Solar inverter sizing:
formula and worked example

Why the inverter is usually smaller than the array

A solar array is rated in DC kilowatts under standard test conditions: a fixed irradiance and cell temperature that a real roof almost never sees. In daily use, heat, haze, panel tilt and the angle of the sun keep output well under that label for most hours. An inverter rated at the full array size would therefore spend nearly all its life working at a fraction of its capacity.

Designers deal with this by pairing the array with a somewhat smaller inverter. The result is a DC/AC ratio above 1.0. You give up a sliver of energy on the sunniest, coolest middle-of-the-day moments, and in return you buy less inverter hardware and run it closer to the load range where it works efficiently. Sizing is the act of picking that AC rating.

Think of the inverter as a doorway sized for the usual crowd rather than the once-a-year peak. A narrower door costs less and is rarely a bottleneck. A door sized for the absolute maximum is rarely full. The same logic applies to every grid-tied system, and it is the reason datasheets list a maximum recommended DC input alongside the AC output.

The decision also interacts with where you live. A system in a cool, clear climate peaks closer to its rating than one in a hot, hazy region, and a south-facing array concentrates output into fewer hours than an east-west split. Those differences are why one ratio does not fit every roof.

The sizing formula and what each input means

• Array kW: the nameplate DC total, found by multiplying the number of modules by the watts per module and dividing by 1,000.
• DC/AC ratio: a dimensionless design choice, also called the loading ratio. A value of 1.2 means the array is rated 20% above the inverter.
• Result: the AC output rating, in kW, that the inverter should have. Round to the nearest available model afterwards.

Two things sit outside the arithmetic: the inverter datasheet limits for maximum DC input voltage and current, and the grid operator's rules on export capacity. Either can override the number the formula gives you.

It helps to keep three ratings separate in your head: the module label, the array total and the inverter output. Confusing the module's single-panel wattage with the array total is a frequent error when reading a quote, and it turns a 6 kW design into a 0.5 kW one on paper.

Worked example: a 6 kW array at a 1.2 ratio

A household plans 6 kW of modules and wants to design at a ratio of 1.2. The question is what AC rating to ask for.

1. 1Take the array rating: 6 kW DC.
2. 2Choose the loading ratio: 1.2.
3. 3Divide: 6 ÷ 1.2 = 5.
4. 4The inverter should be about 5 kW AC.

A 5 kW inverter is a common catalogue size, so no rounding is needed here. With a 6.3 kW array the same ratio would give 5.25 kW, and you would have to choose between a 5 kW unit with a slightly higher ratio or a larger unit.

How to read the ratio you chose

Ratios between roughly 1.1 and 1.3 are a common starting point for fixed rooftop arrays, and some designs go higher where panels face away from the sun's path or sit at a flat tilt. Treat these as conventions that vary by region, equipment and installer, not as rules.

• Near 1.0: almost no clipping, but you pay for inverter capacity that is idle most of the year.
• Around 1.2: a few hours a year of clipping at most, usually a very small share of annual energy.
• Well above 1.3: clipping becomes regular and the yield curve flattens at midday, which may be acceptable if the inverter cost saving is large.

Clipping is simply the inverter refusing to convert DC power above its AC rating. The lost energy is real but small when the ratio is modest, because the array only exceeds the inverter rating during a limited number of peak hours.

Remember too that a higher ratio also means the inverter spends more hours near its full load. That is generally efficient, but it also means more heat and, in hot climates, a need to check the unit's derating behaviour at high ambient temperature.

What happens when the array grows

Because the formula is a straight division, a 10% bigger array raises the required AC rating by exactly 10%: 5 kW becomes 5.5 kW. Changing the ratio instead has a similar scale effect in the other direction. At 1.3 the same 6 kW array needs only about 4.6 kW of inverter, while at 1.0 it needs the full 6 kW.

This is also how you test an installer's proposal. If a quote shows a 6 kW array with a 4 kW inverter, the ratio is 1.5, which is outside the common range and calls for a clear clipping estimate. If it shows 6 kW with 6 kW, ask what you are getting for the extra inverter spend.

Mistakes that distort inverter sizing

• Using the inverter's AC rating where the array's DC rating belongs, which shifts every downstream number by the ratio.
• Ignoring the maximum DC input current, so a high ratio is accepted by the formula but rejected by the inverter's datasheet.
• Forgetting that high-temperature roofs lower module voltage and output, which changes both clipping and string design.
• Sizing to future panel additions without checking whether the inverter supports a bigger array on its DC side.

A quick scenario before you sign a quote

Suppose two quotes arrive for the same 6 kW array, one with a 5 kW inverter and one with a 6 kW inverter. The first has a ratio of 1.2 and the second a ratio of 1.0. Ask each installer for the annual clipping estimate and for the price difference. If the larger unit costs noticeably more and clips only a small share of annual output, the 5 kW option is hard to argue against, unless your grid connection or battery plan needs the extra AC headroom.

A battery changes the picture. In a DC-coupled design the battery can absorb power that would otherwise be clipped, so a higher ratio becomes more attractive. In an AC-coupled design the battery sits behind its own inverter and the primary sizing logic stays as described here.