Solar panels are a weather-dependent technology. Although solar electric panels will generate electricity in all weather, the amount is highly dependent on the irradiance, temperature, visibility, and other factors. It should be no surprise that solar panels work better when it is sunny and not so great in cloudy weather.
The good news is we have tons of historical data that can easily be used to model system performance from month to month and year to year. The annual energy output is very consistent. While panels degrade over time, the annual output is very predictable with little variation. The monthly variation, on the other hand, can be all over the map. In fact, sometimes you have an outlier month, like we had recently in January 2024. Check out this video that shows how weather can impact solar panels.
It’s not all bad, however. As mentioned in the video, you will also get months where almost every day is better than the averages. We tend to remember the streaks of poor weather and storms, but it’s easy to forget how we can have a glorious March or April in Southwest Florida where clouds and rain seem like distant memories.
You will get a predictable output from your solar panels despite good days and bad days. With netmetering, it doesn’t really matter much. Your credits carry over from month to month, and ultimately what matters is the long-term reduction of your energy bills.
Trying unsuccessfully to find the right kind of weather person to ask whether a +/- 5% variation in annual solar production is reasonably due to weather in 95404 (yes, a long way from Florida).
NREL’s PVWATTS manual indicates that the error may be as high as +/- 10% for annual energy totals, but it should average out over the long term. I imagine the error in specific areas may be higher or lower depending on how variable the weather is where you live.