers the temperature factor, so the same job asks for a larger nameplate. AGM, flooded, and gel start at 50% depth of discharge, which makes the required nameplate larger than the LiFePO4 default. That is intentional.

Round up when you buy. The figure is the minimum nameplate that satisfies the formula, not a suggestion to buy a battery that is already tired. Cables, age, and a load that runs harder than your average all eat into it. Match the voltage. Check the continuous-current rating against the load. This page does not draw a series-parallel wiring diagram. How you connect physical batteries is a separate decision. Ask an electrician if the bank feeds a panel.

The cards under the result are example searches and brand catalogs near this kind of battery. They are starting points. They are not hands-on tests. They do not show a price, because this site does not store prices. Open the merchant page and read the current spec. Until an affiliate program is active for this site, those links are ordinary merchant URLs.

Worked examples

60 W for 8 hours on LiFePO4

12 V, 90% depth of discharge, 90% inverter, 77°F. This is the default result on this page.

  1. Usable energy needed = 60 W × 8 h = 480 Wh.
  2. Rated watt-hours = 480 / (0.90 × 1.00 × 0.90) = 592.59 Wh.
  3. Rated amp-hours = 592.59 / 12 = 49.38 Ah.
  4. Shop the next published size above that amp-hour number, at 12 V.

72 W office average for 4 hours on AGM

The 72 W figure is the UPS office template average. AGM at 50% depth of discharge and a 90% inverter, 12 V, 77°F.

  1. Usable energy needed = 72 × 4 = 288 Wh.
  2. Rated watt-hours = 288 / (0.50 × 0.90) = 640 Wh.
  3. Rated amp-hours = 640 / 12 = 53.33 Ah.

Questions

How do you calculate the battery capacity you need?

Multiply the load in watts by the hours you want. That product is usable watt-hours. Divide by depth of discharge, the temperature factor, and inverter efficiency to get rated watt-hours. Divide by voltage to get amp-hours.

What battery does a 60 W load for 8 hours need?

At 12 V, LiFePO4, 90% depth of discharge, 90% inverter efficiency, and 77°F, the estimate is about 592.59 Wh rated, or about 49.38 Ah. Buy a published size above that number, at the same voltage, with enough continuous current. This is an editorial estimate.

Should I round the amp-hour result up?

Yes, for shopping. The figure is the minimum nameplate that satisfies the formula. Cables, age, and a colder night all eat into it. Pick a larger published size than the raw number.

Why do the recommendation cards not show a price?

Prices change, and this site does not store them. The cards are example searches and brand catalogs. Open the merchant page for the current spec and price. Nothing here is a hands-on test.

Is a power station watt-hour rating the same as amp-hours times volts?

You can compare them as energy, with care. A power-station label is the maker's watt-hour figure. This calculator can treat that figure as rated watt-hours and use 100% depth of discharge, or you can lower the slider if you think usable energy is below the label. Makers do not all measure the label the same way.

Does this page design a series or parallel bank?

No. It returns one rated amp-hour number at the voltage you enter. How you wire physical batteries in series or parallel is a separate decision. Match the system voltage, and ask an electrician if the bank feeds a panel.

These figures are editorial estimates. The formula, chemistry defaults, and temperature table are written out on themethodology page.

Updated October 5, 2026