Output calculators vs. real system output

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System
System Posts: 2,511 admin
in Solar Beginners Corner #1
I have been looking at doing a self install of a solar system. I am an electrical engineer undergrad and mechanical engineering PhD and feel comfortable with all the mechanical and electrical work involved (including investigating local building codes), so now I am at the point of determining sizing, equipment, etc.

I found the calculator at the California Solar Initiative (http://www.csi-epbb.com/default.aspx) which seemed great because you could put in actual panels, inverter, and a few installation parameters (e.g. azimuth and tilt angle) to get a figure of output. However, as I have looked at several blogs about installations it seems that the calculator underestimates the actual output by a considerable amount. For example the setup given at http://www.angelfire.com/ca2/roderick/solar/photovoltaic.html says that their first year they really generated 6556 kWh, whereas the CSI calculator gave a figure of 4226 kWh.

So, as a beginner, I have two primary questions:
1) how accurate are a lot of the calculators out there?
2) if I were to design a rail that allowed for east/west tracking (completely open loop based on date and time), what percentage improvement does tracking provide? I see a lot of comments that it improves things, but would like to know how much as a rough percentage.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #2
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    Re: Output calculators vs. real system output

    PV Watts seems to be pretty accurate (if there is a station near you). It is based on actual measurements over ~20 years--so weather is taken into account too. It has options for 0, 1, and 2 axis trackers, plus a spread sheet output with hour by hour estimates based on your input requirements.

    And what is your definition of accuracy... It is easy to have 10% longterm variations due to weather, and upwards of +/-20% if you are in a coastal region that has variable marine layer/weather conditions.

    As a starting point, use the 77% system derating for Grid Tied power, and ~52% for off grid (battery) systems (yes, 1/2 the solar panel ratings actually gets out your AC inverter).

    As always, we first suggest measuring your current power usage (whole house, kill-a-watt type meter, etc.) and addressing those results with targeted conservation (insulation, more ceiling insulation, double pane windows, energy star appliances/air conditioner/heat pump/water heater, etc.).

    Conservation is almost always a better "investment" than spending it on a larger RE power system.

    Also, a big issue for Grid Tied is the whole rate structure for Net Metering, wholesale power replacement, etc... Depending on your local utility's rate plan for solar GT power--it can make an almost 4x difference in $$$$ equivalent in power.

    For example, some utilities only pay you "replacement wholesale power" rates--Around $0.05 per kWH.

    For me, in Northern California, I get 1 year net metering which means that my "credit" (not cash) balance from solar can range from $0.09 to $0.52 per kWH (off peak vs on peak summer afternoons and >~1,000 kWH per month billing).

    For commercial solar GT companies, the power bill vs solar GT is even more complex with demand/reservation charges and other issues. Grid Tied can make for a higher power bill than even "almost free" solar can offset.

    So, you have to decide on your power needs and when "under production" is just a minor annoyance vs a major nightmare (grid tied solar and a bit higher bill; vs and off grid installation in the middle of nowhere and fuel for the backup generator can cost $10's of gallons (or more) delivered.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
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    Re: Output calculators vs. real system output

    don't forget that they base this on past records and system specs iand are going out on a limb that the sunshine, and hence the weather, stays the same in the future. predicting pv output is not an exact science because predicting the weather is also not an exact science. i thought it important enough to point this out as engineers like to be exacting.
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  • System2
    System2 Posts: 6,290 admin
    #4
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    Re: Output calculators vs. real system output

    Thanks for this URL. I hadn't seen that calculator previously (although I probably should have with as much web searching as I have done). We are definitely trying to cut down on our usage. We have started drying all our clothes on clotheslines, turn off appliances/electronics not in use, install efficient lighting, use A/C as little as possible, etc.

    I am definitely looking at a grid-tie system. I don't see the advantage of the extreme extra cost of battery storage (which for my personal situation I see as excessive) to simply say I am "off the grid". That is the biggest reason why I wanted to know about the actual expected output of the system. We currently live in a state where the net metering only allows for banked credits with a 1 year rollover and the retail price is slightly above the national average. We want to make sure we are only generating slightly less power than we use over a 12 month period. I have a stay at home wife with small children, so there is a certain amount that gets used just by being in rooms with lights on, running the TV/computer, cooking, etc.

    If I lived in a state that bought back at retail (and the retail price was high) or had high feed-in tariffs (like Oregon) I would definitely install a system with higher capacity to help pay off the system more quickly. Even if feed-in tariffs go away soon because of up-take, getting in early means you will get it for 2-5 years and hopefully have the system paid off by then. It is amazing to me that for a 4-12 kW system that a professional installation is about 50% the cost of the system. For those who are willing and capable, there is almost no reason to not install yourself.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #5
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    Re: Output calculators vs. real system output
    jpswensen wrote: »
    If I lived in a state that bought back at retail (and the retail price was high) or had high feed-in tariffs (like Oregon) I would definitely install a system with higher capacity to help pay off the system more quickly. Even if feed-in tariffs go away soon because of up-take, getting in early means you will get it for 2-5 years and hopefully have the system paid off by then. It is amazing to me that for a 4-12 kW system that a professional installation is about 50% the cost of the system. For those who are willing and capable, there is almost no reason to not install yourself.

    I don't know of an AHJ anywhere that buys power from customers with PV at the retail rate. Some will let the meter run both ways at the same rate but reset it at the end of the year with no compensation, which is a good deal as long as your yearly production is less than your yearly usage. Some will pay you a pittance every month for your net putback into the grid, which is not so good if your production is higher than your usage in some months. With some the meter runs forward at the retail rate and backward at the wholesale rate, which is a bummer if your PV is producing more than you are using at any time at all.

    Feed in tariffs are another game entirely.

    One argument against self-installation is that in many AHJ's there are certifications the installer must have in order for the system to qualify for rebates, and most places the interconnect can only be done by a licensed electrician. Also, an experienced installer will know the "gotcha's" that inspectors look for to red tag a system.
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #6
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    Re: Output calculators vs. real system output

    Take your time and use PVwatts to get your possible output then look really hard at your utility net metering plan to make sure you are getting what you think. Usually the utility will have some ideas about past installs. I know here the utility is very supportive and will help you estimate what your expected input/output might look like.

    For the rebates & tax credits I think you need at least an electrician to get it accepted. Here they wanted an approved installer. Code guys can be finicky like here, even then it took like 3 trips to clear all the red tags.
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  • System2
    System2 Posts: 6,290 admin
    #7
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    Re: Output calculators vs. real system output
    ggunn wrote: »
    One argument against self-installation is that in many AHJ's there are certifications the installer must have in order for the system to qualify for rebates, and most places the interconnect can only be done by a licensed electrician. Also, an experienced installer will know the "gotcha's" that inspectors look for to red tag a system.

    I have always intended to have the final hookup from the grid tie inverter to the meter to be done by a licensed/bonded/insured electrician. I would also have him check all the work on the rest of the installation. I would be interested to know whether the federal rebate requires a certified installer. I haven't seen that in any of the documentation I have been reading through. Do you know of at least one rebate that requires that so I can go see how it is worded and be on the lookout for it in my state/municipality?
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #8
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    Re: Output calculators vs. real system output
    jpswensen wrote: »
    I have always intended to have the final hookup from the grid tie inverter to the meter to be done by a licensed/bonded/insured electrician. I would also have him check all the work on the rest of the installation. I would be interested to know whether the federal rebate requires a certified installer. I haven't seen that in any of the documentation I have been reading through. Do you know of at least one rebate that requires that so I can go see how it is worded and be on the lookout for it in my state/municipality?

    Here's the documentation of the Austin (Texas) Energy Interconnect Guidelines:
    http://www.austinenergy.com/Energy%20Efficiency/Programs/Rebates/solar%20rebates/interconnectionGuidelines.pdf
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  • bmet
    bmet Solar Expert Posts: 630 ✭✭
    #9
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    Re: Output calculators vs. real system output

    If the screwy weather we're having lately (85 Degrees in November -SW Tx.) keep occuring, those figures are going to be no better than rolling Dice.
    niel wrote: »
    don't forget that they base this on past records and system specs iand are going out on a limb that the sunshine, and hence the weather, stays the same in the future. predicting pv output is not an exact science because predicting the weather is also not an exact science. i thought it important enough to point this out as engineers like to be exacting.
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  • System2
    System2 Posts: 6,290 admin
    #10
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    Re: Output calculators vs. real system output
    ggunn wrote: »
    Here's the documentation of the Austin (Texas) Energy Interconnect Guidelines:
    http://www.austinenergy.com/Energy%20Efficiency/Programs/Rebates/solar%20rebates/interconnectionGuidelines.pdf

    I didn't see anything in that document that "requires" a licensed electrician/installer. It did make clear that there are a lot of regulations that one must be careful to follow at every turn, so I can see how professional help is advisable.
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #11
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    Re: Output calculators vs. real system output
    jpswensen wrote: »
    I didn't see anything in that document that "requires" a licensed electrician/installer. It did make clear that there are a lot of regulations that one must be careful to follow at every turn, so I can see how professional help is advisable.

    Most likely to get it commissioned you may need a licensed electrician. Many states do allow you to do your own wiring however with the proper permits, but sometimes it is just easier going if you have that licensed guy.
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  • Lee Dodge
    Lee Dodge Solar Expert Posts: 112 ✭✭
    #12
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    Re: Output calculators vs. real system output

    I have a section on my website that directly addresses your question, although it is a comparison of PVWatts V.2 predictions, not the model that you used, versus measured energy from three systems all in the same area:

    http://www.residentialenergylaboratory.com/comparison_of_pv_systems.html
    Not sure the link above will work correctly, so I'll list it again here:
    http://www.residentialenergylaboratory.com/comparison_of_pv_systems.html

    The results that I show should not be considered typical, but there is text in the document that reviews literature for some of the typical results. Basically my system produced more than 23.8% above PVWatts V.2 predictions, and two other systems in this area looked similar. More typical results are +-10%.
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  • jagec
    jagec Solar Expert Posts: 157 ✭✭
    #13
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    Re: Output calculators vs. real system output
    jpswensen wrote: »
    I have always intended to have the final hookup from the grid tie inverter to the meter to be done by a licensed/bonded/insured electrician. I would also have him check all the work on the rest of the installation. I would be interested to know whether the federal rebate requires a certified installer. I haven't seen that in any of the documentation I have been reading through. Do you know of at least one rebate that requires that so I can go see how it is worded and be on the lookout for it in my state/municipality?

    It's the city code guys and the power company itself that care about UL listings and using a licensed electrician. If you have any state or local rebates or credits, they might care as well.

    The IRS form just asks (1)How much did you spend on solar electric equipment and installation? (2)Multiply that number by 0.3, there's your credit.
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