Amp-Hour (Ah)

An amp-hour (Ah) is a unit of electrical charge that measures a battery's storage capacity in terms of current delivery over time. A 100Ah battery can theoretically deliver 1 amp for 100 hours, 10 amps for 10 hours, or 100 amps for 1 hour — though real-world performance varies with discharge rate.

Amp-hours tell you how long a battery can power a given load. If your system draws 5 amps continuously, a 100Ah battery (at 100% usable capacity) would last 20 hours. Accounting for real-world depth of discharge limits: at 50% DoD with lead-acid, that same battery provides 10 hours; at 80% DoD with LiFePO4, it provides 16 hours.

For lead-acid batteries, the rated Ah capacity is typically measured at the C/20 rate — a slow 20-hour discharge. At faster discharge rates, the effective capacity drops due to the Peukert effect. A battery rated at 100Ah at C/20 might only deliver 85Ah at C/5 (a 5-hour discharge). LiFePO4 batteries are far less affected by discharge rate, delivering close to their rated capacity even at high C-rates.

To convert amp-hours to watt-hours (a more useful unit for energy calculations), multiply by the system voltage: 100Ah × 12V = 1,200Wh = 1.2 kWh. This conversion makes it easier to compare battery capacity against your daily energy consumption, which is typically measured in kilowatt-hours.

When comparing batteries across different voltages, always convert to watt-hours or kilowatt-hours for an apples-to-apples comparison. A 200Ah 12V battery (2,400Wh) stores the same energy as a 50Ah 48V battery (2,400Wh).