Enter any 3 values (Total Wattage, Safety Factor, Power Factor, or Inverter Size) into the calculator to determine the missing variable.

Inverter Size Calculator

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Inverter Size Formula

The following formula is used to calculate the inverter size for a solar power system:

Inverter Size (VA) = (Total Wattage (W) / Power Factor) * Safety Factor

Variables:

  • Inverter Size is the required inverter apparent power rating in volt-amperes (VA) (often shown as VA or kVA). If the overall power factor is 1, then VA = W.
  • Total Wattage is the sum of the real power (W) ratings of all the devices connected to the inverter.
  • Power Factor (PF) is the ratio of real power (W) to apparent power (VA). It ranges from 0 to 1 for typical AC loads. If you don’t know PF and your loads are mostly resistive, you can use PF ≈ 1.
  • Safety Factor is a factor that accounts for variations in power consumption and helps ensure the inverter can handle peak loads. For many general loads it is often in the range of about 1.2 to 1.5, but motor/compressor loads may require a higher surge capability.

To calculate the inverter size, divide the total wattage by the power factor to get apparent power (VA), then multiply by the safety factor.

What is an Inverter Size?

Inverter size refers to the maximum output power an inverter can supply when converting from DC (Direct Current) to AC (Alternating Current). Inverters are commonly specified by a continuous output rating (often in watts and/or VA) and a separate surge rating for short start-up spikes. The size of the inverter you need depends on the total running power of the devices you want to run and any start-up surges (common with motors and compressors). For example, a 100-watt light bulb plus a 500-watt refrigerator is about 600 watts of running load, but a refrigerator can have a start-up surge that is significantly higher than its running wattage. In practice, you may need an inverter with a continuous rating above 600 W and a surge rating high enough for the refrigerator’s start-up. It’s also important to note that larger inverters typically have higher standby consumption, so it’s best to choose a size that matches your actual needs while still meeting surge requirements.

How to Calculate Inverter Size?

The following steps outline how to calculate the Inverter Size.

  1. First, determine the total real power consumption (W) of all the devices that will be connected to the inverter (use running watts, not just surge watts).
  2. Next, determine (or estimate) the overall power factor (PF) of the loads if you need an apparent power (VA) rating.
  3. Next, calculate the total apparent power (VA) by dividing the total real power by the power factor: VA = W / PF.
  4. Finally, apply a safety factor for headroom and surges: Inverter Size (VA) = VA × Safety Factor. (If PF ≈ 1, you can use watts directly.)
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge.

Total power consumption (Watt) = 5000

Power factor = 0.9

Safety factor = 1.25

Inverter size = (5000 / 0.9) × 1.25 ≈ 6944.44 VA (≈ 6.94 kVA)