[stevehull]

Roostre,

I assumed you have a roof pitch of 14 degrees (3/12 pitch). At least it looks like that from the photos.

Here is the solar radiation data for your specific area using the SLC airport (PVWATTS) using units of kWhr/M**2/day.

This unit is also called a "solar hour" as it represent the average amount of 100% sunshine hours you would get on an average monthly day hitting a panel at 14 degrees facing 180 degrees south.

For January, this means, on the average January day, you have about 2.55 hours of 100% sunshine "hitting" a PV panel at 18 degree pitch facing due south. This PVWATTS summary includes rain/clouds, etc so is a conservative number to start with. For the month, you multiply by the number of days in that month to get the total monthly solar "sunshine hours".

Note the July data where you get almost 8 total hours per day. In summer, you "store away" kWhrs for winter use and then it all gets wiped out in March where you pay some - or loose those banked kWhrs. The key is not to overproduce in the summer so you don't loose too much every March.

For estimates a 10kW system (40 panels producing 250 watts max) would give you about 17,000 kWhrs a year. If history plus your EV use shows a yearly use of 20,000 kWhrs, you would reduce your kWhr consumption by some 85%.

Your EV cars consume 25 kWhrs per day and that accounts for some 9000 kWhrs yearly (25 x 365). This may be a bit high as I counted every day, but on a M-F basis this is still some 6500 kWhrs. For an average call your EV consumption 7500 kWhrs per year. This needs to be added to the historical utility data (as I did above).

But 250 Watt panels in reality only put out 90% of that in real world conditions. That is why I couple 265 W panels with the M215 (that actually puts out 225-230 watts maximum. 90% of 265 = 239 W which is a perfect match for the Enphase M215.


Jan 2.55
Feb 3.65
Mar 4.65
Apr 5.64
May 7.03
Jun 7.34
Jul 7.60
Aug 7.14
Sep 5.95
Oct 4.63
Nov 3.05
Dec 2.10

average of 5.11

Now, we just need an estimate of those monthly kWhrs. If you could even get the YEARLY total, that would be a huge help. We don't want to put up more solar panels than necessary.


Steve
ps I have always gotten this data by asking for a supervisor if need be.