Am I Getting the Correct Amount of Power?

SystemSystem Posts: 2,511 admin
Hi - New to the forum. Looks like a great place for information. I had my system installed in October, and have enjoyed watching the daily KWHs rise as the days get longer, and sun gets higher in the sky.

I have a few questions about the performance of my system, and if it's producing a reasonable amount of power, given the specifics of my installation.

The most power i've seen generated so far is 6.7KW. My system details are as follows:
7.650KW system (34 Sun Power SPR225 panels)
2 - SunPower inverters (1 SPR-3000m and 1 SPR-4000M)
Mounted on south facing roof with no shade.
Lattitude 40° 45'
Roof pitch - 23 degrees.

Also was interested in what the panel "Power Temp Coeficient" is and what the impact is. My panels are -.38% %/degree C. I guess this means that as the ambient temperature goes up the panel output goes down.

Thanks in advance for any insight.



  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Am I Getting the Correct Amount of Power?

    Welcome to the forum.

    Yes as panel temperature goes up output goes down. That's unavoidable.
    Getting 6.7 kW out of 7.6 kW of panel isn't bad. But the real numbers that matter are the kilowatt hours. You can get a fairly good estimate of what you might expect to 'harvest' by using PVWatts:
    It will allow you to adjust variables too, and see if things like changes in panel angle will help maximize production.
  • SlimDieselSlimDiesel Posts: 17Registered Users
    Re: Am I Getting the Correct Amount of Power?

    That is a bit low but then you haven't said how your panels are divided into strings and groups to feed your unequal size inverters. If you have somewhere around 5% (assuming a 95+% efficient inverter) more panel-watts than the inverter's nameplate-watts you should be getting the nameplate output from it.

    For example, I have 18 (2 strings of 9) 230 watt panels also on a 23 degree, shade-free roof (although I am 3 degrees farther north than you). This is 4140 watts of panel and last weekend in a cloud-free sky at solar noon the inverter was showing about 4180 watts from the array and 3950 watts AC out (the 'derating' light was flashing so it that was as high as it would go) from my 3.9kW nameplate inverter. The temp coefficient was in my favor too, since it was a breezy -8 degrees Celsius.

    So maybe you have more panel on one inverter than it can really use and not quite enough on the other just because of the way they had to be distributed.
  • SlimDieselSlimDiesel Posts: 17Registered Users
    Re: Am I Getting the Correct Amount of Power?

    I just ran some numbers and that is indeed probably the case. The specs on the panel are 48.5 Voc so the arrays for each inverter have to be in two parallel strings (4 strings total) so as no to exceed the Vmax of the inverter (500VDC for the 3000, 600VDC for the 4000).

    That means you probably have them connected either as 7x2 + 10x2 or 8x2 + 9x2. In the first case that comes out to 3150 watts and 4500 watts and in the second thats 3600 watts and 4050 watts. For the first case, that's close to the 3000's max DC power of 3250 watts to get max output from it but the 4000 is going to leave about 200 watts on the table from its array (its max input is 4300 watts). For the second case, the 4000 will be a bit short to achieve max output but the 3000 will leave about 350 watts of its array's potential unused.

    Thus, they're probably in a 7x2 ==> 3000 and 10x2 ==> 4000 configuration and you are wasting at least 200 watts off the peak. Not to worry too much... for the inverter that is a bit over-paneled at the peak, they are going to help under less than full sun/solar noon conditions... the 4000 will stay on its max output plateau a bit longer than the 3000 will.
  • System2System2 Posts: 6,290 admin
    Re: Am I Getting the Correct Amount of Power?

    Thanks for the info. Last month was the first where I produced more KWH then I used. It was great to get into the "black" on my net metering. The system was sized to cover my annual usage. I'll be curious after a year how close that estimate is.
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