Inverter Size Calculator
Determine the inverter size needed for your appliance load. Enter circuit values to calculate the selected electrical quantity.
Load & Inverter
Battery Estimation (Optional)
Electronics guide · Electrical power, energy, and wiring
About Inverter Size Calculator
Estimate a minimum inverter apparent-power rating from connected watt load, safety margin, inverter efficiency, and power factor. Optional battery inputs also estimate amp-hour and watt-hour capacity for a chosen backup time.
Power and energy calculations relate quantities such as voltage, current, resistance, time, and load. AC and DC systems can require different assumptions, and a calculated estimate is not a substitute for checking source capability, wire ratings, protection, efficiency, and equipment specifications.
How to Use This Tool
- 1.Add the combined running watt load of the equipment you plan to power.
- 2.Enter a margin, efficiency, and power factor that match the system assumptions; account for motor startup loads separately.
- 3.For battery estimates, enter battery voltage and desired backup hours, then review the VA, W, Ah, and Wh results.
Technical Details to Check
1.Check AC or DC assumptions
In AC calculations, voltage and current may refer to RMS values, and phase relationships can affect real power. Do not apply a DC relationship to an AC system unless the tool's stated assumptions fit.
2.Power is not the same as energy
Watts describe a rate of energy use; watt-hours and joules describe an amount of energy over time. A runtime or consumption estimate needs both a load and a duration or duty profile.
3.Real systems lose energy
Inverters, batteries, wiring, motors, and power supplies are not perfectly efficient. Confirm whether the selected calculation includes efficiency, power factor, voltage drop, or only ideal values.
4.Ratings and protection are essential
An estimated current or power value does not determine a safe wire size, fuse, battery connection, or supply. Follow applicable standards, product ratings, and qualified design guidance.
How the Calculation Works
The tool increases the connected load by its safety-margin percentage, adjusts for inverter efficiency, and divides real power by power factor to estimate VA. Battery energy is based on load and backup duration with efficiency assumptions; amp-hours are derived from the selected battery voltage. The precise recommendation is a sizing estimate, not a design certification.
Worked Example
A 1,000 W load at 0.8 power factor requires 1,250 VA before a reserve margin and efficiency considerations.
Where This Tool Can Help
Compare nominal inverter ratings and make a first-pass backup-energy estimate for a stated load and runtime.
Estimating a load
Use known voltage and current or power figures to understand a device's approximate demand before checking its label and startup behavior.
Planning energy use
Combine a power estimate with realistic operating hours to compare possible consumption, while accounting separately for standby use and efficiency.
Checking a wiring estimate
Use current or voltage-drop results as an early check, then verify conductor size and protection with the correct installation rules and environment.
Comparing supply options
Compare candidate sources on the same voltage, current, power, and duration basis, and include headroom where the equipment specification requires it.
Tips for More Reliable Results
- Confirm whether voltage and current are RMS, peak, nominal, or measured under load.
- Keep watts, watt-hours, joules, and time units distinct when entering values.
- Include efficiency or power factor only when the tool provides that input and you have a suitable value.
- Check conductor, connector, battery, and supply ratings before using an estimate in a real installation.
Limitations and Assumptions
Starting surges, waveform quality, temperature, battery discharge limits, aging, wiring, and inverter overload curves matter. The input load should include all devices and peak conditions. Follow manufacturer guidance and have a qualified electrician verify a real installation.
Electrical power tools estimate values from entered figures and stated assumptions. They do not measure a circuit, model every transient or loss, or certify wiring and equipment as safe. Confirm important designs with manufacturer data and applicable electrical requirements.
Frequently Asked Questions
Why can VA be higher than watts?
Power factor relates real power to apparent power; lower PF requires a larger VA rating for the same watts.
Does the calculation guarantee backup runtime?
No. Battery capacity changes with load, chemistry, age, temperature, and cutoff voltage.
Should I include motor startup power?
Yes. The page does not measure surge demand; account for it from equipment specifications.
Are watts and watt-hours interchangeable?
No. Watts are a rate; watt-hours measure energy used over time.
Does a calculated value make a circuit safe to build?
No. Safety depends on ratings, protection, installation conditions, and applicable standards as well as the arithmetic.