Amp-Hours Measure Charge
An amp-hour is current multiplied by time. In an idealized example, a 100 Ah battery could supply 5 A for 20 hours because 5 A × 20 h = 100 Ah.
This does not mean every 100 Ah battery stores the same energy. Voltage must also be considered.
Watt-Hours Measure Energy
Nominal Wh = nominal V × Ah
A nominal 12 V, 100 Ah battery contains about 1,200 Wh. A nominal 24 V, 100 Ah battery contains about 2,400 Wh. Both have the same amp-hour rating, but the 24 V battery has approximately twice the nominal energy.
Runtime Estimate
Runtime hours = usable Wh × system efficiency ÷ load W
If the 12 V, 100 Ah battery has 80% usable capacity and the system is 90% efficient:
Usable load energy = 1200 × 0.80 × 0.90 = 864 Wh
A constant 200 W load would have an estimated runtime of 864 ÷ 200 = 4.32 hours.
Why Nameplate Energy Is Not Fully Available
- Minimum allowed state of charge can reserve part of the capacity.
- Inverters and converters consume energy and have varying efficiency.
- Temperature, battery age, and cell balance affect available capacity.
- High discharge rates can reduce usable capacity, especially for lead-acid batteries.
- Battery-management systems may disconnect before nominal zero capacity.
Series and Parallel Batteries
Ideal identical batteries in series add voltage while amp-hour capacity remains the same. Identical batteries in parallel keep the same voltage while amp-hour capacities add. In both cases, total nominal watt-hours add.
Battery banks require compatible cells, proper protection, suitable conductors, manufacturer-approved configurations, and safe charging controls.