Battery Runtime Calculator (AC/DC) — Capacity, DoD & Efficiency

This battery runtime calculator helps you estimate how long a battery will power a load, whether it’s a DC device or an AC appliance running through an inverter. In practice, you enter the load (W or A), system voltage, inverter efficiency, and depth of discharge (DoD), and you immediately get runtime, usable energy (Wh), estimated current (A), and capacity (Ah). Therefore, it’s ideal for off-grid systems, RV/van builds, backup power, and tool batteries.

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Why use a battery runtime calculator?

First, it turns guesswork into a quick sizing step, which reduces change orders and callbacks. Second, it separates AC vs DC math, so you don’t mix up inverter efficiency. In addition, it forces a conscious choice of DoD (for example 50% vs 80%), which directly affects lifetime and runtime. Finally, it helps you explain trade-offs to clients and apprentices in minutes.

How to use this battery runtime calculator

1. Choose load type — AC (through inverter) or DC (direct).
2. Enter voltage — typical packs are 12/24/48 V; tool batteries vary.
3. Enter the load — use W for power or A for current (the calculator converts to Wh/Ah).
4. Set DoD — common choices: 50% for lead-acid longevity; 80–90% for LiFePO₄.
5. Set efficiency — use inverter/driver efficiency (e.g., 85–95% for many inverters; 92–96% for quality DC drivers).
6. Finally, click Calculate. If results look off, double-check whether your load is AC or DC, confirm the real power (kW vs kVA), and verify the voltage.

Tip: For AC loads through an inverter, either multiply usable battery Wh by efficiency or, equivalently, divide the AC load by efficiency before computing runtime. Both methods agree.

Formulas at a glance (balanced & simplified)

• Usable battery energy (Wh) = V × Ah × DoD.
• Runtime with DC load (hours) = (V × Ah × DoD × ηDC) ÷ PDC  or  (Ah × DoD) ÷ IDC.
• Runtime with AC load (hours) = (V × Ah × DoD × ηinv) ÷ PAC.
• Estimated current (A) = P ÷ V (apply efficiency to the side that makes sense for your system).

When should you adjust the design?

• Cold weather: capacity (Ah/Wh) drops at low temps; consequently, add margin or use a heated enclosure.
• High surge / starting loads: compressors, pumps and inverters may need headroom; size conductors and fuses accordingly.
• Lead-acid Peukert effect: heavy discharge shortens runtime; therefore, keep discharge rates modest or upsize the bank.
• Inverter efficiency: small inverters at light loads can be less efficient; meanwhile, idle draw also matters.
• Voltage sag and wiring: check conductor size and voltage drop on long runs.

Examples (AC & DC)

Example 1 — 300 W AC load, 24 V system, 100 Ah, inverter 90%, DoD 80%

• Usable DC energy: 24 × 100 × 0.80 = 1,920 Wh.
• Available to AC after inverter loss: 1,920 × 0.90 = 1,728 Wh.
• Runtime: 1,728 ÷ 300 ≈ 5.76 h → about 5 h 46 min.

Example 2 — 8 A DC load, 12 V system, 60 Ah, DoD 50%

• Simple amp-hour method: (60 × 0.50) ÷ 8 = 3.75 h → about 3 h 45 min.
• Power method (optional driver 96%): usable Wh = 12 × 60 × 0.50 = 360 Wh → (360 × 0.96) ÷ (12 × 8) ≈ 3.6 h.

Design notes

• Peukert (lead-acid): higher current shortens effective capacity; LiFePO₄ is much closer to 1.05 and holds capacity better at load.
• Protection & wiring: coordinate OCPD, conductor size, and terminations; verify torque and temperature ratings.
• Safety: for large packs, include fusing, contactor/relay design, pre-charge, and a proper BMS; follow local codes and manufacturer guidance.

Extended reading (internal)

• Power calculator (kW/kVA/kVAR & amps).
• Voltage drop calculator.
• Wire size & ampacity basics.

References (outbound)

• Peukert’s law — Wikipedia.
• Battery University — Discharge characteristics.
• Power inverter — Wikipedia.

Disclaimer: This battery runtime calculator is for education and pre-design checks. Therefore, final designs must consider manufacturer specs, the NEC/IEC and local rules, environmental conditions, and safety margins.

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