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Battery Runtime Formula

A quick-reference formula sheet for calculating battery runtime across all common scenarios. Bookmark this page for fast access to the exact formulas you need.

Core Formulas

Fundamental Runtime

Total Energy (Wh) = Capacity (Ah) × Nominal Voltage (V)

Usable Energy (Wh) = Total Energy × (DoD% / 100)

Effective Load (W) = Actual Load (W) / (System Efficiency / 100)

Runtime (hours) = Usable Energy (Wh) / Effective Load (W)

Ah to Wh Conversion

Capacity Conversion

Energy (Wh) = Capacity (Ah) × Voltage (V)

Capacity (Ah) = Energy (Wh) / Voltage (V)

Current (A) = Power (W) / Voltage (V)

Common pack voltages: 12V (3S/4S), 24V (7S/8S), 36V (10S/11S), 48V (14S/16S).

Inverter Efficiency

AC Load Runtime

Effective Load (W) = AC Load (W) / Inverter Efficiency

Runtime (h) = Usable Energy (Wh) / Effective Load (W)

Typical inverter efficiency: 85–92% at 50–80% load. Drops to 75–80% at 10% load.

Temperature Derating

Cold Weather Correction

Derated Energy = Usable Energy × Temperature Factor

Temperature	LFP Factor	Lead-Acid Factor
25°C	1.00	1.00
10°C	0.95	0.90
0°C	0.90	0.80
-10°C	0.80	0.65
-20°C	0.70	0.50

Peukert Effect (Lead-Acid Only)

Lead-Acid Derating

Effective Ah = Rated Ah × (C-rate / 0.05C)^(n-1)

Where n is the Peukert exponent (1.1–1.3 for AGM, 1.3–1.6 for flooded). At 0.05C (20-hour rate), the battery delivers its full rated capacity. At higher rates, effective capacity decreases.

Lithium batteries have a Peukert exponent near 1.0, meaning they deliver nearly full capacity at any reasonable discharge rate.

Quick Reference Table

Scenario	Formula
DC load, no inverter	Ah × V × DoD / Load
AC load through inverter	Ah × V × DoD / (Load / η_inv)
With temperature derating	Ah × V × DoD × TempFactor / Load
Solar + battery system	Ah × V × DoD / (Load / (η_inv × η_wiring))

Try It

Use the Runtime Calculator to apply these formulas to your system.

Open Runtime Calculator

Use the Inverter Battery Calculator for AC system sizing.

Inverter Battery Calculator

How to Calculate Battery Runtime

The step-by-step engineering method explained in detail.

Read Guide →

12V vs 24V vs 48V Runtime

How voltage choice affects system efficiency and runtime.

Read Guide →

Frequently Asked Questions

What is the basic battery runtime formula?

Runtime (hours) = (Capacity Ah × Voltage V × DoD%) ÷ (Load W ÷ Efficiency%). For example: (100Ah × 12.8V × 0.80) ÷ (200W ÷ 0.90) = 4.6 hours.

How do I convert Ah to Wh for runtime calculations?

Multiply Ah by the nominal voltage: Wh = Ah × V. A 100Ah 12.8V battery stores 1,280 Wh. Apply DoD to get usable Wh: 1,280 × 0.80 = 1,024 Wh usable.

What efficiency value should I use?

For AC loads through an inverter, use 85–92% depending on inverter quality and load level. For DC loads, use 95–99% (wiring losses only). For solar systems, include charge controller efficiency (95–98%).

How do I account for temperature in runtime formulas?

Apply a temperature derating factor: 0.95 at 10°C, 0.90 at 0°C, 0.80 at -10°C, 0.70 at -20°C. Multiply the usable energy by this factor before dividing by load.