How it’s calculated
An amp-hour rating says how much charge a battery delivers: 100 Ah supplies 5 A for 20 hours. Runtime is capacity divided by current, reduced to the share you can actually use (depth of discharge). A load in watts is converted to battery current with I = P ÷ (V × efficiency).
Real batteries deliver less charge at high current. Peukert’s empirical law captures this with an exponent k and the rating period H (20 h for most lead-acid batteries). With k = 1 there is no correction.
Example: a 12 V, 100 Ah lead-acid battery runs a 60 W load through a 90% inverter. I = 60 ÷ (12 × 0.9) = 5.56 A. Ideal runtime to 50% DoD = 100 ÷ 5.56 × 0.5 = 9.0 h. With k = 1.2 at the 20 h rate: t = 20 × (100 ÷ (5.56 × 20))^1.2 × 0.5 = 8.8 h.
Temperature, age and state of health also reduce capacity; cold lead-acid batteries can lose a third or more.
Frequently asked questions
How long will a 100 Ah battery last?
Divide Ah by the current: at 5 A, 20 hours in theory. Limit the depth of discharge (about 50% for lead-acid) and it is 10 hours.
What is a Peukert exponent?
An empirical constant describing how capacity shrinks at higher discharge currents. 1.0 is ideal; lead-acid batteries are typically 1.1–1.3. Battery datasheets or monitor manuals often list it.
Do batteries in series increase runtime?
Series connection raises voltage but keeps Ah the same. For a fixed wattage load, the higher voltage means less current, so runtime in watt-hours terms is the same as the total energy stored.
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Sources
- Electric Circuits I (Kuphaldt), 11.3 Battery Ratings — Workforce LibreTexts
- University Physics Vol. 2, 10.1 Electromotive Force — OpenStax
Formulas are taken from the free public references above. Results are provided “as is” for informational and educational purposes only. See our disclaimer.
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