Understanding your daily output
The real swing between clear and overcast days
A clear day typically produces about 3.5 times more output than a fully overcast day for the same system. In June at 38 degrees latitude, a system averaging 37 kWh a day might produce around 46 kWh on a clear day and only 13 kWh on an overcast one. This range is normal and doesn't signal a problem with your system. A DOE federal PV performance study compared measured daily production against modeled estimates across 75 systems to establish realistic day-to-day expectations.
Why a daily figure adds something the monthly number doesn't
A monthly average smooths over a lot of day-to-day variation that matters for things like sizing a battery for daily backup or understanding why your production app shows wildly different numbers from one day to the next. If you check your production app and see a number well below the monthly average, it's worth checking whether that day was simply cloudier than usual before assuming something's wrong. That same day-to-day swing also changes what each kilowatt-hour is worth, since a system that produces less on cloudy stretches delivers fewer billable units overall. The cost per kWh produced calculator turns a daily output figure like this one into a per-unit cost.
What counts as each weather day type
A partly cloudy day with intermittent sun breaks typically produces around 85% of the month's average daily output, since scattered clouds still let a good amount of both direct and diffuse light through over the course of the day. Check the cloud cover reduction tool for a deeper look at how cloud density specifically changes this figure. NOAA's own research on clouds and solar output even documents cases where broken cloud fields briefly push output above clear-sky levels, a phenomenon called cloud lensing, though that's the exception rather than the rule on a typical partly cloudy day.
Whether the day-type pattern holds across seasons
The relative difference between clear, partly cloudy, and overcast days stays roughly similar across months, but the baseline average shifts a lot with the season, so a clear day in December still produces less than a clear day in June. The day-type ratios in this tool apply on top of whichever month you select.
What to do with this number next
Check the peak sun hours calculator for a related daily metric, or head back to the monthly estimator to see how this day fits into the bigger picture.