Reference · Electrical · IEC 60086 · BCI Group · Solar PV

Battery Size

Standard battery sizes — from CR2032 button cells to industrial 8D lead-acid and Tesla Megapack — defined by IEC 60086 (consumer cells), ANSI C18, and BCI Group specifications (automotive). This page covers the full battery sizes chart, the formula linking Wh ↔ Ah ↔ V, the differences between Li-ion 18650, 21700, and 4680 formats, sizing for solar PV, and a calculator that converts any required runtime into the right battery size.

Battery size calculator

The embedded calculator converts between load wattage, runtime, voltage, and required Ah — with depth-of-discharge and Peukert corrections for the chemistry you pick. Useful for the battery size calculator solar use case: enter the daily kWh demand and battery voltage to get the required nameplate capacity.

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

Figure 1 — Battery size comparison: CR2032 button cell to Tesla Megapack utility battery

Battery sizing formulas

Eq. 01 — Required capacity from load and runtime SI · IEEE Std 485 (lead-acid sizing)

C_{Ah} = \frac{P_{load} \cdot t_{hours}}{V_{batt} \cdot \text{DoD}}

C_Ah: nameplate battery capacity, Ah
P_load: average load power, W
t: required runtime, h
V_batt: battery nominal voltage, V
DoD: usable depth of discharge (lead 0.5; LFP 0.9), —

Eq. 02 — Energy capacity (Wh) SI · IEC 60086

E_{Wh} = C_{Ah} \cdot V_{nominal}

E_Wh: energy capacity, Wh
V_nominal: nominal cell or pack voltage, V

Mid-discharge voltage is what gives the standard Wh figure; under heavy load the actual delivered Wh is less because the terminal voltage drops with the C-rate. Manufacturer datasheets show the discharge curve at 0.2 C, 0.5 C, 1 C, 2 C — sizing should use the curve nearest the actual application.

Eq. 03 — Peukert correction (lead-acid only) SI · Peukert 1897 · IEEE Std 485

C_{actual} = C_{nominal} \cdot \left( \frac{t_{actual}}{t_{rated}} \right)^{1-1/k}

k: Peukert exponent (1.1–1.3 for AGM, 1.05 for LFP), —
t_rated: rated discharge time (typically 20 h or 10 h), h

Lead-acid loses capacity at higher discharge rates. A 100 Ah battery rated at 20 h (5 A draw for 20 h) delivers only ~ 60 Ah at 1 h (60 A draw). Lithium iron phosphate is essentially flat — Peukert exponent ~ 1.05, so the rated and high-rate capacities are nearly identical.

Standards: IEC 60086, ANSI C18, BCI Group

Three families of standards govern battery sizes worldwide:

IEC 60086 — Primary batteries (the international consumer-cell standard). Defines AAA, AA, C, D, 9 V, button cells (CR2032, LR44, etc.) and their dimensions, voltages, and capacity test methods. Adopted as ANSI C18.1 in the US.
BCI (Battery Council International) Group sizes — Standardises automotive and deep-cycle lead-acid box dimensions and terminal positions. Groups 24, 27, 31, 4D, 8D are the most common. Used in North America; Europe uses DIN 72311 sizes (e.g. L1, L2, L3).
IEC 60896 — Stationary lead-acid batteries (UPS, telecom). Defines float and cycle-service rating methods.
UL 1973 / IEC 62619 — Lithium-ion battery safety standards for stationary and motive (BESS) applications.
SAE J537 / J240 — Automotive battery test methods (CCA cold-cranking amps, RC reserve capacity).

Battery sizes chart — full reference

Consumer alkaline / Li-ion / button cells (IEC 60086)

Name	IEC code	Voltage	Dimensions (mm)	Capacity	Use
AAA	LR03 (alkaline) / FR03 (Li)	1.5 V	10.5 × 44.5	1 200 mAh	Small electronics, remotes
AA	LR6 / FR6	1.5 V	14.5 × 50.5	2 700 mAh	Most-common consumer cell
C	LR14	1.5 V	26.2 × 50	8 000 mAh	Flashlights, toys
D	LR20	1.5 V	34.2 × 61.5	18 000 mAh	Large flashlights, boom boxes
9 V	6LR61	9 V	26.5 × 17.5 × 48.5	600 mAh	Smoke alarms, multimeters
CR2032	CR2032	3 V	20 × 3.2	220 mAh	Watches, motherboards (RTC)
CR123A	CR17345	3 V	17 × 34	1 500 mAh	Cameras, smoke detectors, flashlights
CR2025	CR2025	3 V	20 × 2.5	170 mAh	Key fobs, slim watches
LR44 / AG13	LR44	1.5 V	11.6 × 5.4	110 mAh	Calculators, hearing aids

Cylindrical lithium-ion cells

Format	Diameter × Length	Voltage	Typical capacity	Use
14500	14 × 50 mm	3.7 V	700–900 mAh	Small flashlights (AA-replacement Li-ion)
18650	18 × 65 mm	3.7 V	2 500–3 500 mAh	Laptops, power tools, vapes, early Teslas
21700	21 × 70 mm	3.7 V	4 000–5 000 mAh	Tesla Model 3/Y, modern power tools, e-bikes
26650	26 × 65 mm	3.7 V	5 000–6 500 mAh	High-drain flashlights, RC vehicles
32700	32 × 70 mm	3.2 V (LFP)	6 000 mAh	LFP packs, low-rate energy storage
4680	46 × 80 mm	3.7 V	~ 26 000 mAh	Tesla 2022+ (5× energy of 18650)

Automotive lead-acid (BCI Group sizes)

Group	Dimensions L × W × H (mm)	Voltage	Capacity	Use
Group 24	260 × 173 × 225	12 V	70–85 Ah	Mid-size cars, light RVs
Group 27	307 × 173 × 225	12 V	85–100 Ah	Marine, larger RVs
Group 31	330 × 172 × 240	12 V	100–125 Ah	Heavy trucks, commercial RVs
Group 34	260 × 173 × 200	12 V	55–70 Ah	Compact cars (GM, Chrysler)
Group 35	230 × 175 × 225	12 V	55–70 Ah	Many imports (Toyota, Honda, Subaru)
Group 65	300 × 190 × 195	12 V	75–85 Ah	Ford full-size trucks
Group 75	230 × 180 × 195	12 V	55–60 Ah	GM compact
Group 78	260 × 180 × 195	12 V	75–85 Ah	GM mid-size
Group 4D	527 × 222 × 250	12 V	175–200 Ah	Industrial, large diesels
Group 8D	530 × 280 × 250	12 V	230–270 Ah	Heavy industrial, trucking

Stationary / UPS / telecom

Class	Voltage	Capacity	Use
SLA UPS battery	12 V	7 / 9 / 18 / 28 Ah	Desktop and small server-room UPS
Sealed lead-acid telecom string	48 V (4 × 12 V)	100–200 Ah/cell	Telecom equipment back-up
Industrial flooded lead-acid	2 V cells, strung in series	100–3 000 Ah	Substation control DC, large UPS
Rack LFP module	48 V	50 / 100 / 200 Ah	Rack-mount BESS, datacenter back-up

EV / utility-scale

System	Voltage	Capacity	Notes
Tesla Model 3 SR pack	355 V nominal	~ 50 kWh	4 416 cells × 2 170 (NMC)
Tesla Model S Plaid pack	400 V nominal	100 kWh	7 920 × 18650 cells
Hyundai Ioniq 5 pack	800 V	77.4 kWh	Pouch cells, 350 kW DC charging
Tesla Powerwall 3	~ 380 V DC pack	13.5 kWh	Residential storage
Tesla Megapack II XL	~ 800 V DC	3.9 MWh	20-ft container; utility BESS
Moss Landing Phase III	—	3 GWh	World\'s largest BESS as of 2024

How to pick the right battery size, step by step

Compute the load (W) and required runtime (h). For a 60 W LED lamp running 4 hours: energy = 60 × 4 = 240 Wh. For a 5 kW solar inverter at 50 % average load for 8 night-time hours: 5 000 × 0.5 × 8 = 20 000 Wh = 20 kWh. The energy figure drives the Wh capacity of the battery.
Pick a battery voltage that matches the load. 1.5 V (alkaline AA/AAA) for small electronics. 3.7 V (Li-ion 18650 / 21700) for laptops and power tools. 12 V (lead-acid Group 24/27/31) for cars, RVs, marine. 24 V or 48 V (battery banks) for solar / off-grid. 350–800 V (EV packs) for electric vehicles.
Convert energy to required Ah. Ah = Wh / V. A 240 Wh load on a 12 V battery needs 240 / 12 = 20 Ah. For a 20 kWh solar bank on 48 V: 20 000 / 48 = 417 Ah. Round up — never specify a battery at exactly the required Ah, leave 20–30 % margin.
Apply depth-of-discharge (DoD) and Peukert correction. Lead-acid: usable capacity is only 50 % of nameplate (DoD ≤ 50 % for long life). Lithium iron phosphate (LiFePO₄): 80–90 % usable. So a 100 Ah lead-acid battery delivers ~ 50 Ah usable; a 100 Ah LFP delivers ~ 90 Ah. At high discharge rates (C-rate > 0.5), lead-acid Peukert effect reduces capacity another 20–40 %.
Pick the standard battery size that matches. Look up the standard size that meets your Ah / V requirement: BCI Group 24/27/31 (12 V automotive), Group 4D/8D (12 V industrial), 100 Ah / 200 Ah / 400 Ah LFP modules (rack-mount solar), or stack of 18650 / 21700 / 4680 cells (custom packs).
Confirm physical fit. Battery sizes are standardised by external dimensions — Group 24 is 260 × 173 × 225 mm, Group 31 is 330 × 172 × 240 mm. Verify the tray, cable terminals, and ventilation match before ordering. Lithium banks come in 19" rack-mount form factors for direct integration with inverter cabinets.

Worked example: solar off-grid cabin

Off-grid cabin with 5 kWh average daily energy demand. Owner wants 3 days autonomy on cloudy weather. Pick LFP (lithium iron phosphate) at 48 V system voltage.

Step	Calculation	Result
Daily energy	—	5 kWh/day
Required usable energy (3 days autonomy)	5 × 3	15 kWh usable
LFP DoD	—	90 %
Nameplate energy	15 / 0.9	16.7 kWh
System voltage	—	48 V
Required Ah at 48 V	16 700 / 48	~ 350 Ah at 48 V
Pick standard size	Two 200 Ah LFP modules in parallel	400 Ah, 19.2 kWh nameplate
If lead-acid instead (DoD 50 %)	15 / 0.5 / 48	~ 625 Ah at 48 V
Lead-acid cost vs LFP cost	roughly equal up-front; LFP 4–6× longer life	LFP wins lifecycle

The 350 Ah requirement maps neatly to two 200 Ah modules (a common rack-mount LFP size). Lead-acid would require 625 Ah of much heavier batteries plus more frequent replacement, making LFP\'s up-front premium pay back within 2–3 years.

Battery chemistry comparison

Chemistry	Energy density (Wh/kg)	Cycle life	DoD	Cost ($/kWh)	Best for
Lead-acid (flooded)	30–40	300–500	50 %	100–150	Off-grid backup, ICE starter
Lead-acid (AGM)	30–50	500–1 000	50–80 %	200–300	UPS, premium starter
NiMH	60–120	500–1 000	80 %	300–500	Hybrid vehicles, AA cells
LFP (LiFePO₄)	90–160	3 000–5 000	90 %	200–300	Solar storage, EV (BYD, modern Tesla)
NMC (Li-ion)	200–270	1 000–2 000	80–90 %	130–200	EVs, laptops, premium electronics
NCA (Li-ion)	220–260	1 000–2 000	80 %	150–250	Older Tesla packs, power to