Can I use the full capacity?

Lead-acid batteries should not be discharged below 50% if you want reasonable cycle life, so a 100 Ah lead-acid battery gives about 600 usable Wh. LiFePO4 tolerates 80–90% depth of discharge.

How long will a 100 Ah battery run a fridge?

A 12 V compressor fridge drawing 45 W averages roughly 540 Wh per day, so a 100 Ah lead-acid battery covers about one day within safe discharge limits.

Why do off-grid systems use 24 V or 48 V?

Higher voltage carries the same power at lower current, which means thinner cables and lower losses. This is why large systems move to 48 V.

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

Conversion Formula

Wh = Ah × 12 V

Multiply amp-hours by the system voltage to get watt-hours. This calculator assumes a 12 V system, standard for leisure, marine and automotive batteries. Use 24 V or 48 V for larger off-grid systems.

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 Amp-hours to Watt-hours Conversion

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.

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.

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 amp-hours to watt-hours manually:

Start with your value in amp-hours

Example: You want to convert 20 Ah

Apply the conversion formula

Use the formula: Wh = Ah × 12 V

Calculate the result

20 Ah = 240 Wh

✓

Verify your answer

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

Quick Mental Math Tip

Multiply amp-hours by the system voltage to get watt-hours. This calculator assumes a 12 V system, standard for leisure, marine and automotive batteries. Use 24 V or 48 V for larger off-grid systems.

Common Conversions

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

Amp-hours to Watt-hours conversion table at 12 V
Amp-hours (Ah)	Watt-hours (Wh)	Watt-hours → Amp-hours
7	84	7 Wh = 0.5833 Ah
12	144	12 Wh = 1 Ah
20	240	20 Wh = 1.6667 Ah
50	600	50 Wh = 4.1667 Ah
75	900	75 Wh = 6.25 Ah
100	1200	100 Wh = 8.3333 Ah
150	1800	150 Wh = 12.5 Ah
200	2400	200 Wh = 16.6667 Ah
300	3600	300 Wh = 25 Ah
400	4800	400 Wh = 33.3333 Ah

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.