How does sizing a solar inverter work?

Solar inverters are one of the most important components of a solar panel system. They're responsible for converting direct current (DC) electricity from your solar panels to alternating current (AC) electricity to power your appliances. When designing your solar panel system, the size of your inverter will play an essential role in overall electricity production. In this article, we'll discuss what impacts solar inverter sizing.

Solar inverters come in all different sizes, big and small. Similar to solar panels, the size of an inverter can be rated in watts (W). When it comes to solar inverter sizing, installers will consider three primary factors: the size of your solar array, geography, and site-specific conditions.

Size of your solar array

The size of your solar array is the most important factor in determining the appropriate size for your solar inverter. Because your solar inverter converts DC electricity coming from the array, it needs to have the capacity to handle all the power the array produces.

As a general rule of thumb, the size of your inverter should be similar to the DC rating of your solar panel system; if you are installing a 6 kilowatt (kW) system, you can expect the proposed inverter to be around 6000 W, plus or minus a small percentage.

Inverter manufacturers typically list sizing guidelines for the array capacity their inverters can be paired with on their product spec sheets. If the size of the solar array paired with their inverter is outside the stated guidelines, manufacturers may void their warranty offering.

Geography

Geography also plays a vital role in sizing your solar inverter due to its impact on the production of your solar panel system. Properties in Arizona have higher solar irradiances (i.e., larger amounts of solar radiation) than properties in Vermont; as such, a rooftop 6 kilowatt (kW) system in Arizona should produce more power than a similarly sized system further north.

Because these two systems will produce different amounts of DC electricity at a given time, the inverters needed to handle that electricity load can also be different sizes. In areas with more sunshine and moderate temperatures, inverters will likely be sized closer to the solar array's overall wattage so it can handle close to the maximum power output of the array at any given point. Alternatively, if your solar array experiences lower amounts of solar radiation or high temperatures that decrease panel efficiency, it's less likely to produce that maximum power output defined by the DC rating under standard testing conditions (STC). A smaller, undersized inverter may get the job done in these scenarios.

Site-specific factors

The site and design specifics of your solar array will impact the size of your solar inverter. Like geography, the tilt and azimuth your solar array is installed at will affect how much electricity the system can produce. Environmental factors (such as shading, dust, etc.) will significantly affect how much sunshine reaches the array.

Solar installers will account for these considerations, equipment efficiencies, and more when estimating the overall production of your solar panel system. All will contribute to the overall derating factor of your system, which is used to help determine what your solar panel system will produce in a real-life scenario (as opposed to the STC specs determined in a lab.) Solar panel systems that experience more shade, sit at a sub-optimal tilt, or face east rather than due south have higher derating factors than systems on sunny, south-facing roofs.

Solar panel systems with higher derating factors will not hit their maximum energy output and can afford smaller inverter capacities relative to the size of the array.

The size of your solar inverter can be larger or smaller than the DC rating of your solar array, to a certain extent. The array-to-inverter ratio of a solar panel system is the DC rating of your solar array divided by the maximum AC output of your inverter. For example, if your array is 6 kW with a 6000 W inverter, the array-to-inverter ratio is 1. If you install the same-sized array with a 5000 inverter, the ratio is 1.2. Most installations will have a ratio between 1.15 to 1.25; inverter manufacturers and solar system designers typically do not recommend a ratio higher than 1.55.

Below are some examples of solar inverter products and their maximum DC power output recommendation:

A higher array-to-inverter ratio may work for your system if your solar panels do not produce their maximum power output due to the abovementioned factors. The benefit of oversizing your solar array relative to the inverter capacity is that lower-wattage inverters will be less expensive than their larger counterparts. But it's not advisable to oversize your array too much as it can cause clipping. Clipping occurs when your solar panels produce too much DC for the inverter to handle at a given time. When this happens, the inverter will limit the amount of energy it's converting, resulting in power losses from your solar panel system.

On the other hand, you don't want to install a solar inverter that's too big (i.e., has a lower array-to-inverter ratio) because your inverter will be most efficient if it's running close to its overall capacity. If the inverter is too large compared to the array, it will not produce the desired amount of electricity.

With microinverters, the conversion from DC electricity to AC electricity occurs at each panel. Microinverters are smaller than large central inverters devoted to handling power for an entire system. As such, the size of a microinverter corresponds to the energy output of the solar panel it's converting power for rather than the DC rating of the entire system.

Similar to central inverters, manufacturers of microinverters will list guidelines as to the maximum DC rating a panel should have if tied to their microinverter product. Clipping will occur if you connect a higher wattage panel than the microinverter specs indicate.

The best way to ensure your solar inverter is sized appropriately is to work with a qualified, reputable solar installer. You can sign up on the EnergySage Marketplace to receive multiple quotes online from pre-screened, vetted installers. These EnergySage Approved installers will use design tools to ensure your solar array and inverter are sized according to your electricity needs, solar equipment, property, and geography.