Amp Hours vs Watt Hours: Why Wh Is the Number That Matters

Watt-hours (Wh) measure the energy a battery stores. Amp-hours (Ah) measure charge, and they only mean something at a stated voltage. The link is Wh = volts × Ah. That's why a 5 Ah tool pack holds 90 Wh at 18 V but 280 Wh at 56 V, more than three times as much. When you're comparing batteries across brands, tools or device types, compare watt-hours.

This guide explains the difference. For the conversions themselves, use the amp-hours to kWh converter or the mAh to Wh converter.

Amp hours vs watt hours: what's the difference?

Amp-hours (Ah) measure electric charge: current × time. A 5 Ah battery can, in principle, deliver 5 amps for one hour, or 1 amp for five hours.

Watt-hours (Wh) measure energy: power × time. A 280 Wh battery can deliver 280 watts for one hour, or 28 watts for ten hours.

Energy is what actually runs a mower, spins a drill or keeps a fridge cold, and you only get energy from charge once you know the voltage:

Wh = V × Ah

Ah = Wh ÷ V

kWh = Wh ÷ 1,000

The water analogy works here: amps are flow rate, volts are pressure. A count of how much water flowed (amp-hours) tells you nothing about the work it can do until you know the pressure behind it.

Why is Ah alone misleading?

Two packs, both labeled 5 Ah:

• 18 V nominal (a "20V max" pack): 18 × 5 = 90 Wh
• 56 V pack: 56 × 5 = 280 Wh

Same amp-hours, about 3.1 times the energy. In a mower or blower, that's roughly three times the runtime at the same power draw.

So:

• Within one platform (same voltage), Ah is a fine shortcut: a 6 Ah pack holds more than a 4 Ah pack.
• Across platforms, Ah is meaningless on its own. Convert to Wh first.

What does "20V max" mean vs nominal voltage?

Lithium-ion cells reach about 4.2 V when fully charged and run at about 3.6 V nominal under load. Tool makers often market the peak:

"20V max" and "18V" packs are the same cell arrangement with different labels. Wh figures are normally calculated at nominal voltage, so use 18 V for a 20V max pack and 36 V for a 40V max pack.

Brands with higher-voltage platforms (56 V, 60V max, 80 V and others) don't all label the same way. The Wh rating printed on the pack is the number to trust. Use the label voltage only when there's no Wh rating.

How much energy is in common tool batteries?

This is just arithmetic: nominal voltage (or label voltage where noted) × Ah. Check the label on your pack.

Not every Ah size exists on every platform. The table shows the math, not a catalog.

Runtime follows from Wh:

Runtime (hours) ≈ Wh ÷ watts drawn

A 90 Wh pack in a tool drawing 300 W lasts about 90 ÷ 300 = 0.3 h, or 18 minutes, in theory. Real runtime is a bit shorter: the pack's electronics cut off before it's completely empty, and capacity drops when it's hot or cold. See how long a lawn mower battery lasts and what CFM means on a leaf blower for runtime worked out on real yard tools.

What does it cost to fill a battery?

Wh tells you that too. WattSimple uses $0.1648/kWh, the US average residential rate, and we assume chargers are about 85% efficient. That's our assumption; real chargers vary.

Cost to recharge = Wh ÷ 0.85 ÷ 1,000 × rate

Use your own rate in the electricity cost calculator. For mowers specifically, see how to charge a lawn mower battery.

How do power stations list capacity?

Portable power stations list capacity in Wh because that's what you size them by. Some also list Ah and the battery's internal voltage. The Wh = V × Ah relationship still holds.

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The Jackery Explorer 2000 v2, for example, lists 45.6 Ah at 44.8 V: 45.6 × 44.8 ≈ 2,042 Wh, which matches its rated capacity.

Note that 45.6 Ah is the figure at the pack's internal ~45 V. It isn't the same thing as 45.6 Ah from a 12 V battery, which would be only about 550 Wh. Running AC loads through the inverter also costs some energy, so plan on getting a bit less than the rated Wh at the outlet. See portable power station explained and how to size a portable power station. The amp-hours to kWh converter also estimates runtime for a given load.

The same applies to 12 V deep-cycle and RV batteries. A 12 V LiFePO4 battery is 12.8 V nominal (four cells at 3.2 V), so a 100 Ah unit holds about 1,280 Wh. For home-scale batteries, the home battery sizing calculator works in kWh from the start.

Power banks: why mAh at 3.7 V, not 5 V?

Phone power banks are rated in milliamp-hours (mAh) at the internal cell voltage, usually 3.6 to 3.7 V. They are not rated at the 5 V USB output.

Wh = mAh × cell voltage ÷ 1,000

Because the bank steps 3.7 V up to 5 V, the charge it can deliver at 5 V is smaller than the label suggests. A 10,000 mAh bank has at most about 10,000 × 3.7 ÷ 5 = 7,400 mAh at 5 V before conversion losses, and real output is lower still. That's normal, not a defect. Apart from conversion losses, the watt-hours are the same on both sides; only the voltage and the amp-hour figure change.

If you work out a bank's Wh using 5 V, you overstate it by about a third. To convert any bank, use the mAh to Wh converter.

Can you fly with these batteries?

The FAA's limits are written in watt-hours, which is one more reason to know the Wh of what you carry. Under the FAA's PackSafe guidance for lithium-ion batteries:

• Spare batteries and power banks go in carry-on bags only, never in checked bags. If your carry-on is gate-checked, take them out and keep them with you.
• Up to 100 Wh per battery, with no FAA quantity limit for personal use.
• 101 to 160 Wh, with airline approval: up to two spare batteries per person.
• Protect the terminals from short circuits: retail packaging, tape over the contacts, or a case.
• Airlines can be stricter, and many are, especially on power banks. Check with yours before you fly.

Applying that to the tables above: most phone power banks and smaller 12V max and 20V max tool packs come in under 100 Wh. A 20V max 6 Ah pack (108 Wh) or 8 Ah pack (144 Wh) would fall in the 101 to 160 Wh tier that needs approval. Most mower-class packs (a 56 V, 5 Ah pack is 280 Wh) and nearly all power stations are above 160 Wh, past where the passenger allowance stops. Batteries installed in devices follow separate FAA rules, and damaged or recalled batteries aren't allowed on board at all.

The bottom line

Wh = V × Ah. Watt-hours measure energy, and amp-hours only mean something at a stated voltage, so compare batteries across platforms by Wh. Convert "max" voltage to nominal first (20V max is 18 V, 40V max is 36 V), and trust the Wh printed on the pack. Work out power bank mAh at 3.7 V, not 5 V. Runtime is roughly Wh ÷ watts, and the cost to recharge is roughly Wh ÷ charger efficiency × your rate.

For the conversions themselves, use amp-hours to kWh or mAh to Wh. For yard-tool decisions, see electric vs gas lawn mower.