Why solar installers might oversize a solar panel array

Please see an updated version (2024) of this article here >

There could be multiple reasons why solar installers might recommend an inverter size that is smaller than your solar array size, but don't be alarmed. For instance, a 3kW rated inverter with 4kW's worth of solar panels (e.g. 16 x 250W solar panels). Normally, 3kW's worth of solar panels would be matched with an inverter rated for 3kW. Confused? Solar engineers are always looking to improve system performance. Let me explain further €¦

Solar panels don't necessarily produce as much power as their rating; a 200W panel seldom produces 200W of solar power. Solar panels receive their rating value in Standard Test conditions (STC), the ideal test environment. The STC is 25 degrees, with 1,000W per meter square of sunlight. Roof angles and solar panel tilts can also affect the production of power.

Therefore a solar array that is 4kW will seldom produce 4kW, even on a sunny day the loss can be minimal. Below is a graph of a solar power system's daily power generation(estimated). Notice that only a small portion of the energy is lost with a smaller 3kW inverter.

Reasons for oversizing a solar array include:

• A  3kW inverter is cheaper than a 4kW inverter
• Although the input of the system will be 3kW, over 12 months the total power loss is marginal.
• Due to power losses (e.g. dirt on panels, power losses via the wiring), a solar power system won't produce the stated rating value.
• Additional solar panel capacity allows the inverter to run at a higher average efficiency (inverters are most efficient when running at full, or near full capacity).
• If your local lines company restricts your system size, e.g. maximum 3kW rated system, you can have a 3kW inverter and oversized solar array.

The power loss from oversizing a solar array will be much less over 12 months, in proportion to what is shown in the graph above. On cloudy, dark days the production of the system won't reach 4kW.

Inverter manufacturers will determine if a system can be oversized, and the limit to which it can be exceeded. If the specified amount is exceeded, the warranty is void. Generally, a system can't be oversized by more than 30%.

Hopefully, we've helped with understanding on how oversizing arrays can be used to your advantage.  Reputable solar installation companies have the best advice and recommendations, such as whether to oversize an array or not.

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