Battery Capacity Calculator
What a battery holds and how long it runs — amp-hours into watt-hours, shaved by depth of discharge and the inverter's tax.
Battery Capacity Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Continuous-load model — no compressor duty cycling
- Peukert derate for fast lead-acid discharge not applied
In short: A 200 Ah battery at 12 V holds 2,400 Wh — 2.4 kWh, whatever the label's amp-hours suggest. Draw half of it (a lead-acid's honest 50% depth of discharge) and 1,200 Wh remain; run a 150 W fridge through a 90% inverter and the clock reads about 7 hours 12 minutes. Every link in that chain — volts, chemistry, inverter — takes a cut, which is why amp-hours alone have never sized anything.
Formula
Wh = Ah × V · usable = Wh × DoD · runtime = usable × η ÷ W
Amp-hours are charge; watt-hours are energy, and only energy runs appliances. Depth of discharge is the chemistry's honesty tax — lead-acid ages fast below half empty, lithium rides to eighty or ninety. The inverter takes its percentage on the way out. What is left, divided by the load in watts, is the clock.
Worked Example
- Enter the pack's amp-hours and nominal voltage.
- Set the depth of discharge your chemistry tolerates.
- Read the runtime for your continuous load.
Defaults: 200 Ah at 12 V, 50% DoD, a 150 W load through a 90% inverter — 2.4 kWh of pack, 1.2 kWh usable, 7 hours 12 minutes of fridge. Switch the chemistry line to 80% and the same pack runs 11.5 hours.
Strengths & Limits Of This Model
Where this engine is strong
- The whole chain shown: Wh, DoD, inverter, clock
- Chemistry DoD presets stop the flat-cycling mistake
Where it stops
- Cycling losses and temperature drift live outside
Practical Use Cases
Off-grid fridges
the biggest continuous draw
RV and van builds
amp-hour labels into real hours
Backup planning
which load survives the outage
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Battery Capacity Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why multiply amp-hours by voltage at all?
Because amp-hours count charge, not energy, and loads consume power. A 100 Ah 12 V battery holds 1,200 Wh; the same amp-hours at 48 V hold 4,800 Wh. Comparing Ah across voltages is the classic shopping mistake.
What does depth of discharge really cost?
Lead-acid wants to keep half its charge — cycle it flat and the plates sulphate early. LiFePO₄ tolerates 80% or more daily. The battery's usable size is its label times the number your chemistry survives.
Where did my 7 hours go? The label said 200 Ah.
Half stayed in the battery by design, and the inverter ate another tenth of what left. 200 Ah becomes 2,400 Wh, then 1,200 usable, then 1,080 at the load — every conversion is on this page, in order.
What is the Peukert effect?
Lead-acid delivers fewer amp-hours the faster you pull them: a 200 Ah pack rated at the 20-hour rate might give 150 at a two-hour pace. Lithium barely cares. Treat high-rate lead-acid runtimes here as optimistic by a fifth or more.
How long does a 100 Ah battery run a fridge?
At 12 V that is 1,200 Wh; a 30 W chest fridge through a 90% inverter reads about 18 hours at 50% DoD. Compressor duty cycles stretch it further — this page assumes the load runs flat out.
Does temperature matter?
Yes — capacity drops in the cold, sharply below freezing, and lead-acid feels it more than lithium. Cold banks plus the inverter's tax can trim a quarter off the nameplate in winter.
Can I size a solar array from this?
The reverse direction is the solar page: energy needed divided by sun hours divided by system efficiency. This page tells you what the pack holds; the array page tells you what refills it.
Why does the inverter efficiency matter so much?
It is a percentage of everything you use. Ten points of inverter efficiency is ten percent more runtime for free — cheaper than any battery upgrade you will ever price.