Watts, volts and the number on the label that is not quite true

The four quantities, in the order they matter

Milliamp-hours on their own are close to meaningless for comparison, because they measure charge rather than energy. A 5,000 mAh pack at 7.4 volts holds twice the energy of a 5,000 mAh pack at 3.7 volts. Both listings say 5,000 mAh in the same size font.

The label voltage trap

Here is the part that catches people out, and it is not the buyer's fault. In the United States, lithium tool packs are commonly labeled with their maximum charge voltage rather than their working voltage. DEWALT states the distinction on its own packaging: "Maximum initial battery voltage (measured without a workload) is 20 volts. Nominal voltage is 18."

Both numbers are true. The 20 volts is what the pack reads the instant it comes off the charger with nothing connected; the 18 volts is what it actually runs at. But if you do the watt-hour arithmetic with the label figure, every result is about 11% too high.

The DEWALT 5.0 Ah row is worth a second look. At the label voltage it computes to exactly 100 Wh, which is the FAA's carry-on threshold. At nominal it is 90 Wh, comfortably clear. Same pack, two different answers to a question that matters at an airport. Flying with heated clothing batteries covers the rules properly.

Watts: the number nobody publishes

Energy tells you what is in the tank. Watts tells you how fast the garment empties it. Runtime is the first divided by the second, and it is the second that heated apparel makers do not print.

There are two honest ways around the gap, and both use figures the makers do publish voluntarily.

Method one: the pack's output ceiling

Some packs publish an output current. ororo, for instance, publishes 2.5 amps for its main apparel packs. Multiply by the voltage and you have the maximum wattage that pack can deliver: 7.4 × 2.5 = 18.5 watts.

The garment cannot draw more than the pack will give, so this sets a floor on runtime. 38.5 Wh ÷ 18.5 W = 2 hours 05 minutes. That is the shortest possible runtime from that pack, whatever setting you use and whatever the listing claims.

Method two: invert the advertised hours

If a maker advertises a runtime and publishes a capacity, the runtime states the draw. Milwaukee claims up to 8 hours from an M12 2.0 Ah pack: 21.6 Wh ÷ 8 h = 2.7 watts. ororo claims up to 10 hours from its 37.6 Wh PD 5K: 37.6 ÷ 10 = 3.8 watts.

Those are the draws the marketing claims assume. They are small — a few watts is a nightlight, not a heater — and they are almost certainly the lowest setting. Which is the entire point: the advertised number describes a trickle, and you switched the garment on because you were cold.

How zones spend the budget

A zone is one heating element. Zones are wired in parallel, so each one draws its share and the total draw is roughly the sum. Turning on six zones instead of three does not make each element hotter — it means the pack is being emptied roughly twice as fast at the same per-element output, or each element runs cooler at the same total draw.

Which of those two happens depends on how the garment's controller is designed, and no maker publishes that either. What you can say with confidence is the conclusion: more zones on the same pack is not more heat. It is the same energy covering more area. Heating zones explained works through which zones are worth the share.

Why the arithmetic gives you a ceiling, not a promise

Everything above is a best case. Three things push real runtime below it, and they all push the same direction:

• Cold reduces usable capacity. A lithium pack in freezing conditions delivers less than its rating — which is inconvenient, given what the pack is for.
• Age reduces capacity. Every charge cycle takes a little. A pack two winters old holds meaningfully less than its label.
• Wind and movement increase demand. The controller is holding a target temperature, so more heat loss means more draw. The manufacturer Dr. Warm, writing about its own products, describes runtime claims as a roof rather than a promise, and puts the wind and lab-to-field gaps in the tens of percent.

We publish the ceiling because it is the honest, checkable number. Anything below it varies with your day. Anything above it is not physically available.