mAh, Wh, Lumens and Lux: The Honest Units Behind Batteries and Bulbs
By the VoltConvert team · June 12, 2026
Two corners of everyday electrical life run almost entirely on misunderstood units: batteries (mAh and Wh) and lighting (lumens and lux). Marketing departments thrive in that confusion. Here's the decoder.
mAh: a number that means nothing without voltage
Milliamp-hours count charge, not energy — and energy is what charges your phone. The honest unit is the watt-hour: Wh = mAh × V ÷ 1000. Power-bank mAh figures use the internal cell voltage of 3.7 V, so a "20,000 mAh" bank stores 74 Wh. Convert any battery with the mAh to Wh calculator, and reverse it with Wh to mAh when a spec sheet speaks the other dialect.
Why your power bank delivers fewer charges than advertised
Stored energy isn't delivered energy. Boosting 3.7 V cells up to 5 V (or 9–20 V for fast charge) burns 20–35% in conversion and heat. So the 74 Wh bank delivers perhaps 50–60 Wh — against a phone battery of 15–20 Wh, that's a realistic 3–4 charges from a "20,000 mAh" promise that arithmetic alone would call 5+. This is physics, not fraud — but a bank promising 5+ full charges of a modern phone at a miracle price has skipped physics class.
The airline rule travelers actually face
Aviation rules limit lithium batteries in watt-hours: under most carriers' rules, up to 100 Wh travels freely in carry-on, 100–160 Wh needs airline approval, and spare batteries are barred from checked bags outright. Since power banks advertise mAh, the gate-counter math is exactly the conversion above — 20,000 mAh × 3.7 V = 74 Wh, fine; a 30,000 mAh monster at 111 Wh may need approval. Check your number before the airport does it for you, and confirm your specific airline's current policy.
Lumens: buy light, not watts
For a century "60-watt bulb" described brightness by accident — watts measured the cost, and brightness tagged along. LEDs broke the link: today's bulbs produce 80–120 lumens per watt, so an 800 lm LED sips 8–10 W where the old incandescent burned 60. Lumens are the light; watts are just the bill. Rough anchors: 450 lm replaces an old 40 W bulb, 800 lm a 60 W, 1,600 lm a 100 W.
Lux: whether the room is actually bright enough
Lumens describe the bulb; lux describes the surface — one lux is one lumen spread over a square meter. Guidance used across US and UK lighting practice puts living spaces around 100–200 lux, kitchens and offices at 300–500, and detail work higher. A single 1,600 lm fixture over a 12 m² home office averages ~130 lux — numerically explaining why one ceiling light never feels like enough for desk work. The lumens to lux calculator does the division (in m² or ft²) and the page lists target levels by room.
Four units, one theme: the advertised number is rarely the number that matters. Convert mAh to Wh before buying or flying, and think in lumens-then-lux before buying light. The calculators make each conversion a ten-second habit.