Inverter Battery Runtime Guide
Most battery loads in RV, marine, and home backup systems pass through an inverter. The inverter converts DC battery power to AC for standard appliances, but it consumes 8–15% of the energy in the process. Understanding this conversion loss is essential for accurate runtime calculations.
How Inverter Efficiency Affects Runtime
An inverter rated at 90% efficiency means 10% of the battery energy is lost as heat during DC-to-AC conversion. This directly reduces runtime by 10%. For a 1000W load, the battery actually supplies 1,111W — the extra 111W is the inverter's consumption.
Inverter efficiency varies with load level. Most inverters are optimized for 50–80% of their rated capacity. Running a 3000W inverter at 100W may only achieve 75–80% efficiency, while running it at 2000W achieves 90–92%. Match your inverter size to your typical load range for best results.
Inverter Runtime Formula
For DC loads (no inverter), remove the efficiency step. DC loads draw directly from the battery with only wiring losses (1–5%).
Worked Example
Given: 200Ah 12.8V LFP, 2000W inverter (90% eff), 500W AC load
Step 1: Usable energy:
Step 2: Effective load:
Step 3: Runtime:
Without the inverter (DC load), runtime would be 4.1 hours — the inverter reduces it by 10%.
Inverter Efficiency by Load Level
| Load % | Typical Efficiency | Effective Load at 500W |
|---|---|---|
| 10% | 75–80% | 625–667 W |
| 25% | 82–85% | 588–610 W |
| 50% | 88–91% | 549–568 W |
| 75% | 90–92% | 543–556 W |
| 100% | 88–90% | 556–568 W |
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Frequently Asked Questions
How do I calculate inverter battery runtime?
Divide the battery's usable energy (Ah × V × DoD) by the effective load (load watts ÷ inverter efficiency). A 200Ah 12.8V LFP at 80% DoD with a 1000W load through a 90% efficient inverter runs for approximately 1.8 hours.
What size inverter do I need for my battery?
Match the inverter's continuous wattage rating to your maximum expected load. For a 200Ah 12V battery, a 2000W inverter is typical. The battery's maximum discharge rate (C-rate) must also support the inverter's peak current draw.
Does inverter efficiency vary with load?
Yes. Inverters are most efficient at 50–80% of their rated capacity. At very light loads (5–10%), efficiency drops to 70–80%. At full load, efficiency is typically 85–92%. This means light loads consume proportionally more battery energy.
Can I run an air conditioner from a battery through an inverter?
Yes, but the startup surge (3–5× rated power) requires a battery and inverter sized for peak current. A 1500W AC unit needs at least a 3000W inverter and a battery bank capable of delivering 300–400A momentarily.