Understanding actual vs expected production
Why comparing to a fixed expectation beats gut feeling
It's easy to notice a system "feels" like it's not producing much, but that impression is unreliable without a baseline to compare against. Your original design estimate, whether from PVWatts or your installer's proposal, gives you that baseline, the same reference point used when weighing options in the inverter efficiency curve comparison tool. Comparing actual monitored kWh against that expected figure for the same period turns a vague feeling into a concrete number you can act on.
Why the thresholds in this tool aren't arbitrary
Solar production naturally varies with weather, even across otherwise identical periods, so a small gap from expected is normal and not a sign of a problem. This tool flags anything within about 5% of expected as normal variance, a 5-10% gap as worth monitoring over the next period or two, and a gap beyond 10% as worth investigating, roughly reflecting how much day-to-day and month-to-month weather variability typically accounts for on its own.
Why a single low period usually isn't the whole story
One cloudy month can easily produce a 10-15% gap from expected without anything being wrong with the system. What separates ordinary weather variance from a real issue is persistence: if the gap shows up again the next period, or compounds across several consecutive periods, that consistency is the signal worth acting on rather than any single reading. Comparing the same month year-over-year, which controls for seasonal sun angle differences, is one of the more reliable ways to catch a genuine decline, and a persistent gap is exactly what the monthly bill reduction estimator can help translate into an actual dollar impact on your bill.
What a persistent gap might mean
A consistent underperformance gap can point to several possible causes: new shading from tree growth, a failed string or MPPT channel, dirt or debris buildup, an inverter operating above its ideal temperature range (worth checking against the inverter noise and heat derate calculator), or occasionally a monitoring or metering discrepancy rather than an actual production problem. If you suspect the monitoring data itself is unreliable, the monitoring system cost comparison tool covers what it costs to upgrade to a more capable platform. This tool flags that something is worth a closer look; diagnosing the specific cause is a separate step, often best done with your installer or the monitoring platform's diagnostic tools.
What this tool doesn't cover
This is a simple period-over-period comparison, not a full performance ratio analysis or degradation model. It doesn't account for irradiance data specific to your exact site and dates, so treat the status flag as a starting point for investigation rather than a definitive diagnosis. If wiring or resistive losses are a suspected factor, cross-check with the AC cable loss calculator and DC cable loss calculator; for a persistent, unexplained gap beyond that, the Department of Energy's inverter and grid services overview or your installer can help you pull more detailed diagnostic data than a simple actual-vs-expected comparison provides.