OneCalcApp LogoOneCalcAppConvert. Calculate. Simplify.

Inverter Sizing Calculator

Select an inverter rating from connected load or array size, with surge headroom, DC/AC ratio check and output current.

Inputs

Formula

  • Grid-tied: P_inv = P_array / DC:AC ratio
  • Off-grid: P_inv = continuous load × (1 + expansion), surge capacity ≥ load × surge factor
  • I_ac = P / (V) for 1-ph or P / (√3 V) for 3-ph

Grid-tied inverters are deliberately undersized against the array — a DC/AC ratio of 1.1 to 1.3 raises annual energy harvest because the array only reaches full output for a few hours a year, and mild clipping costs less than the extra inverter capacity. Off-grid inverters are sized the opposite way: they must carry the largest simultaneous load plus motor starting surges.

Step-by-step calculation

  1. Required rating

    array kWp / DC:AC ratio

    5 kW

  2. Standard rating

    next size up

    5 kW

  3. Output current

    P / V

    21.74 A

Worked example

  • 6 kWp array at a 1.2 DC/AC ratio → 5 kW inverter.
  • Output current at 230 V single phase = 5 000 / 230 = 21.7 A.
  • Clipping loss at ratio 1.2 is typically under 1.5% of annual yield.

Assumptions

  • Standard test-condition array rating; clipping estimated for a typical clear-sky profile.

Tips

  • A DC/AC ratio of 1.15–1.25 is the economic optimum for most rooftop systems.

Warnings

  • Never exceed the inverter's maximum DC input voltage at the lowest expected site temperature.
  • Off-grid inverters must be rated for motor surge — a 1 hp pump can draw 5× running current on start.

Standards & references

  • IEC 62109
  • IEC 61727
  • IS 16221

Frequently asked questions

What size inverter do I need for a 6 kW solar system?

A 5 kW inverter, giving a DC/AC ratio of 1.2. This maximises annual harvest with negligible clipping.

Can the inverter be smaller than the array?

Yes, and it usually should be. Arrays rarely produce their nameplate output, so a 1.1–1.3 ratio increases yield per rupee invested.

How do I size an off-grid inverter?

Add the largest set of loads that can run at once, add a margin for growth, then check that the surge rating covers motor starting currents.

Related calculators

Solar Generation Calculator

Estimate daily, monthly and annual generation from a PV system using peak sun hours and performance ratio, with CO₂ savings and bill savings.

Battery Backup Calculator

Calculate backup runtime from battery capacity, or the battery bank size needed for a required number of backup hours.

AC Cable Sizing Calculator

Size the AC cable from a solar inverter to the distribution board, sized on inverter output current with a 1% voltage-drop target.

DC Cable Voltage Drop Calculator

Solar DC string and battery cable voltage drop with the 1% / 2% design limits used for PV array and battery circuits.