What is a battery reserve capacity?

Reserve Capacity (RC) is the time in minutes a fully charged battery delivers 25 A at 80 °F (27 °C) before voltage drops to 10.5 V. Used as a standardised metric for car-starter batteries in particular. RC ≈ Ah × 2.4 (rough conversion). Useful for comparing similar-class batteries; not directly applicable to deep-cycle sizing — use Ah at the appropriate discharge rate instead.

What is a battery reserve capacity (alt phrasing)?

Same as above. The standard "what is a battery reserve capacity" search returns the SAE J537 RC definition: minutes of 25 A discharge before 10.5 V cut-off.

What is the capacity of a battery?

What is the capacity of a battery: it is the total electric charge the battery can deliver under specified conditions, measured in ampere-hours (Ah). Multiply by nominal voltage to get the energy in watt-hours (Wh). The unit of capacity of battery is the ampere-hour for charge or the watt-hour for energy.

What is a battery capacity?

A battery capacity is the same quantity — Ah at a stated discharge rate. A 100 Ah battery delivers 100 A for one hour, or 5 A for 20 hours (the C/20 rated discharge for lead-acid). Practical usable capacity is reduced by the depth-of-discharge limit and Peukert effect.

How to measure capacity of battery?

How to measure capacity of battery — two methods. Discharge test: fully charge, then discharge at the rated current (e.g. C/20 = 5 A for a 100 Ah battery) and measure time until cut-off voltage. Capacity = current × hours. Coulomb counter: battery monitor with shunt sensor integrates current over time. The discharge test is the gold standard.

How to measure capacity of a battery?

Same as above — "how to measure capacity of a battery" uses the same discharge-test or coulomb-counter procedure.

How do you measure battery capacity?

Two methods. Discharge test: fully charge, then discharge at the rated current (e.g. C/20 = 5 A for a 100 Ah battery), measure time until cut-off voltage. Capacity = current × hours. Industry standard but requires hours of testing. Coulomb counter: battery monitor with shunt sensor (Victron BMV, Renogy BT) integrates current over time. Live readout. The discharge test is the gold standard for capacity verification.

How to check capacity of battery?

How to check capacity of battery — for a quick health check use a load tester ($30–100) that applies a known current for 15 seconds and reads voltage drop; for an accurate Ah figure run a full discharge test as above.

How to check the capacity of a battery?

Identical procedure — "how to check the capacity of a battery" reduces to the same load-test or full-discharge-test methods.

How do you check battery capacity?

For lead-acid: a load tester (cheap, $30–100) applies a known current for 15 seconds and reads voltage drop — gives a rough state-of-health indication, not exact Ah. For lithium: BMS-equipped batteries report SoC and SoH directly via Bluetooth or display. For accurate capacity verification: full discharge test or specialised capacity tester ($150–500 for hobbyist, thousands for laboratory).

How to calculate capacity of battery?

How to calculate capacity of battery (and the equivalent "how to calculate capacity of a battery" / "how to calculate the capacity of a battery"): for sizing, Ah = (load_amps × runtime_hours) / DoD, then divide by Peukert factor and add safety margin. The calculator above does this automatically.

How to test capacity of battery?

A controlled discharge test: charge fully, then discharge at the rated current to the cut-off voltage and integrate current × time. For lead-acid run at C/20 (e.g. 5 A for a 100 Ah battery); for LFP run at C/10 or higher. Specialised testers automate this and produce a capacity report.

How do you calculate the capacity of a battery?

For sizing (you know the load, want to find Ah): Ah = (load_amps × runtime_hours) / DoD, then divide by Peukert factor and add safety margin. The calculator above does this automatically — pick "Sizing" mode, enter your load, runtime, voltage, DoD, and chemistry. For verifying an existing battery: discharge test as described above.

How do safety margins work in battery sizing?

Add 20–25% extra Ah beyond the calculated minimum for: ageing (battery loses ~20% capacity over its rated cycle life), temperature variation (capacity drops in cold), load growth (real-world loads usually creep upward), and uncertainty in load estimation. For critical applications (medical, telecom backup), use 50% margin and 80% DoD limit even for lithium.

What size battery do I need for an off-grid cabin?

Rough rules of thumb: 1 kWh per appliance per day average use. A weekend cabin with lights + fridge + small electronics typically needs 2–4 kWh/day = ~80–160 Ah at 24 V. Add 2 days autonomy for cloudy weather: 320 Ah. Pick LFP at 80% DoD with 20% margin: ~480 Ah at 24 V = ~12 kWh storage = ~$3500–5000 in batteries. Use the Sizing mode of the calculator above with your specific numbers.

How long do typical batteries last?

In years (depends on cycles per year): Lead-flooded at 50% DoD daily: ~3 years (500 cycles). AGM at 50% DoD daily: ~4 years. LFP at 80% DoD daily: ~8 years (3000 cycles). LFP at 50% DoD daily: ~15+ years. Cycle life is much higher at lower DoD — there is a strong trade-off between cost-per-cycle and capacity utilisation.

Tutorial + Tool · Battery sizing · Solar / UPS / Marine / RV

Battery capacity sizing

Battery capacity covers two related ideas: rated Ah (what the battery delivers) and Reserve Capacity (RC, the SAE J537 metric used for car batteries). Sizing calculator preset, RC vs CCA reference, C-rate explainer, six chemistries with Peukert-aware sizing. Reviewed by a licensed PE.

Use the battery capacity calculator

The Sizing tab is selected by default. Enter required runtime, load (in W or A), system voltage, and chemistry — the calculator returns the required Ah, the equivalent Wh / kWh, and how many standard 100 Ah modules in parallel you need.

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

Battery capacity — at a glance

Reserve Capacity (RC): Minutes a fully charged battery delivers 25 A at 27 °C before voltage drops to 10.5 V (SAE J537). Used for car-starter batteries; rough conversion: RC ≈ Ah × 2.4.
RC vs CCA: CCA = cold cranking amps (engine start, –18 °C). RC = sustained 25 A run-time. CCA is starting power; RC is endurance after the alternator fails.
C-rate definition: C-rate = current as a multiple of capacity. 1C = full discharge in 1 h; C/20 = standard 20 h lead-acid rated discharge; C/5 = 5 h LFP rating.
50% DoD rule: Lead-acid sized so daily discharge stays at 50% DoD or less: triples cycle count vs 80% DoD. LFP is sized at 80–90% DoD without cycle penalty.
Required Ah formula: Ah = (load_amps × runtime_hours) / DoD, then divide by Peukert factor and add 20–25% safety margin.
Capacity by chemistry (rated): AGM and flooded lead at C/20; LFP and Li-ion at C/5 or C/3. Reading capacity at the wrong rate gives 20–30% wrong number for lead-acid (Peukert).
How to measure capacity: Discharge test: charge fully, discharge at rated current to cut-off voltage, integrate I × t. Or use a coulomb-counter battery monitor (Victron BMV) for live SoC.
Unit of capacity: Ampere-hour (Ah) for charge; watt-hour (Wh = Ah × V) for energy. Smaller cells in milliampere-hours (mAh): 2 500 mAh = 2.5 Ah.

The capacity sizing formula

Eq. 01 — Required capacity for a given runtime SI · Inverse of Peukert formula

C = \frac{I \cdot t}{\mathrm{DoD} \cdot f_{Peukert}} \cdot (1 + m)

C: required rated capacity, Ah
I: load current (P / (V × η_inv) for AC), A
t: required runtime, h
DoD: depth-of-discharge fraction, —
f_Peukert: Peukert factor at this rate, —
m: safety margin (e.g. 0.25), —

Worked example: 500 W backup for 8 hours

UPS-style backup: 500 W AC continuous load, 8 hours, 24 V LFP, 80% DoD, 95% inverter efficiency, 25% safety margin.

Step	Calculation	Result
DC current at 24 V (with inverter loss)	500 / (24 × 0.95)	21.9 A
Linear Ah needed	(21.9 × 8) / 0.80	219 Ah
LFP Peukert factor	k = 1.05, near linear	~1.0 (negligible)
Add safety margin	219 × 1.25	274 Ah
Round to standard modules	3 × 100 Ah parallel	300 Ah
Total energy stored	300 × 24	7.2 kWh

How to size a battery, step by step

List the load to be supplied. Steady continuous power (W) plus any peaks. Use the average for sizing, not the peak — peaks need momentary capacity not Ah headroom.
Set the required autonomy time. How many hours / days the battery must last between recharge. Solar systems: 1–3 days backup. UPS: 5–30 minutes. Off-grid cabin: 2–5 days for cloudy weather.
Pick the battery voltage. Higher V = lower current = smaller wires. Common: 12 V (small / mobile), 24 V (mid), 48 V (off-grid solar / EV class).
Pick the chemistry and DoD. Lead 50%, AGM/Gel 80%, LFP 80–90%. Higher DoD = less capacity needed but fewer cycles. Cost per kWh × cycle life is the right comparison.
Compute required Ah. Ah = (load_W × hours) / (V × DoD × inverter_eff × Peukert_factor). Add 20–25% safety margin for ageing.
Round up to standard module sizes. Real batteries come in 50 / 100 / 200 / 300 Ah modules. Pick a count that gives ≥ required Ah, parallel them.

Variants and special cases

Reserve capacity (RC)

Time in minutes a fully charged battery delivers 25 A before voltage drops to 10.5 V. Standard for starter batteries; converted via RC ≈ Ah × 2.4. Use Ah at C/20 rate for deep-cycle sizing instead — RC is too rate-specific.

Related concepts on this site

Capacity quick reference

Topic / question	Quick answer
Capacity of battery formula	Sizing: Ah = (I × t) / DoD; energy: Wh = Ah × V. The capacity of battery formula is the same for every chemistry — chemistry only changes the DoD limit and Peukert exponent.
Calculate capacity of battery	The "calculate capacity of battery" workflow: Ah = (load_amps × runtime_hours) / DoD; multiply by 1.20–1.25 for ageing margin.
Cable capacity calculator / current carrying capacity of cables / current carrying capacity cable	For cable current capacity (NEC ampacity per Table 310.16, IEC 60364, BS 7671): use the wire-size calculator — that page is the dedicated cable capacity calculator.
Rating capacity	The rating capacity of a battery is the manufacturer-specified Ah at the standardised C-rate (typically C/20 for lead-acid, C/5 for LFP). Real capacity at a different rate is reduced by the Peukert effect.
Motor capacity	Motor "capacity" = rated horsepower or kW from the nameplate, the maximum continuous mechanical output. For motor branch sizing see the MCA / MOP calculator.
Floor loading capacity calculator	Structural floor loading is unrelated to battery capacity — for floor live-load capacity per IBC / ASCE 7-22 see the ASCE 7 reference page.
What factors affect carrying capacity	For batteries: chemistry, temperature, age (cycle count), discharge rate (Peukert), and DoD limit. For cables: ambient temperature, conductor count in the raceway, insulation type, and installation method (NEC 310.16 derating).
Unit of capacity of battery	The standard unit of capacity of battery is the ampere-hour (Ah) for charge and the watt-hour (Wh) for energy. Smaller cells are rated in milliampere-hours (mAh).

Frequently asked questions

What is a battery reserve capacity?: Reserve Capacity (RC) is the time in minutes a fully charged battery delivers 25 A at 80 °F (27 °C) before voltage drops to 10.5 V. Used as a standardised metric for car-starter batteries in particular. RC ≈ Ah × 2.4 (rough conversion). Useful for comparing similar-class batteries; not directly applicable to deep-cycle sizing — use Ah at the appropriate discharge rate instead.
What is a battery reserve capacity (alt phrasing)?: Same as above. The standard "what is a battery reserve capacity" search returns the SAE J537 RC definition: minutes of 25 A discharge before 10.5 V cut-off.
What is the capacity of a battery?: What is the capacity of a battery: it is the total electric charge the battery can deliver under specified conditions, measured in ampere-hours (Ah). Multiply by nominal voltage to get the energy in watt-hours (Wh). The unit of capacity of battery is the ampere-hour for charge or the watt-hour for energy.
What is a battery capacity?: A battery capacity is the same quantity — Ah at a stated discharge rate. A 100 Ah battery delivers 100 A for one hour, or 5 A for 20 hours (the C/20 rated discharge for lead-acid). Practical usable capacity is reduced by the depth-of-discharge limit and Peukert effect.
How to measure capacity of battery?: How to measure capacity of battery — two methods. Discharge test: fully charge, then discharge at the rated current (e.g. C/20 = 5 A for a 100 Ah battery) and measure time until cut-off voltage. Capacity = current × hours. Coulomb counter: battery monitor with shunt sensor integrates current over time. The discharge test is the gold standard.
How to measure capacity of a battery?: Same as above — "how to measure capacity of a battery" uses the same discharge-test or coulomb-counter procedure.
How do you measure battery capacity?: Two methods. Discharge test: fully charge, then discharge at the rated current (e.g. C/20 = 5 A for a 100 Ah battery), measure time until cut-off voltage. Capacity = current × hours. Industry standard but requires hours of testing. Coulomb counter: battery monitor with shunt sensor (Victron BMV, Renogy BT) integrates current over time. Live readout. The discharge test is the gold standard for capacity verification.
How to check capacity of battery?: How to check capacity of battery — for a quick health check use a load tester ($30–100) that applies a known current for 15 seconds and reads voltage drop; for an accurate Ah figure run a full discharge test as above.
How to check the capacity of a battery?: Identical procedure — "how to check the capacity of a battery" reduces to the same load-test or full-discharge-test methods.
How do you check battery capacity?: For lead-acid: a load tester (cheap, $30–100) applies a known current for 15 seconds and reads voltage drop — gives a rough state-of-health indication, not exact Ah. For lithium: BMS-equipped batteries report SoC and SoH directly via Bluetooth or display. For accurate capacity verification: full discharge test or specialised capacity tester ($150–500 for hobbyist, thousands for laboratory).
How to calculate capacity of battery?: How to calculate capacity of battery (and the equivalent "how to calculate capacity of a battery" / "how to calculate the capacity of a battery"): for sizing, Ah = (load_amps × runtime_hours) / DoD, then divide by Peukert factor and add safety margin. The calculator above does this automatically.
How to test capacity of battery?: A controlled discharge test: charge fully, then discharge at the rated current to the cut-off voltage and integrate current × time. For lead-acid run at C/20 (e.g. 5 A for a 100 Ah battery); for LFP run at C/10 or higher. Specialised testers automate this and produce a capacity report.
How do you calculate the capacity of a battery?: For sizing (you know the load, want to find Ah): Ah = (load_amps × runtime_hours) / DoD, then divide by Peukert factor and add safety margin. The calculator above does this automatically — pick "Sizing" mode, enter your load, runtime, voltage, DoD, and chemistry. For verifying an existing battery: discharge test as described above.
How do safety margins work in battery sizing?: Add 20–25% extra Ah beyond the calculated minimum for: ageing (battery loses ~20% capacity over its rated cycle life), temperature variation (capacity drops in cold), load growth (real-world loads usually creep upward), and uncertainty in load estimation. For critical applications (medical, telecom backup), use 50% margin and 80% DoD limit even for lithium.
What size battery do I need for an off-grid cabin?: Rough rules of thumb: 1 kWh per appliance per day average use. A weekend cabin with lights + fridge + small electronics typically needs 2–4 kWh/day = ~80–160 Ah at 24 V. Add 2 days autonomy for cloudy weather: 320 Ah. Pick LFP at 80% DoD with 20% margin: ~480 Ah at 24 V = ~12 kWh storage = ~$3500–5000 in batteries. Use the Sizing mode of the calculator above with your specific numbers.
How long do typical batteries last?: In years (depends on cycles per year): Lead-flooded at 50% DoD daily: ~3 years (500 cycles). AGM at 50% DoD daily: ~4 years. LFP at 80% DoD daily: ~8 years (3000 cycles). LFP at 50% DoD daily: ~15+ years. Cycle life is much higher at lower DoD — there is a strong trade-off between cost-per-cycle and capacity utilisation.

Sources and methodology

IEEE. IEEE Std 485 — Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications, 2020.
IEC. IEC 60896 — Stationary lead-acid batteries.
NEC 2023 — Article 480 Storage Batteries (installation requirements).