About

Designing independent power systems-whether for off-grid solar, recreational vehicles (RVs), or marine applications-requires translating total energy (Watt-hours) into battery bank capacity (Amp-hours). Batteries are sold by Ah capacity at a specific nominal voltage, yet consumption loads are calculated in Wh. Without this conversion, it is impossible to correctly size a battery bank to support a known load duration.

This tool performs the necessary division of energy by voltage to determine charge capacity. Note that battery voltage is not constant; it drops as the battery discharges. Calculations should use the nominal voltage for general sizing (e.g., 12V for a lead-acid block, 3.7V for a lithium cell). Accurate sizing usually requires adding a 20-50% safety margin to the result to prevent deep discharge cycles that shorten battery life.

Formulas

Charge capacity is derived from the definition of electrical power over time:

Q_Ah = E_WhV

Where E_Wh is energy in Watt-hours, V is the system Voltage, and Q_Ah is the resulting charge capacity in Ampere-hours.

Reference Data

System Voltage	Energy (Wh)	Calculated Capacity (Ah)	Typical Use Case
3.7 V	10 Wh	2.7 Ah	Single 18650 Cell
5 V	50 Wh	10 Ah	USB Power Bank
12 V	1,200 Wh	100 Ah	RV Deep Cycle Battery
12 V	2,400 Wh	200 Ah	Small Solar Storage
24 V	2,400 Wh	100 Ah	Off-grid Cabin
48 V	5,000 Wh	104 Ah	Home Power Wall
48 V	10,000 Wh	208 Ah	EV Battery Module
400 V	75,000 Wh	187.5 Ah	Electric Vehicle (Full)

Frequently Asked Questions

Use the nominal voltage of your battery bank. Common values are 12V, 24V, or 48V for solar/marine systems, and 3.7V for individual lithium-ion cells. Do not use the "fully charged" voltage (e.g., 14.4V), as this will underestimate the required capacity.

Watt-hours measure total energy work. Amp-hours measure the "amount" of electron flow. If you double the voltage (pressure), you need half the current flow (Ah) to deliver the same total energy.

No. If you are powering AC appliances through an inverter, you should increase your required Wh input by roughly 15% (divide by 0.85) before converting to Ah to account for conversion losses.

C-rate describes how fast a battery is discharged relative to its capacity. A 100Ah battery discharged at 100A is 1C. This tool calculates capacity, but ensure your battery chemistry can handle the discharge current (Amps) required by your load.