Hi everyone,

I'm looking for help calculating the PV system wattage/size as I'm beginning to research an upcoming project.

I've used a number of online solar power calculators in the past but would love to see the actual mathematical formula and breakdown for calculating a system's wattage based on my monthly kwh usage.

I'd love to see what factors people use to compensate for efficiency as I know people like to also round the system wattage by approx 20% or so. I've already found a breakdown of minimum sun hours by region and will be plugging that into the formula



Thanks guys!

I think the number you're actually looking for is the accumlated daily/monthly kwh for your location & mounting angle, not instantaneous power output?

It's not a mathematical formula- it's highly variable dependent on actual weather. To some extent it's dependent upon the type of photovoltaic you're using (silicon drops off dramatically below some light intensity, and is more angle-sensitive than most thin-film/amorphous silicon technologies.) The efficiency of your inverter or charger isn't the same under all loading conditions either, eh? PV also needs to be significantly derated at higher temps too, also hard to quantify in a simple formula model.

But for the raw average daily incident energy per square meter for you location you can refer to the data here:

http://rredc.nrel.gov/solar/pubs/redbook/

Pick the data for the state & city nearest you, the mounting angle of your panels, and use the panel manufacturer's efficiency & panel-area numbers to compute YOUR daily-by-month or annual uptake- it'll be pretty close (on average. :-) ) Subtract 10-15% if you live in a hot climate where the highest-gain months correlate with typical afternoon temps over 90F. Add 10-15% if you live at altitudes over 6k' with lots of sunny snow-cover.

But it's all just a "pretty-good guesstimate" in the end- you won't ever get a full 2-significant digits of accuracy out of it.

Thanks Dana1, I appreciate the feedback and definitely wasn't expecting anything to give me an accurate conversion as I obviously left out a number of variables (battery bank, charge controller, offsetting the bill completely, inefficiencies etc).

I think I've come up with a fairly basic estimate that at least will put me in the right ballpark in case an installer tries to over quote me.

If you haven’t already been over to Wind & Sun's forums there are a lot of knowledgeable RE people there. Once you get some numbers you could post them there and get some feedback. Most of the users there have systems themselves, some grid tied batteryless, some grid tied with batteries (what I have) and some off the grid.

thanks guys, i really appreciate it!

I found this link when trying to find a solar power wattage sizing calculator.

Anyone know what formula they're using to calculate the system wattage sizing alone based on monthly kwh and sun hour?

Would really appreciate it!

Hi @ All

Good informations on this website: http://www.sma.de/en_US.html

PV Watts is a great program for sizing systems and output. It uses long term climate data along with options you add to give accurate annual output. It allows for a series of variables like  efficiency,angle and orientation. I don't know the formular they use but it appears to be pretty accurate based on systems I  have seen actual data on amount collected verses modeling.

http://www.nrel.gov/rredc/pvwatts/version2.html

Tom Pittsley
[email protected]
www.eebt.org