Solar DC to AC Ratio Formulas

Solar DC to AC ratio formulas: the array size, the ratio of array power to inverter power and the inverter size for a target ratio.

The formulas

Pdc is the array rating at standard test conditions in kWp and Pac the AC rating of the inverter in kW. The array seldom reaches its rated power because of the heat, the sun angle and the losses, so an inverter smaller than the array is common, with a ratio of about 1.1 to 1.4 in many designs. A larger ratio gives more energy in the morning and evening and clips the power at noon in clear weather. The best ratio depends on the site, the tilt, the tariff and the inverter price, so the figures here are examples. Check the maximum DC input voltage and current of the inverter as well.

Worked example: 24 modules of 550 W on a 10 kW inverter

1. Pdc = 24 × 550 / 1000 = 13.2 kWp
2. r = 13.2 / 10 = 1.32
3. For a ratio of 1.25 the inverter is 13.2 / 1.25 = 10.56 kW

Ratio for a 13.2 kWp array

Frequently asked questions

Is a ratio above 1 allowed?

Yes, if the inverter data sheet allows it. The inverter limits its output to its rating and the extra array power is clipped at the peak of the day.

What does clipping cost?

On most sites a small part of the yearly energy at a ratio near 1.2. It rises with the ratio, so use a simulation for the site.

Does the temperature change the ratio?

A hot module gives less than its rating, so the real ratio at noon is below the figure. The solar panel temperature article shows the loss.

What else limits the array size?

The maximum DC voltage (from the cold string voltage) and the maximum current per input of the inverter.

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