Ah, Wh, mAh — the battery capacity units nobody explains
Amp-hours count electrons. Watt-hours count energy. To compare batteries fairly, always multiply by voltage.
Phone batteries are labelled in mAh. Laptop batteries in Wh. Car batteries in Ah. EV batteries in kWh. Every capacity spec picks a different unit and none are directly comparable without knowing the nominal voltage.
The relationship
Wh = Ah × V > mAh × V (in V) / 1000 = Wh > 1 kWh = 1,000 Wh = 1,000 Ah at 1 V (or 100 Ah at 10 V, or 10 Ah at 100 V)
Chemistry-specific voltages
Alkaline: 1.5 V/cell nominal. Lead-acid: 2.0 V/cell (12 V battery = 6 cells). NiMH: 1.2 V/cell. Lithium-ion (LCO, NMC, NCA): 3.7 V/cell nominal. LiFePO₄: 3.2 V/cell. Understanding the voltage lets you back out the true energy content from any Ah rating.
Worked example — comparing two 'big' batteries
A 100 Ah lead-acid car battery: 100 Ah × 12 V = 1,200 Wh = 1.2 kWh. A 100 Ah LiFePO₄ home-battery module: 100 Ah × 12.8 V = 1,280 Wh. But usable capacity differs — lead-acid discharge to 50% for cycle life, so real usable = 600 Wh. LiFePO₄ discharge to 90%: usable = 1,150 Wh. Same Ah rating, 2× usable energy.
Phone vs laptop vs EV
iPhone 15 Pro: 3,274 mAh × 3.87 V = 12.67 Wh. MacBook Pro 14: 70 Wh (= 5,527 mAh at 12.67 V or 18,919 mAh at 3.7 V — depends on which voltage is quoted). Tesla Model 3 Long Range: 82 kWh = 82,000 Wh. Ratios: laptop is 5.5× a phone, EV is 1,170× a laptop.
C-rating and real capacity
Battery capacity is voltage-and-current-dependent. A '100 Ah at C/20' battery may deliver only 60 Ah at C/1 (one-hour discharge). Lead-acid loses capacity dramatically at high discharge rates (Peukert's law). Lithium is much flatter — 5% loss from C/2 to 2C is typical.
The five-second recap
Wh = Ah × V. Always multiply by voltage before comparing capacities. Chemistry sets the voltage; capacity in mAh alone is meaningless.
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FAQ
Why does my 5,000 mAh power bank not fully charge my 3,000 mAh phone?+
The power bank is 5,000 mAh at 3.7 V = 18.5 Wh. Charging through USB requires stepping up to 5 V, which loses 15–20%. Net delivered energy: ~15 Wh. Your phone needs 3,000 × 3.87 / 1000 = 11.6 Wh — so you should get one full charge plus 30% left over. If you get less, the power bank is under-rated.
Are 18650 cells still standard?+
For legacy laptops and vaping, yes. Modern EVs use 2170 (Tesla Model 3/Y) or 4680 cells. Cell format is a mechanical choice; capacity is chemistry-dependent.
How many kWh in a car battery?+
Lead-acid starter: ~1 kWh. Hybrid: 1.5–2 kWh. Plug-in hybrid: 10–20 kWh. Full EV: 50–150 kWh.
This article was written by The Turbo Unit Converter engineering desk (Battery & storage) and last reviewed on 2026-06-29 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.