PV Watts Calculation

Ian S
Ian S Solar Expert Posts: 35 ✭✭
in Advanced Solar Electric Technical Forum #1
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 11,768-13,006 kWh.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #2
    Re: PV Watts Calculation
    Ian S wrote: »
    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 11,768-13,006 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.
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  • Ian S
    Ian S Solar Expert Posts: 35 ✭✭
    #3
    Re: PV Watts Calculation
    inetdog wrote: »
    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.
    Well, I'm not sure what statistical data PV Watts uses but based on historical data for June from this site, it appears that daytime temperatures were maybe a little higher than normal. June is a dry month with an average of 0.02" rain. None was recorded this past June. I'm only using the derate factor a kind of a "fudge" factor to try to get a prediction of the rest of the year's performance based on only one month actual data so far.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #4
    Re: PV Watts Calculation
    Ian S wrote: »
    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 11,768-13,006 kWh.
    PVWatts 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?
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  • Juram
    Juram Solar Expert Posts: 36 ✭
    #5
    Re: PV Watts Calculation

    Avg. derate factor for SunPower in Arizona is 0.90-0.92. With good southern exposure you can utilize a factor of about 1.85 x (DC system size) to fairly accurately discover production.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #6
    Re: PV Watts Calculation
    Juram wrote: »
    Avg. derate factor for SunPower in Arizona is 0.90-0.92. With good southern exposure you can utilize a factor of about 1.85 x (DC system size) to fairly accurately discover production.
    That's interesting. Why would it be any different for Arizona than anywhere else? Climatic conditions are folded into the data upstream of the derate.
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  • Ian S
    Ian S Solar Expert Posts: 35 ✭✭
    #7
    Re: PV Watts Calculation
    ggunn wrote: »
    PVWatts 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?
    Actually, as part of my lease, I am guaranteed that first year production I quoted. They have a table for each year of the entire 20 year period of the lease. Of course, if my production falls short, they only pay me $0.03/kWh of shortfall. However, I figure they would be conservative in their estimate since they are on the hook. I do not have the derate that the installer assumed. I realize of course that there could be anomalous months of production and as time progresses, I'll get a better handle on overall performance of my specific system. However, by historical standards, last month was pretty typical of a June in Phoenix. Now July is looking perhaps atypical compared to the last few marginal monsoon years - we just had a period of four consecutive days when it rained, something that hasn't occurred since 2008 - but may be more in line with the decades-long average.
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  • Ian S
    Ian S Solar Expert Posts: 35 ✭✭
    #8
    Re: PV Watts Calculation
    Juram wrote: »
    Avg. derate factor for SunPower in Arizona is 0.90-0.92. With good southern exposure you can utilize a factor of about 1.85 x (DC system size) to fairly accurately discover production.
    Interesting! I hope you're right because I'll be quite happy with 12.7 MWh of annual production. That will get rid of all my peak charges and a lot of my off-peak.
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  • Drastyn
    Drastyn Registered Users Posts: 2 ✭
    #9
    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.

    Hey Ian, yes PV Watt is accurate, what I suggest you is compare the PV Watt estimation with PV Compare real outputs, you can insert your area , inclination and orientation and try to find pv-plants in the same area with similar characteristics. I don’t know if you can find also a Sunpower plant , obviously in that case ti will be perfect! However, this is just a “something extra" coz PV Watt is more than enough.
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