Advanced solar energy power + ROI calculators?

jms_az
jms_az Registered Users Posts: 12 ✭
in General Solar Power Topics #1
I'm trying to decide the right size system for me + would like to do my own due diligence in addition to what the installation companies recommend.

Are there any advanced calculators / math toolboxes for solar energy power + ROI calculations, that include the math equations they utilize? (so I could run some calculations myself)

I found the one here: http://www.solar-estimate.org/?page=solar-calculator but I need to see what the incremental cost/benefit is for making a system size larger or smaller. I live in the Phoenix area and can fit maybe 6-7kW on my south-facing roof slope (have more area on east/west slopes but unsure whether it's worth adding the extra area).

But I am tired of calculators that I'm not sure if they're making the right assumptions (and more tired of salespeople telling me I want a 10kW or a 12kW system) so I would like to figure out what makes sense myself.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: Advanced solar energy power + ROI calculators?

    Welcome to the forum Jason,

    The two things to get a handle on first is your daily/monthly electrical loads (kWH per day/month) and your billing plan.

    It turns out that for Grid Tied Solar, your utility's billing plan can have a huge impact on your rates of return... And the utilities are starting to change the billing plans to be less "customer friendly". There may now be a race to get your solar GT system installed/turned on and grandfather one of the older/more consumer friendly plans. The new plans will make GT solar (and even conservation) less attractive.

    For example, in Northern California I have 1 year net metering with time of use power... I "buy" power at ~$0.10 per kWH and sell my "excess" summer power at $0.30 per kWH (If I used/generated a lot of electricity, my summer TOU afternoon rates are near $0.50 per kWH). And I have a $4.50 per month minimum bill.

    So--I "sell" to my utility $0.30 per kWH summer afternoons and "buy" power off peak at $0.10 per kWH... And I only pay $4.50 per month (assuming I sell more than I buy).

    That is a "consumer friendly plan". What is now happening in some regions of the US, the utiltiies are looking at, or have implemented a plan were you buy power at $0.15 (just a guess) per kWh and they buy your excess at $0.03 to $0.07 per kWH). And they have a $40-$100 per month monthly "service charge".

    In this case, you get very little money for excess power you generate and have a large, minimum monthly bill (for infrastructure, billing, service) that may be ~50% of your "normal" electrical bill.

    So, no matter what you do, you still have that fixed monthly charge and, at best, get a few pennies per kWH "in the bank".

    The ROI calculators--They have a hard time dealing with time of use plans (it all depends on how much and what time of day you use power, and what time you generate excess power).

    So, I cannot give you a generic answer. And I have not looked at ROI calculators for a decade--So I don't know what they take into account. In general, Solar GT systems cost of power is pretty cheap--on the order of the cost of utility power (and in some areas like mine, cheaper than utility power).

    Some Arizona utilities are starting to fight and implement the highly minimum monthly service charge billing plans (I will argue that these are probably justifiable in terms of utility costs for infrastructure and avoiding driving up electricity costs for their non-solar customers/businesses)....

    Have you studied your rate plans and understand how the utility will charge/credit your bill with GT solar? And will they grandfather the basic rate plan for 20 years?

    I still believe that conservation (energy star appliances, lots of insulation, turning stuff off when not used, shading of windows/walls/roofs, etc.) can all have as good or better rates of return vs GT solar (conserve first, spend money for GT solar second).

    We can certainly help you with the calculations of how much energy the system will produce--And if you can tell us your utility, there may be some posters here that can help you understand your rate plan.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • jms_az
    jms_az Registered Users Posts: 12 ✭
    #3
    Re: Advanced solar energy power + ROI calculators?

    >We can certainly help you with the calculations of how much energy the system will produce--And if you can tell us your utility, there may be some posters here that can help you understand your rate plan.

    Recommendations are helpful but I really want to get into some heavy number crunching.

    My utility is SRP (Salt River Project) and they have smart meters on many homes in my area so I can look up daily and even hourly usage. I would like like to calculate average power production for various sized systems based on the direction of my roof (part of it is a few degrees from south, I'm not sure of the slope angle but I can measure that too) and how the insolation and power production (temperatures here often exceed 100 degrees between May and September, so that cuts down the solar efficiency) varies with the day of the year and hour of the day, so that I can see what size system makes sense to me and which time-of-use plan would make the most sense. (SRP has a 3-6pm time of use plan, as well as a 1-8pm May-Oct and a 5-9am,5-9pm Nov-Apr time of use plan.)
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #4
    Re: Advanced solar energy power + ROI calculators?

    OK--Then use PV Watts setup for each "plane" of solar array/gt inverter. PV Watts has an output hour by hour CSV output. In general, if your degrees are withing 10 degrees of true, you should be "close enough" for solar work:

    http://rredc.nrel.gov/solar/calculators/pvwatts/version1/

    Here is a 10 kW fixed array for Phoenix, default settings, for the first three days of the year. Remember that the data is probably +/- 10% for long term (decade averages)--And daily/weekly actually numbers may be a lot a lot less (my array will output only ~5% or less for a couple days straight for dark weather).

    The data below is "real data" from a statistically "typical" day from the hourly measurement period of several decades (the first few days are from 1988).
    "PVWATTS: Hourly PV Performance Data"
    "City:","PHOENIX"
    "State:","Arizona"
    "Lat (deg N):", 33.43
    "Long (deg W):", 112.02
    "Elev (m): ", 339
    "Array Type:", "Fixed Tilt"
    "Array Tilt (deg):", 33.5
    "Array Azimuth (deg):", 180.0
    "DC Rating (kW):", 10.0
    "DC to AC Derate Factor:", 0.770
    "AC Rating (kW):", 7.7

    "Year", "Month", "Day", "Hour", "AC Power (W)"
    1988, 1, 1, 01:00, 0
    1988, 1, 1, 02:00, 0
    1988, 1, 1, 03:00, 0
    1988, 1, 1, 04:00, 0
    1988, 1, 1, 05:00, 0
    1988, 1, 1, 06:00, 0
    1988, 1, 1, 07:00, 0
    1988, 1, 1, 08:00, 0
    1988, 1, 1, 09:00, 310
    1988, 1, 1, 10:00, 4560
    1988, 1, 1, 11:00, 5661
    1988, 1, 1, 12:00, 6722
    1988, 1, 1, 13:00, 7219
    1988, 1, 1, 14:00, 6349
    1988, 1, 1, 15:00, 3245
    1988, 1, 1, 16:00, 1439
    1988, 1, 1, 17:00, 222
    1988, 1, 1, 18:00, 0
    1988, 1, 1, 19:00, 0
    1988, 1, 1, 20:00, 0
    1988, 1, 1, 21:00, 0
    1988, 1, 1, 22:00, 0
    1988, 1, 1, 23:00, 0
    1988, 1, 1, 24:00, 0
    1988, 1, 2, 01:00, 0
    1988, 1, 2, 02:00, 0
    1988, 1, 2, 03:00, 0
    1988, 1, 2, 04:00, 0
    1988, 1, 2, 05:00, 0
    1988, 1, 2, 06:00, 0
    1988, 1, 2, 07:00, 0
    1988, 1, 2, 08:00, 0
    1988, 1, 2, 09:00, 2298
    1988, 1, 2, 10:00, 3849
    1988, 1, 2, 11:00, 4824
    1988, 1, 2, 12:00, 6738
    1988, 1, 2, 13:00, 6043
    1988, 1, 2, 14:00, 4781
    1988, 1, 2, 15:00, 5371
    1988, 1, 2, 16:00, 2328
    1988, 1, 2, 17:00, 488
    1988, 1, 2, 18:00, 0
    1988, 1, 2, 19:00, 0
    1988, 1, 2, 20:00, 0
    1988, 1, 2, 21:00, 0
    1988, 1, 2, 22:00, 0
    1988, 1, 2, 23:00, 0
    1988, 1, 2, 24:00, 0
    1988, 1, 3, 01:00, 0
    1988, 1, 3, 02:00, 0
    1988, 1, 3, 03:00, 0
    1988, 1, 3, 04:00, 0
    1988, 1, 3, 05:00, 0
    1988, 1, 3, 06:00, 0
    1988, 1, 3, 07:00, 0
    1988, 1, 3, 08:00, 0
    1988, 1, 3, 09:00, 2408
    1988, 1, 3, 10:00, 4397
    1988, 1, 3, 11:00, 5822
    1988, 1, 3, 12:00, 6355
    1988, 1, 3, 13:00, 6159
    1988, 1, 3, 14:00, 4851
    1988, 1, 3, 15:00, 1685
    1988, 1, 3, 16:00, 505
    1988, 1, 3, 17:00, 4
    1988, 1, 3, 18:00, 0
    1988, 1, 3, 19:00, 0
    1988, 1, 3, 20:00, 0
    1988, 1, 3, 21:00, 0
    1988, 1, 3, 22:00, 0
    1988, 1, 3, 23:00, 0
    1988, 1, 3, 24:00, 0
    ...

    Is that what you are looking for?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • jms_az
    jms_az Registered Users Posts: 12 ✭
    #5
    Re: Advanced solar energy power + ROI calculators?

    Excellent! Yah, I'd like to run the numbers myself + if they are close to what I'm getting for the summary stats of the solar-estimates.com site or other calculators, then I can play around with the time of use stuff.

    Are these based on average data (i.e. including the effects of cloudy days for the lat/lon in question) or peak power assuming sunny days?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #6
    Re: Advanced solar energy power + ROI calculators?

    As I understand... They looked at ~20 years worth of January 1st's at that physical lat/lon (basically a weather station with a pyrometer). And then used the data from the "most typical" day. And figured out which one was "typical" for that twenty year period (including weather/haze/smog/etc.).

    Then they used software to account for panel position vs sun location in sky (these are all flat panel based calculations--and 1 or 2 axis flat panel trackers)...

    Then they looked and January 2nd's...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #7
    Re: Advanced solar energy power + ROI calculators?

    Well AZ is different than California in that they credit kWh and not $$$$. SRP has the issue of zeroing out the credit in April just before the max load season. Check the SRP web site for more applicable information. here is the net metering rider:
    http://www.srpnet.com/environment/earthwise/pdfx/RenewNetMeterRider0510.pdf
    And SRP FAQ:
    http://www.srpnet.com/environment/earthwise/solar/electricfaq.aspx

    There are several others here on SRP, I can only speak for how APS works in detail. At least APS does the annual zero out in Dec. allowing one to build up some credit up in the spring for summer consumption, APS credit is applied to the TOU time frames that the power was generated, I.E. On-peak generation can only offset on-peak loads.
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  • jms_az
    jms_az Registered Users Posts: 12 ✭
    #8
    Re: Advanced solar energy power + ROI calculators?
    solar_dave wrote: »
    Well AZ is different than California in that they credit kWh and not $$$$. SRP has the issue of zeroing out the credit in April just before the max load season. Check the SRP web site for more applicable information. here is the net metering rider:
    http://www.srpnet.com/environment/earthwise/pdfx/RenewNetMeterRider0510.pdf

    Anyone here with SRP net metering using a TOU plan? If you generate power during on-peak loads does it only carry over to cancel out other on-peak loads, or does 1kWh = 1kWh for the purposes of net metering?
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  • Jburgess
    Jburgess Solar Expert Posts: 130 ✭✭✭
    #9
    Re: Advanced solar energy power + ROI calculators?

    Excess peak production is only used to other offset peak usage. The same for off peak with SRP.
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  • photonik
    photonik Registered Users Posts: 2
    #10
    Re: Advanced solar energy power + ROI calculators?

    Jason,
    you could try this one: http://pvcalc.org/pvcalc. Equations are not included, but there are many options which are explained and you get various graphs.
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  • solarix
    solarix Solar Expert Posts: 713 ✭✭
    #11
    Re: Advanced solar energy power + ROI calculators?

    You can sign up with Ongrid Solar that costs $100/month for using about the best solar estimator around.
    There is a lot of complexity in doing these numbers, I spent 2 months building my own spreadsheet that only works here with my utility.
    Lot cheaper to use Andy Black's.
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  • courtneydevine
    courtneydevine Registered Users Posts: 1 ✭
    #12
    solar_dave said:
    Re: Advanced solar energy power + ROI calculators?

    Well AZ is different than California in that they credit kWh and not $$$$. SRP has the issue of zeroing out the credit in April just before the max load season. Check the SRP web site for more applicable information. here is the net metering rider:
    http://www.srpnet.com/environment/earthwise/pdfx/RenewNetMeterRider0510.pdf
    And SRP FAQ:
    Resource page

    There are several others here on SRP, I can only speak for how APS works in detail. At least APS does the annual zero out in Dec. allowing one to build up some credit up in the spring for summer consumption, APS credit is applied to the TOU time frames that the power was generated, I.E. On-peak generation can only offset on-peak loads.


    Thanks Dave, these SRP resources are really helpful. It clarified lots of doubt for my future solar installation planning. 

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