Considering Solar in RI

[riken]

I've gotten quotes from 6 companies and I've narrowed it down to 3 - Direct, RGS, SolarFlair. Direct and RGS quotes include LG300 panels, SolarFlair - Canadian CS6K-280M. All suggest Enphase M250 Micro Inverters. I have a 14 degree, 200 azimuth roof they will go on. My question is are 300 W panels overkill considering that I'll have only 250 W inverters or because of the loss of efficiency due to the poor roof angle the 300 W panels are a good thing? If I'm understanding azimuth charts correctly at best I'll get about 70% efficiency in the winter and 90% in the summer at high noon.

[J.P.M.]

I've gotten quotes from 6 companies and I've narrowed it down to 3 - Direct, RGS, SolarFlair. Direct and RGS quotes include LG300 panels, SolarFlair - Canadian CS6K-280M. All suggest Enphase M250 Micro Inverters. I have a 14 degree, 200 azimuth roof they will go on. My question is are 300 W panels overkill considering that I'll have only 250 W inverters or because of the loss of efficiency due to the poor roof angle the 300 W panels are a good thing? If I'm understanding azimuth charts correctly at best I'll get about 70% efficiency in the winter and 90% in the summer at high noon.

On inverter sizing: Run PVWatts w/ 10 % system losses, change to a 1.2 AC to DC size ratio from the advanced parameters and choose the hourly output option. Then go back and change the AC - DC to 1.0 and look for differences in the annual output from the 1.2 ratio. That'll be a SWAG at annual clipping penalty.

At that az., tilt and location, I doubt a 300 Watt panel will get close to 250 Watts output much, if at all.

Why not use a properly sized string inverter ? If nothing else, usually or often a bit less initial cost, and some would argue more reliability for lots of reasons.

[riken]

Thanks for the info, my head is spinning. I think I need to do some more homework. I did as you suggested and the difference on June 21st noon was about 4 W and December 21 noon was about 7 W. Of course I don't know what that means. Are you saying 300 W panels are not overkill?

[J.P.M.]

Thanks for the info, my head is spinning. I think I need to do some more homework. I did as you suggested and the difference on June 21st noon was about 4 W and December 21 noon was about 7 W. Of course I don't know what that means. Are you saying 300 W panels are not overkill?

You're most welcome.

No. Overkill, in one sense, is when a system is less than optimally sized on the high side so that it fails to provide the least long term life cycle cost mix of POCO and PV provided power for the design load it needs to meet.

I'm simply suggesting that at your location and array orientation, a 300 W panel will not put out 300 W for more than a few min. or so per year, if ever, and while it may put out 250 W more often, it will probably not do so often enough or long enough to make a 250 W micro undersized. That is, 300 W panels at your location and orientation probably won't clip too much w/a 250 W micro.

But, a system with a correctly sized string inverter will probably not clip at all, for what's probably a few less $$ up front, and for perhaps a lower probability of problems down the road as a consequence of fewer parts (1 inverter instead of 1 inverter per panel), better protection for the electronics (in the semi controlled environment of a garage for example, rather than a bunch of electronics on a roof with a 100 F.+ annual temp. variation and exposure to the elements and critters on a roof), and easier access for serviceability if/when inverter problems arise. Besides, the novelty of individual panel monitoring usually wears off quickly, and so, if 1 panel of say 12-20 or so fails, the small loss in output may go unnoticed. If a string inverter fails, the chances of that failure going unnoticed are probably a lot less.

FWIW, on homework: Knowledge is power. I'd suggest, as I often do, to download (for free) "Solar Power Your Home for Dummies" from the net, or spring for ~ $25 for an updated version at bookstores or Amazon. You may want to delay committing until you get more informed.