Battery Inverter Run Time Calculator: How Long Will It Last?

This calculator estimates how long a battery will power a load through an inverter, based on battery capacity, system voltage, load wattage, and inverter efficiency. You enter those four values, and it returns the estimated run time in hours and minutes, along with the total watt-hours available. The result is a planning estimate, not a guarantee, because real-world conditions like battery age, temperature, and discharge rate affect actual performance.

Formula

The run time is calculated using the fundamental energy relationship: Run Time (hours) = (Battery Capacity in Ah × Battery Voltage in V × Inverter Efficiency) / Load Wattage. In symbols: t = (C × V × η) / P, where:

• C = battery capacity in amp-hours (Ah)
• V = battery voltage (V) – typically 12V, 24V, or 48V
• η = inverter efficiency as a decimal (e.g., 0.90 for 90%)
• P = continuous power draw of the load in watts (W)

The numerator (C × V × η) gives the usable energy in watt-hours (Wh). Dividing by the load wattage yields the number of hours the battery can sustain that load. This is the standard DC-to-AC conversion formula used by most off-grid and RV power planners.

Worked Example

Using the default values from the calculator: a 100 Ah, 12V battery, a 300 W load, and a 90% efficient inverter. First, calculate the usable energy: 100 Ah × 12 V = 1200 Wh (raw battery energy). Multiply by inverter efficiency: 1200 Wh × 0.90 = 1080 Wh usable. Now divide by the load: 1080 Wh ÷ 300 W = 3.6 hours. The calculator shows 3.6 hours (3 hours and 36 minutes) and 1080 Wh total energy. This matches the block output exactly.

What the Result Really Means

The run time figure is a theoretical maximum under ideal conditions. It assumes the battery is fully charged, the load is constant, and the inverter operates at its stated efficiency. In practice, three factors reduce this number:

• Depth of discharge (DoD): Lead-acid batteries should not be discharged below 50% to avoid damage, effectively halving usable capacity. Lithium batteries can often go to 80–90% DoD.
• Peukert effect: The faster you discharge a lead-acid battery, the less total energy it delivers. A 100 Ah battery at a 5A draw delivers more total energy than at a 50A draw.
• Inverter idle consumption: Many inverters draw 10–30W just to stay on, even with no load. For small loads, this can be significant.

For a realistic estimate, multiply the calculated time by 0.5 for lead-acid batteries (50% DoD) or by 0.8 for lithium (80% DoD). For example, the 3.6 hours from above becomes about 1.8 hours for lead-acid or 2.9 hours for lithium.

How to Choose the Right Battery and Inverter

Start with your load. List every device you plan to power, find its wattage (printed on the label or plug), and add them up. That total is your minimum inverter wattage, but add a 20–30% safety margin for startup surges from motors or compressors. Then decide how long you need to run that load. Multiply the load wattage by the desired hours to get watt-hours. Divide by the battery voltage and by the inverter efficiency to get the required amp-hours. Finally, apply the depth-of-discharge factor: for lead-acid, double the amp-hours; for lithium, multiply by 1.25. This gives you the minimum battery capacity.

Common Mistakes and Misconceptions

The most frequent error is confusing watts and amp-hours. A 100 Ah battery at 12V stores 1200 Wh, but at 24V it stores 2400 Wh – the amp-hour rating alone doesn’t tell you energy. Another mistake is ignoring inverter efficiency; cheap inverters may be only 75–80% efficient, while quality units reach 90–95%. Also, many people assume a 1000W inverter always draws 1000W – it only draws what the load demands. Finally, don’t forget that battery capacity is rated at a specific discharge rate (usually 20 hours for lead-acid); drawing current faster reduces usable capacity.

Comparing Battery Types for Inverter Use

Lead-acid batteries (flooded, AGM, gel) are cheaper upfront but heavier, have a 50% DoD limit, and degrade faster with deep discharges. Lithium iron phosphate (LiFePO4) batteries cost more initially but offer 80–90% DoD, longer cycle life (2000–5000 cycles vs. 300–1000 for lead-acid), and lighter weight. For occasional use, lead-acid is often adequate. For daily off-grid use, lithium pays off over time. The calculator works for both, just adjust the DoD factor and the usable capacity accordingly.

Costs and Efficiency Considerations

Inverter efficiency directly affects run time and energy costs. A 90% efficient inverter wastes 10% of your battery energy as heat. Over a year of daily use, that can add up to hundreds of watt-hours wasted. Higher-efficiency inverters usually cost more but may save money in the long run if you cycle batteries frequently. Also consider standby power: an inverter left on but unused can drain a battery over days. If you only need power occasionally, a unit with a low idle draw or a remote off switch is worth the extra cost.

Safety and Practical Limits

Never discharge a lead-acid battery below 50% regularly – it shortens lifespan and can cause damage. Lithium batteries have built-in management systems that cut off at low voltage, but relying on that cutoff is not ideal. Also, ensure your wiring and fuses are rated for the current: amps = watts ÷ volts. For a 1000W load on a 12V system, that’s over 83A, requiring thick cable (e.g., 4 AWG or larger) and appropriate fuses. Overloading an inverter or using undersized wire can cause overheating and fire risk. Always follow the manufacturer’s specifications for both inverter and battery.

Alternatives to a Single Battery

If your run time comes up short, you have options: add a second battery in parallel (same voltage, doubled capacity), increase system voltage (e.g., from 12V to 24V, which halves current and reduces losses), or reduce the load by using more efficient appliances. Solar panels can recharge during the day, effectively extending run time indefinitely if sized correctly. A generator is another backup, but it adds noise and fuel costs. The calculator helps you compare these options by showing the effect of changing capacity, voltage, or load.

Frequently Asked Questions

How long will a 100Ah battery run a 500W inverter? Using the formula with a 12V battery and 90% efficiency: 100 × 12 × 0.9 = 1080 Wh, divided by 500W = 2.16 hours. But with a 50% DoD for lead-acid, plan for about 1 hour of usable time.

Does inverter size affect run time? No – the inverter’s wattage rating is its maximum, not its constant draw. A 2000W inverter running a 100W load will draw about 100W (plus idle losses), same as a 300W inverter. The efficiency difference between units matters more than rated size.

Can I run a refrigerator on a battery inverter? Yes, but refrigerators have startup surges (often 3–5 times running watts) and cycle on/off. Use the running wattage (typically 100–200W for a modern fridge) for run time, but ensure the inverter can handle the surge. A 100Ah battery at 12V would run a 150W fridge for about 6 hours (with 50% DoD), but only if the inverter handles the startup.

What is the difference between a modified sine wave and pure sine wave inverter? Modified sine wave inverters are cheaper but can cause issues with sensitive electronics, motors, and some appliances. Pure sine wave inverters produce cleaner power, compatible with everything, but cost more. For run time, both have similar efficiencies, but pure sine wave may be slightly less efficient at low loads.

How do I convert amp-hours to watt-hours? Multiply amp-hours by battery voltage. For example, a 200Ah 24V battery has 200 × 24 = 4800 Wh. This is the total energy, before inverter efficiency losses.

Can I use a car battery for an inverter? Car batteries are designed for short, high-current bursts, not deep discharge. Using one for inverter loads will damage it quickly. Use a deep-cycle battery (marine, RV, or golf cart) or a lithium battery designed for cycling.

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in bc | Battery Capacity | Ah | 100 | 1 | 10000
in bv | Battery Voltage | V | 12 | 1 | 600
in lw | Load Wattage | W | 300 | 1 | 50000
in ie | Inverter Efficiency | % | 90 | 10 | 100
out h | Run Time | hours | (bc * bv * ie / 100) / lw | 2
out wh | Total Energy | Wh | bc * bv * ie / 100 | 1