How do I size a battery bank from daily kWh?

Convert daily energy to watt-hours, divide by system voltage, then divide by usable depth of discharge to get required amp-hours.

Does temperature affect capacity?

Yes. Lead-acid loses roughly 20% of capacity at 0 °C, and lithium should not be charged below freezing without a heater.

Is Ah comparable between battery types?

Only at the same voltage, and only after accounting for usable depth of discharge. Watt-hours of usable energy is the fairer comparison.

Watt-hours to Amp-hours | Wh to Ah Battery Calculator

Conversion Formula

Ah = Wh ÷ 12 V

Divide watt-hours by the system voltage of 12 V to get amp-hours, the capacity figure printed on lead-acid and LiFePO4 battery labels.

This formula uses the system voltage currently set in the calculator (12 V). Change it above and this formula updates with it — the relationship is not a fixed unit pair.

Complete Guide to Watt-hours to Amp-hours Conversion

What is a Watt-hours?

A watt-hour (Wh) is an energy rating that already includes voltage, making it the only way to compare batteries across different system voltages and chemistries.

What is a Amp-hours?

An amp-hour (Ah) is a charge rating: one ampere delivered for one hour. Lead-acid, AGM and LiFePO4 batteries are labelled in amp-hours because that is how battery capacity has been specified since the earliest automotive systems.

How the Conversion Works

Energy equals charge multiplied by voltage, so watt-hours equal amp-hours times system voltage. A 100 Ah battery in a 12 V system stores 1,200 Wh; the same battery in a 24 V pair-configuration stores 2,400 Wh at the pack level.

Why This Conversion Matters

Off-grid, marine and campervan systems are designed around daily energy consumption in watt-hours, but batteries are sold in amp-hours. Sizing a system without converting between the two is how people end up with a bank that dies halfway through the night.

Scientific Context

Rated capacity depends on discharge rate. Lead-acid capacity is normally quoted at the 20-hour rate; drawing the same battery harder reduces delivered capacity substantially, an effect described by Peukert's law. Lithium chemistries are far less sensitive to discharge rate.

Step-by-Step Conversion Guide

Follow these steps to convert watt-hours to amp-hours manually:

Start with your value in watt-hours

Example: You want to convert 600 Wh

Apply the conversion formula

Use the formula: Ah = Wh ÷ 12 V

Calculate the result

600 Wh = 50 Ah

✓

Verify your answer

Use our calculator above to double-check your manual calculation.

Quick Mental Math Tip

Divide watt-hours by the system voltage of 12 V to get amp-hours, the capacity figure printed on lead-acid and LiFePO4 battery labels.

Common Conversions

Calculated at 12 V (system voltage). Change the value in the calculator above and this table recalculates.

Watt-hours to Amp-hours conversion table at 12 V
Watt-hours (Wh)	Amp-hours (Ah)	Amp-hours → Watt-hours
100	8.3333	100 Ah = 1200 Wh
250	20.8333	250 Ah = 3000 Wh
600	50	600 Ah = 7200 Wh
1200	100	1200 Ah = 14400 Wh
1800	150	1800 Ah = 21600 Wh
2400	200	2400 Ah = 28800 Wh
3600	300	3600 Ah = 43200 Wh
4800	400	4800 Ah = 57600 Wh

When You'll Need This Conversion

Campervan and marine systems

Size a leisure battery bank against your daily watt-hour consumption.

Off-grid solar storage

Match panel output, daily loads and battery amp-hours at your chosen system voltage.

UPS and backup power

Convert backup battery amp-hours into runtime at a known load.

Solar system design

Decide between 12 V, 24 V and 48 V architectures based on current and cable sizing.

Pro Tips for Accurate Conversions

Always state the system voltage

An amp-hour figure without voltage cannot be turned into energy.

Design on usable, not nameplate, capacity

Lead-acid: 50%. LiFePO4: 80–90%.

Go 48 V above about 3 kW

Lower current means thinner cable, less loss and safer connections.

Common Mistakes to Avoid

Comparing 12 V and 24 V amp-hour ratings

The same Ah number represents twice the energy at 24 V.

Convert both to watt-hours before comparing.

Planning around 100% discharge

Deep discharge destroys lead-acid batteries within months.

Size the bank so daily use stays within the safe depth of discharge.

Forgetting inverter losses

DC-to-AC conversion costs 10–15% of stored energy.

Divide AC load energy by 0.85 when sizing the battery.

History & Background

Amp-hour ratings date to the lead-acid battery of 1859 and survived because automotive and marine systems have used them for over a century.

Key Milestones

1859

Gaston Planté invents the lead-acid battery, still the reference for Ah ratings.

1897

Peukert publishes his law relating discharge rate to delivered capacity.

1996

LiFePO4 chemistry is described, later dominating leisure and off-grid storage.

2020s

Drop-in lithium replacements push watt-hour comparisons into the mainstream.

Global Usage Today

Amp-hours remain the labelling standard for automotive, marine and off-grid batteries worldwide, while watt-hours dominate portable electronics and EVs.