How to Calculate Inverter Battery Capacity and Runtime

To calculate inverter battery capacity, first work out how much energy your appliances need in watt-hours (Wh), then allow for the usable portion of the battery and the losses between the battery and the appliances. To estimate runtime, divide the usable battery energy by the average battery-side power draw. The inverter's watt rating alone cannot tell you how long a battery will last.

1. Understand watts, watt-hours and amp-hours

• Watts (W) measure power at a given moment. Add the running watts of the devices that will operate together to estimate the inverter load.
• Watt-hours (Wh) measure energy. A steady 100W load used for 3 hours consumes 300Wh at the load.
• Amp-hours (Ah) describe battery charge capacity at a specified voltage. Compare batteries using Wh when their voltages differ.

For example, a battery labeled 12V, 100Ah has approximately 1,200Wh of nominal energy. A battery labeled 24V, 100Ah has approximately 2,400Wh. Use the actual nominal voltage on the battery label: a 12.8V, 100Ah battery is approximately 1,280Wh. If the manufacturer provides a Wh rating, start with that figure.

A larger Ah number does not by itself prove that a battery can run a larger load. The battery's permitted discharge current and the inverter's output capability must also be sufficient.

2. Calculate inverter battery capacity for a target runtime

1. List the devices and their actual running power. Use labeled watts or measure typical use with a suitable power meter. A laptop's charging demand can change; a refrigerator cycles. If a label lists only AC volts and amps, their product is VA and may differ from actual watts.
2. Calculate energy for each device. Multiply running W by the hours it will run, then add the Wh values.
3. Choose the usable battery fraction. This is the portion of nominal capacity you plan to use between the starting charge and your chosen reserve or cutoff. Follow the battery maker's limits and allow for condition, temperature and discharge rate. There is no universal 80% rule.
4. Allow for total battery-to-load efficiency at the expected load. Prefer measured information for the actual setup. Peak efficiency is not a constant value at every load.

In these formulas, enter 80% as 0.80 and 90% as 0.90. If you already have a measured usable Wh figure, do not reduce it by the same usable-capacity fraction again.

Worked example: 310W for 3 hours

Suppose five 30W lights and two 80W laptops actually draw their stated power throughout the session:

• Simultaneous load: 5 × 30W + 2 × 80W = 310W.
• Energy needed at the appliances: 310W × 3h = 930Wh.
• For this calculation only, assume 80% usable battery capacity and 90% total battery-to-AC efficiency.
• Required nominal battery energy: 930Wh ÷ (0.80 × 0.90) ≈ 1,292Wh.

These are arithmetic sizing figures before any additional planning reserve. They are not a recommendation to substitute one voltage for another: the battery system must match the selected inverter's DC input. Confirm the actual battery's usable energy, discharge rating and compatibility before choosing a size.

3. Estimate how long an existing battery will last

The most useful starting point is the average power drawn from the battery by the complete connected setup:

Runtime (h) ≈ usable battery energy (Wh) ÷ average battery-side power (W)

A suitable battery monitor can help measure this draw. If the measurement covers the inverter, connected loads and wiring, their losses are already in the measured value. Do not apply another inverter-efficiency reduction to the same measurement.

If you only know the AC load and a suitable total-efficiency estimate, use:

Runtime (h) ≈ nominal V × Ah × usable fraction × total efficiency ÷ AC load W

Using a hypothetical 12V, 100Ah battery, 80% usable capacity, 90% total efficiency and a steady 310W AC load gives: 12 × 100 × 0.80 × 0.90 ÷ 310 ≈ 2.8 hours. This is an illustrative estimate, not a tested runtime for a WZRELB inverter or any particular battery.

4. Plan small-device backup by energy use

For a router, laptop and fan, start with a device-by-device energy budget. The powers below are hypothetical examples; substitute measurements for your own equipment.

The battery must supply more than this 400Wh load budget after losses and your chosen reserve. For a phone or laptop using a compatible USB output, measure or estimate that specific power path instead of assuming it has the same efficiency as AC output.

For a refrigerator or other cycling load, measure Wh over a representative operating period and use its duty cycle or daily energy consumption. Check startup demand separately. Multiplying the appliance's maximum power by 24 hours can misrepresent a normal day.

5. Include inverter losses and idle consumption once

An inverter can use energy while switched on with no appliance drawing power. This matters particularly for small loads and long standby periods. Use the idle consumption for the exact model and settings, or measure it; do not reuse another inverter's number.

Idle energy (Wh) = idle power (W) × hours left on without a load

If the inverter is loaded for part of a day and left on without a load for the remainder, account for the load-period energy and the separate idle-period energy. A direct battery-side measurement across the full period includes both.

Avoid double counting: total DC-to-AC efficiency measured at a given load already includes the inverter's own losses during that loaded period. Do not add the same idle loss again for those hours. A published maximum-efficiency figure also should not be applied to every load, especially a very small one.

Battery age, temperature, high discharge current, wiring losses and the battery or inverter cutoff can shorten runtime. Lead-acid capacity can vary with discharge rate. Use the battery maker's discharge information and check the estimate under representative conditions.

6. Calculate nominal energy in tool batteries

Use the battery's stated Wh or its nominal voltage and Ah. For example, DEWALT identifies its 20V MAX batteries as 18V nominal. A DCB205 5Ah battery therefore represents approximately 18V × 5Ah = 90Wh nominal, before reserve and conversion losses.

For several compatible packs, add their individual nominal Wh values. This adds energy for planning; it does not describe the inverter's internal battery wiring or guarantee a particular output or runtime.

For WZRELB ETP models, confirm the compatible battery model, required battery count and connected load using the ETP battery inverter guide. Capacity and available output are separate checks. Choose the appropriate ETP500 product configuration or ETP600 product configuration only after checking those requirements.

7. Check these items before selecting the battery and inverter

• DC input: match the battery system to the exact inverter version. Products can offer several fixed voltage versions in one listing; those options do not by themselves mean automatic voltage detection.
• AC output and frequency: select the voltage and 50Hz or 60Hz version required by your appliances.
• Continuous and startup power: check both the running load and the required startup power and duration. There is no single startup multiplier for all refrigerators, pumps or motors. See why an inverter can trip when a fridge starts.
• Battery discharge capability: confirm that the cells and battery management system (BMS) can deliver the continuous and momentary current required. Enough nominal Wh alone does not establish that a load will start.
• Battery-bank configuration: series and parallel arrangements change voltage and Ah differently. For two matching batteries labeled 12V, 100Ah, series gives 24V, 100Ah; parallel gives 12V, 200Ah. Both total about 2,400Wh nominal. Only use configurations permitted by the battery and inverter manufacturers.

Have your battery model and quantity, appliance running watts, startup requirements, desired hours of use and selected inverter version ready. Contact info@powerreliable.com if you need help checking a WZRELB configuration.

Frequently asked questions

How long will a 100Ah battery run an inverter?

You also need its nominal voltage, usable energy and the connected load. At the same voltage and usable capacity, a lower average battery draw generally lasts longer. Use the calculation above instead of assigning one runtime to every 100Ah battery.

Does a 3,000W inverter always draw 3,000W?

No. Its rating describes output capability. Battery draw depends on the actual load, operating mode and losses. Use the manufacturer's load-efficiency information or measure the setup for a runtime estimate.

Will a larger battery fix an inverter that cannot start a refrigerator?

Check the cause first. Battery discharge limits, voltage drop and the inverter's startup capability are separate constraints. More nominal capacity alone does not guarantee a successful start.

Can I use the calculated time as a guaranteed backup duration?

No. An estimate depends on the stated assumptions. Verify the exact battery, charge level, loads, settings and cutoff conditions, and leave an appropriate reserve for your intended use.

Technical references

The following manufacturer references explain general battery and inverter principles. Their model-specific efficiency or idle-consumption figures are not WZRELB specifications.

• Victron Energy: output rating, operating temperature and efficiency — efficiency and no-load losses.
• Victron Energy: Wiring Unlimited — battery-bank energy and series/parallel principles.
• DEWALT: DCB205 20V MAX 5Ah battery — maximum versus nominal battery voltage.