What is the battery capacity?

What is the battery capacity: it is the total electric charge a battery can deliver, measured in ampere-hours (Ah). A 100 Ah battery can deliver 1 A for 100 hours, 5 A for 20 hours, or 100 A for 1 hour (in theory; Peukert losses reduce real-world capacity at high rates). Energy capacity in Wh = Ah × nominal V.

What is a battery capacity?

A battery capacity is the same quantity — the rated ampere-hour figure on the label, given at a specified discharge rate (typically C/20 for lead-acid, C/5 for LFP). Nominal V × Ah gives energy in Wh.

What is battery capacity?

Battery capacity is the total electric charge a battery can deliver, measured in ampere-hours (Ah). A 100 Ah battery can deliver 1 A for 100 hours, 5 A for 20 hours, or 100 A for 1 hour (in theory; Peukert losses reduce real-world capacity at high rates). Energy capacity in Wh = Ah × nominal V — used to compare batteries across voltages and to size for energy-based loads.

How do you measure battery capacity?

How do you measure battery capacity — by a controlled discharge test: fully charge, then draw a known current until the cut-off voltage, and measure elapsed time. Capacity = current × time. Faster alternatives: use a battery monitor (Victron BMV, Renogy BT) for live SoC and a coulomb-counter integration; or a standalone capacity tester for ~$150–500.

How to measure a battery capacity?

How to measure a battery capacity — same procedure as above. For a more practical workflow run the discharge test at the rated discharge rate (C/20 for lead-acid, C/5 for LFP), and accept that ageing and temperature can shift the measured Ah by 10–20 % from the nameplate.

How to measure battery capacity?

Discharge test: fully charge, then draw a known current until cutoff voltage, measure elapsed time. Capacity = current × hours. Industry standard but slow. For lithium with a BMS, the BMS reports SoC and SoH (state-of-health) directly. For lead-acid, a battery tester applying a known load gives state-of-health estimate quickly.

How to determine battery capacity?

From the label (rated capacity at the rated discharge rate). Or by discharge test as above. For an unknown / unmarked battery: weigh it (specific energy of chemistry × weight ≈ Wh, then divide by V for Ah), or test at C/20 and time discharge to cutoff voltage.

How to check battery capacity?

Three options. (1) Battery monitor / coulometer (Victron BMV ~$200) — live SoC and Ah counted in/out. (2) Standalone capacity tester (~$150-500) — performs full discharge and reports Ah. (3) Manual load test — fully charge, apply known load, time to cutoff voltage. Method 1 is ongoing monitoring; methods 2 and 3 are point-in-time verification.

What is reserve capacity?

Reserve Capacity (RC) is the time in minutes a fully charged battery delivers 25 A at 80°F before voltage drops to 10.5 V. Standard for car-starter batteries. Conversion: Ah ≈ RC / 2.4. RC is for comparing similar batteries; for deep-cycle sizing use Ah at appropriate discharge rate instead.

Calculator · Battery · Ah / Wh / DoD / SoC / Reserve Capacity

Battery capacity

What batteries capacity actually means: ampere-hours, watt-hours, reserve capacity, state of charge, depth of discharge — and how each is measured. The page covers the calculation of battery capacity (sizing formula and embedded calculator), the formula for battery capacity, the standard battery capacity equation, and the methods for determining battery capacity used by engineers and battery monitor BMS firmware. Embedded sizing calculator below.

Use the battery capacity calculator

CALC.008 Battery · Runtime + Sizing + Energy · 6 chemistries · Peukert

Chemistry[Chem]

Battery voltage[V]

Depth of discharge[DoD]

Runtime

— h

—

Linear runtime (no Peukert): —
Peukert-corrected runtime: —
Usable energy at DoD: — Wh
Total stored energy: — Wh
C-rate (load / capacity): —
Estimated cycles to 80%: —
Estimated weight: — kg
Standard 100 Ah modules: —

FORMULA · t = (C × DoD) / I × Peukert SOURCE · IEEE 485 · IEC 60896 · PEUKERT 1897

The battery capacity formula

Eq. 01 — Capacity definitions SI · Standard battery engineering

E_{Wh} = C_{Ah} \cdot V \qquad C_{usable} = C_{rated} \cdot \mathrm{DoD}

C_Ah: capacity in ampere-hours, Ah
V: nominal voltage, V
E_Wh: energy capacity, Wh
DoD: depth-of-discharge fraction, —
C_usable: usable capacity per cycle, Ah

Worked example

A 100 Ah, 12 V LFP battery at 80% DoD limit:

Quantity	Result
Total energy stored	100 × 12 = 1200 Wh = 1.2 kWh
Usable energy per cycle	1200 × 0.80 = 960 Wh = 0.96 kWh
Usable Ah per cycle	100 × 0.80 = 80 Ah
Approximate weight (LFP, 110 Wh/kg)	~11 kg

Battery capacity quick reference

Topic	Quick answer
Battery capacity calculation / calculation of battery capacity	Sizing form: required Ah = (load_amps × runtime_hours) / DoD; energy form: Wh = Ah × V. The full battery capacity calculation workflow is on the capacity sizing page.
Formula for battery capacity / battery capacity equation	Two equations: E_Wh = C_Ah × V and C_usable = C_rated × DoD. The formula for battery capacity is the same across chemistries; only the DoD limit and Peukert exponent change.
Determining battery capacity	From the label (rated Ah at a specified C-rate) or by discharge test. For unknown / unmarked batteries use a coulomb-counter battery monitor or a dedicated capacity tester.
Car battery capacity	Typical car battery capacity (12 V starter): 50–100 Ah, BCI Group 24/35/65/75/78. Reserve Capacity (RC) of 90–180 minutes at 25 A discharge.
Lithium battery capacity / li ion battery capacity	Common lithium battery capacity sizes: 18650 (3.7 V × 3 Ah ≈ 11 Wh), 21700 (3.7 V × 5 Ah ≈ 18 Wh), 4680 (3.7 V × 25 Ah ≈ 92 Wh). LFP prismatic cells: 100–304 Ah at 3.2 V, stacked into 12/24/48 V banks for solar / EV.
Battery capacity unit of measure	Charge: ampere-hour (Ah), or milliampere-hour (mAh) for small cells. Energy: watt-hour (Wh) or kilowatt-hour (kWh). Reserve Capacity: minutes at 25 A.

Frequently asked questions

What is the battery capacity?: What is the battery capacity: it is the total electric charge a battery can deliver, measured in ampere-hours (Ah). A 100 Ah battery can deliver 1 A for 100 hours, 5 A for 20 hours, or 100 A for 1 hour (in theory; Peukert losses reduce real-world capacity at high rates). Energy capacity in Wh = Ah × nominal V.
What is a battery capacity?: A battery capacity is the same quantity — the rated ampere-hour figure on the label, given at a specified discharge rate (typically C/20 for lead-acid, C/5 for LFP). Nominal V × Ah gives energy in Wh.
What is battery capacity?: Battery capacity is the total electric charge a battery can deliver, measured in ampere-hours (Ah). A 100 Ah battery can deliver 1 A for 100 hours, 5 A for 20 hours, or 100 A for 1 hour (in theory; Peukert losses reduce real-world capacity at high rates). Energy capacity in Wh = Ah × nominal V — used to compare batteries across voltages and to size for energy-based loads.
How do you measure battery capacity?: How do you measure battery capacity — by a controlled discharge test: fully charge, then draw a known current until the cut-off voltage, and measure elapsed time. Capacity = current × time. Faster alternatives: use a battery monitor (Victron BMV, Renogy BT) for live SoC and a coulomb-counter integration; or a standalone capacity tester for ~$150–500.
How to measure a battery capacity?: How to measure a battery capacity — same procedure as above. For a more practical workflow run the discharge test at the rated discharge rate (C/20 for lead-acid, C/5 for LFP), and accept that ageing and temperature can shift the measured Ah by 10–20 % from the nameplate.
How to measure battery capacity?: Discharge test: fully charge, then draw a known current until cutoff voltage, measure elapsed time. Capacity = current × hours. Industry standard but slow. For lithium with a BMS, the BMS reports SoC and SoH (state-of-health) directly. For lead-acid, a battery tester applying a known load gives state-of-health estimate quickly.
How to determine battery capacity?: From the label (rated capacity at the rated discharge rate). Or by discharge test as above. For an unknown / unmarked battery: weigh it (specific energy of chemistry × weight ≈ Wh, then divide by V for Ah), or test at C/20 and time discharge to cutoff voltage.
How to check battery capacity?: Three options. (1) Battery monitor / coulometer (Victron BMV ~$200) — live SoC and Ah counted in/out. (2) Standalone capacity tester (~$150-500) — performs full discharge and reports Ah. (3) Manual load test — fully charge, apply known load, time to cutoff voltage. Method 1 is ongoing monitoring; methods 2 and 3 are point-in-time verification.
What is reserve capacity?: Reserve Capacity (RC) is the time in minutes a fully charged battery delivers 25 A at 80°F before voltage drops to 10.5 V. Standard for car-starter batteries. Conversion: Ah ≈ RC / 2.4. RC is for comparing similar batteries; for deep-cycle sizing use Ah at appropriate discharge rate instead.

Sources and methodology

IEC 60896 — Stationary lead-acid batteries.
IEEE Std 485 — Sizing lead-acid batteries.
BCI (Battery Council International) — RC standard.