Watt-hours are not printed on every label—you derive them from amp-hours and voltage. This guide shows how to calculate watt-hours from amp-hours with the nominal volts your pack actually runs at.
Benefits
- Formula: Wh = Ah × nominal voltage.
- Works for 12 V AGM, 48 V solar, and 3.7 V lithium cells.
- Reverse check: Ah = Wh ÷ V when sizing from an energy target.
How it works
- Read amp-hours from the battery nameplate or datasheet.
- Multiply by nominal system voltage—not peak charge voltage alone.
- Read watt-hours; divide by 1,000 for kWh on large banks.
FAQ
How do I calculate watt-hours from amp-hours?
Wh = Ah × V. Example: 280 Ah LiFePO4 at 12.8 V nominal → 280 × 12.8 = 3,584 Wh (3.584 kWh). Use the voltage the load sees during discharge, typically nominal pack voltage.
Should I use fully charged or nominal voltage?
Use nominal or average discharge voltage for energy estimates—12.8 V for LiFePO4 12 V packs, 12 V for lead-acid, 48 V for a 48 V nominal bus. Full-charge voltage overstates Wh slightly; nominal is standard for sizing comparisons.
What if my spec only lists mAh?
Convert to Ah first: mAh ÷ 1,000. Then Wh = Ah × V. A 10,000 mAh 11.1 V (3S) pack → 10 Ah × 11.1 V = 111 Wh. Same math, smaller numbers.
Technical specifications
- Wh = Ah × nominal_V.
- kWh = (Ah × V) ÷ 1,000.
- Usable Wh ≈ rated Wh × depth_of_discharge.
- Related: ah-to-wh-converter, wh-to-ah, battery-bank-size.
Multiply two numbers you already have
Every deep-cycle label lists amp-hours. Your system architecture sets voltage—12 V RV, 24 V boat, 48 V off-grid. Watt-hours are the product. No lookup table required: 100 Ah × 12 V is always 1,200 Wh. The calculation is linear, so double Ah or double V and Wh doubles.
Pick the voltage that matches discharge
A 12 V lead-acid bank spends most of its useful cycle near 12.0–12.6 V under load; LiFePO4 12 V packs nominal at 12.8 V. Using 14.4 V full-charge float inflates Wh versus what the inverter actually draws down. For conservative runtime, some planners use minimum discharge voltage—document your assumption when comparing quotes.
From Wh to actionable planning numbers
Once Wh is calculated, divide by load watts for hours of runtime, or by $/Wh for pack cost. A 3,584 Wh bank feeding 400 W average load yields ~8.96 h before empty—before inverter loss and BMS cutoff. Calculate Wh from Ah first; every downstream battery spreadsheet starts there.