Watt Hour Calculator – Convert mWh, Wh, kWh, mAh, Ah

We built this calculator because we needed it ourselves. Every day, our battery engineering team compares different packs – voltages, capacities, energy, power. And every day, we saw the same mistakes repeated.

Three problems we kept running into:

1. mAh ≠ energy – A 20,000 mAh power bank at 3.7 V stores 74 Wh, while a 20,000 mAh e‑bike battery at 36 V stores 720 Wh. Same mAh number, nearly 10× (9.73×) the actual energy. We needed a tool that shows the real number instantly.
2. Wh vs. W – time disappears – Many assume 100 Wh can deliver 100 W "forever". It can't. 100 Wh = 100 W for 1 hour, or 10 W for 10 hours. Forgetting time leads to wrong chargers and poor range estimates.
3. Cross‑type conversions are awkward – "How many watts can a 500 Wh pack sustain over 2 hours?" requires dividing by time. "How many Ah is 100 Wh at 48 V?" requires voltage. Most converters just give an error. Ours asks for the missing value and explains why.

What this calculator does (and how we use it):

• It's a pure formula tool – no black magic, no fake "AI calibration".
• It shows the full chain: mWh, kWh, J, MJ, Ws – so we can quickly cross-check specs.
• It asks for the missing parameter (Voltage or Time) when you try cross‑type conversions, and shows exactly why it's needed.
• We added our own MOTAWILL battery as a clickable example (73V, 45Ah → 3285Wh). That's a real pack from our swappable battery system. Click it – the calculator auto‑fills the numbers. That's exactly how we verify specs internally before signing off on an order.

Why you can trust this tool:

• Same inputs always give the same outputs – reproducible, verifiable math.
• All formulas are public and auditable: Wh = Ah × V, W = Wh ÷ h.
• We built it for our own daily work, stress-tested it against real battery datasheets, then handed it to the web team. What you're using now is the same version we rely on.

–– MOTAWILL Battery Engineering Team
We use this tool before finalizing every battery order. It's simple, honest, and it saves us from the kind of unit-confusion mistakes that cost real money.