PV Watts Calculation
Ian S
Solar Expert Posts: 35 ✭✭
I have my new Sunpower prepaid lease system (6.9 kW) up and running and I'm anxious to see how it's doing compared to what the original estimate/guarantee of annual power claimed. I can put all the factors into the PV Watts calculation but the only one that's a real question mark is the derate factor. Can I "bootstrap" an estimate of the derate by using the default derate for PV Watts then using my actual June production here in Phoenix to apply a correction based on the June estimate provided by the default PV Watts calculation? I know it assumes that this past June is an "average" one but still may be a useful way to get an estimate early on of my system's performance. It also depends on how good PV Watts is at predicting the relative production when comparing month to month for a given location. I assume that's pretty accurate.
FYI, when I did the calculation i.e. June_actual/June_default*Derate_default, I got a new derate factor of 0.92 and an annual PV Watts production estimate of 12,873 kWh. This compares to my first year guaranteed annual production range of 11,76813,006 kWh.
FYI, when I did the calculation i.e. June_actual/June_default*Derate_default, I got a new derate factor of 0.92 and an annual PV Watts production estimate of 12,873 kWh. This compares to my first year guaranteed annual production range of 11,76813,006 kWh.
Comments

Re: PV Watts CalculationI have my new Sunpower prepaid lease system (6.9 kW) up and running and I'm anxious to see how it's doing compared to what the original estimate/guarantee of annual power claimed. I can put all the factors into the PV Watts calculation but the only one that's a real question mark is the derate factor. Can I "bootstrap" an estimate of the derate by using the default derate for PV Watts then using my actual June production here in Phoenix to apply a correction based on the June estimate provided by the default PV Watts calculation? I know it assumes that this past June is an "average" one but still may be a useful way to get an estimate early on of my system's performance. It also depends on how good PV Watts is at predicting the relative production when comparing month to month for a given location. I assume that's pretty accurate.
FYI, when I did the calculation i.e. June_actual/June_default*Derate_default, I got a new derate factor of 0.92 and an annual PV Watts production estimate of 12,873 kWh. This compares to my first year guaranteed annual production range of 11,76813,006 kWh.
Although this *might* give you a better estimate of the derate resulting from your panel type, inverter type, panel temperature, etc., I would feel better about it if you could at least compare the average daytime temperature and clear sky percentage for this June with the averages used by PV Watts.SMA SB 3000, old BP panels. 
Re: PV Watts CalculationAlthough this *might* give you a better estimate of the derate resulting from your panel type, inverter type, panel temperature, etc., I would feel better about it if you could at least compare the average daytime temperature and clear sky percentage for this June with the averages used by PV Watts.

Re: PV Watts CalculationI have my new Sunpower prepaid lease system (6.9 kW) up and running and I'm anxious to see how it's doing compared to what the original estimate/guarantee of annual power claimed. I can put all the factors into the PV Watts calculation but the only one that's a real question mark is the derate factor. Can I "bootstrap" an estimate of the derate by using the default derate for PV Watts then using my actual June production here in Phoenix to apply a correction based on the June estimate provided by the default PV Watts calculation? I know it assumes that this past June is an "average" one but still may be a useful way to get an estimate early on of my system's performance. It also depends on how good PV Watts is at predicting the relative production when comparing month to month for a given location. I assume that's pretty accurate.
FYI, when I did the calculation i.e. June_actual/June_default*Derate_default, I got a new derate factor of 0.92 and an annual PV Watts production estimate of 12,873 kWh. This compares to my first year guaranteed annual production range of 11,76813,006 kWh.
0.77 is a fairly pessimistic derate; on systems we are monitoring we typically see equivalent derates of over 0.8. 0.92 is exceptionally good, and I wouldn't expect that sort of performance month in and month out. If you base your expectations on a 0.92 derate you will probably be disappointed. You can certainly do what you described as long as you keep in mind that the only real data is that which is seen in retrospect.
I assume your installer used PVWatts to quote an estimate (I doubt they called it a guarantee) of what your production would be. Did they quote you the derate that they used? 
Re: PV Watts Calculation
Avg. derate factor for SunPower in Arizona is 0.900.92. With good southern exposure you can utilize a factor of about 1.85 x (DC system size) to fairly accurately discover production. 
Re: PV Watts CalculationAvg. derate factor for SunPower in Arizona is 0.900.92. With good southern exposure you can utilize a factor of about 1.85 x (DC system size) to fairly accurately discover production.

Re: PV Watts CalculationPVWatts declared accuracy is +/ 10%, which gives a lot of wiggle room. You have to take any calculations based on a month or two's numbers with a large grain of salt; you are making short term comparisons to 30 years of accumulated data.
0.77 is a fairly pessimistic derate; on systems we are monitoring we typically see equivalent derates of over 0.8. 0.92 is exceptionally good, and I wouldn't expect that sort of performance month in and month out. If you base your expectations on a 0.92 derate you will probably be disappointed. You can certainly do what you described as long as you keep in mind that the only real data is that which is seen in retrospect.
I assume your installer used PVWatts to quote an estimate (I doubt they called it a guarantee) of what your production would be. Did they quote you the derate that they used? 
Re: PV Watts CalculationAvg. derate factor for SunPower in Arizona is 0.900.92. With good southern exposure you can utilize a factor of about 1.85 x (DC system size) to fairly accurately discover production.

Ian S said:Can I "bootstrap" an estimate of the derate by using the default derate for PV Watts then using my actual June production here in Phoenix to apply a correction based on the June estimate provided by the default PV Watts calculation? I know it assumes that this past June is an "average" one but still may be a useful way to get an estimate early on of my system's performance. It also depends on how good PV Watts is at predicting the relative production when comparing month to month for a given location. I assume that's pretty accurate.
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