Charge Is the Label, Energy Is the System
Buying a battery by amp hours alone is like buying fuel by the gallon without asking what grade — the volume is real, but the energy in it depends on the chemistry. For batteries the chemistry is encoded in voltage: a lead-acid cell sits near 2 V, a lithium cell near 3.7 V, and a pack is cells in series. Two packs can both say 100 Ah and differ by a factor of four in total energy, purely from how many cells are stacked. Multiply by the pack voltage and the labels become comparable — this is the conversion every solar, marine, RV, and vehicle system runs before anything else.
watt hours = amp hours × volts
Wh = Ah × V
Exact, by the definition of the watt-hour — use the battery's nominal voltage.
Battery Packs You Will Actually Meet
| amp hours | volts | watt hours | What it powers |
|---|---|---|---|
| 4 Ah | 12 V | 48 Wh | Small power tool battery — drill, impact driver. |
| 20 Ah | 48 V | 960 Wh | E-bike pack — roughly a 1 kWh ride. |
| 50 Ah | 12 V | 600 Wh | Mid-size marine/AGM battery. |
| 100 Ah | 12 V | 1,200 Wh | Standard deep-cycle — the RV/solar workhorse. |
| 25 Ah | 48 V | 1,200 Wh | E-bike long-range pack — same energy as the 100 Ah 12 V battery. |
The last two rows are the conversion's point in one glance: 100 Ah at 12 V and 25 Ah at 48 V both hold 1,200 Wh — identical energy, radically different labels. That is why an e-bike with a "small" 25 Ah pack and an RV with a "big" 100 Ah battery carry the same energy. It is also why 48 V is winning in solar and e-bike systems: the same watt hours at four times the voltage means a quarter of the current, which means thinner wire and less loss — the voltage drop question, not a battery question at all.
Engineering Context
The reverse direction — a known Wh rating converted back to the Ah label — is watt hours to amp hours, the power-bank comparison page. The energy this page computes connects directly to the energy hub: watt hours to kWh puts a pack in electricity-bill terms, kWh to joules in scientific terms. Sizing the solar panel that refills the pack, or the inverter that drains it, both start from watts — see watts to amps and amps to watts. The wire between them is the voltage drop page. All roads lead back to the electric hub.
More: Wh to Ah · Wh to kWh · Voltage Drop · Electric Hub
Related Unit Converters
Frequently Asked Questions
How do I convert amp hours to watt hours?
Multiply by the battery's voltage: watt hours (Wh) = amp hours (Ah) × volts (V). A 100 Ah 12 V deep-cycle battery holds 100 × 12 = 1,200 Wh — enough to run a 100 W device for 12 hours before fully discharging.
Why is watt hours the right way to compare batteries?
Because watt hours is energy — the thing a load actually consumes — while amp hours is only charge, which means nothing until you multiply by the voltage it flows at. A 20 Ah battery at 12 V holds 240 Wh; at 48 V it holds 960 Wh, four times the energy under the same amp-hour label. Wh normalizes that: compare any two packs by Wh and the voltage noise disappears.
How long will a battery run my appliances?
Divide the battery's watt hours by the load's watts: runtime (h) = Wh ÷ W. A 1,200 Wh pack running a 100 W refrigerator-compressor style load lasts 12 hours. For lead-acid, derate to 50% of rated capacity (depth-of-discharge limit); for lithium you can use 80–90%. A 100 Ah lead-acid at 12 V is therefore a practical 600 Wh, not 1,200 Wh.