getting Solar evaluated...need some guidance

[prhamilton]

Sorry, got some threads crossed and thought you were in California.

California is unique in energy pricing(right now) in that we pay 0.13 kWh for our first 300 kWh. Then we 0.16 for our next 150 kWh. Then we pay .33 after that. So with my usage, my effective rate is 0.24 per kWh. Someone who uses less power might see their rate as low as 0.13 and a heavy users might have a rate approaching .33 per kWh. My comment was to try to keep usage in the first tier where the rates are favorable.

Sorry to muddy the waters with California specific stuff. If your rates are a flat 0.16 kWh then it makes the calculations pretty easy. Then you need to figure out what rate they credit you back. I think MA has pretty friendly net metering rules.

[peakbagger]

Mass has some very sweet incentives that vary by utility and I believe that Feed in Tariffs are available. One of my coworkers paid out of pocket for a system and maxed out his roof with panels and he figures with the 30% fed rebate, the local incentives and the FITs that his payback is four years and then he earns 5% per year.

[jlatenight]

Ok, the system he sized is 5880w DC, est. 7120kwh/yr. My TSRF is 80% on average. Panels are 24x245w Trina panels. When it's all said and done, after rebates and incentives, he figures I'd be positive $78.85/mo the first year. Based on today's electricity rates (which by the way are going up 30% in two months, and they just went up 37% in Nov), for the life of the system (25 years, but will most likely be more), I'll have a total positive value of $60,506 The cost of the system is reduced 50% by tax credits, incentives, & rebates. That's good right?

[pleppik]

A guy is on my roof taking measurements and assessing its solar potential as we speak. I have a meeting today at 2pm to review his findings. Can you guys come up with a handful of key questions to ask the guy? I guess the main thing to ask besides price is how much wattage the system will generate. What are other questions to ask? Thank's a lot for everyone's help!! -Jon

Ask every dumb question you can think of and try to really understand everything about where his numbers come from.

[pleppik]

Ok, the system he sized is 5880w DC, est. 7120kwh/yr. My TSRF is 80% on average. Panels are 24x245w Trina panels. When it's all said and done, after rebates and incentives, he figures I'd be positive $78.85/mo the first year. Based on today's electricity rates (which by the way are going up 30% in two months, and they just went up 37% in Nov), for the life of the system (25 years, but will most likely be more), I'll have a total positive value of $60,506 The cost of the system is reduced 50% by tax credits, incentives, & rebates. That's good right?

OK, now to go PVWatts and punch in his numbers and see if it comes up with something close for the production.

My gut check is that the production estimate sounds a bit high for a fixed orientation system in MA. You can get that, but the orientation of the array would have to be nearly optimal with no shading. He should have given you the azimuth and elevation of your roof, and the shading factor from the site survey, so you can get an idea if these numbers are realistic.

Also, are you enough of a finance geek to be familiar with the Net Present Value calculation and what it means? If not, then that $60,506 number might as well be from Mars.

[J.P.M.]

Ok, the system he sized is 5880w DC, est. 7120kwh/yr. My TSRF is 80% on average. Panels are 24x245w Trina panels. When it's all said and done, after rebates and incentives, he figures I'd be positive $78.85/mo the first year. Based on today's electricity rates (which by the way are going up 30% in two months, and they just went up 37% in Nov), for the life of the system (25 years, but will most likely be more), I'll have a total positive value of $60,506 The cost of the system is reduced 50% by tax credits, incentives, & rebates. That's good right?

Sounds like a lot of money. What's it going to cost up front ? Depending on your situation what you describe could be a great deal, but I'd dig into the details and verify. I think I may be seeing some smoke and mirrors. As I suggested, not everyone tells the truth or the whole truth all the time. Trust but verify on your own, especially about (projected) utility rate increases and things like "total positive value" - what is that exactly ? SREC's in there ? Cost of funds ? tax considerations? assumptions ? And how is it calculated ? Many vendors in San Diego shouted about 30% - 40% rate increase last Sept. Published rate tariffs proved that mathematically impossible. A little digging and Socratic questioning showed that most of that was hype, word play or just plain B.S. believed by people all too eager to repeat what they read/hear.

Caveat Emptor.

I'd get more quotes and more education on the subject, and take some time.

BTW : What's TSRF ?

[jlatenight]

OK, now to go PVWatts and punch in his numbers and see if it comes up with something close for the production.

My gut check is that the production estimate sounds a bit high for a fixed orientation system in MA. You can get that, but the orientation of the array would have to be nearly optimal with no shading. He should have given you the azimuth and elevation of your roof, and the shading factor from the site survey, so you can get an idea if these numbers are realistic.

Also, are you enough of a finance geek to be familiar with the Net Present Value calculation and what it means? If not, then that $60,506 number might as well be from Mars.

Ok, went to PVwatts, assuming I filled it out right, came out to 5963 kwh/yr.... way less than what he said (7120kwh/yr). How accurate is PVwatts?

[inetdog]

Ok, went to PVwatts, assuming I filled it out right, came out to 5963 kwh/yr.... way less than what he said (7120kwh/yr). How accurate is PVwatts?

PVWatts is very accurate but depends on three assumptions:

1. The insolation is based on historical average weather patterns at one point in the approximate area you look up. Actual and future conditions may vary.
2. PVWatts assumes a standard performance coefficient for the conversion between insolation and the panel output. It is probably low for current high-end equipment, but you can change that number as an input.
3. It assumes that you have an unobstructed horizon to horizon view. Based on that factor, there is nothing that you can do in a specific case to improve that. Except maybe mount the panels at the top of a south-facing cliff.

[pleppik]

Ok, went to PVwatts, assuming I filled it out right, came out to 5963 kwh/yr.... way less than what he said (7120kwh/yr). How accurate is PVwatts?

Probably more accurate than a salesperson.

So this is where the rubber meets the road. I'm not an expert, but the people on this forum who have posted thousands of times will probably say that PVWatts is pretty good for a simple consumer tool. You should be able to get the number from PVWatts to come within a few percent of what the site survey estimates, if the site survey was done correctly.

There are a bunch of different parameters in PVWatts you can tweak, like the shading factor, inverter efficiency, etc. You should ask your salesperson to explain why there's this big discrepancy, and help you understand the parameters he's using to come up with your production estimate.

If he can't give a good reason for any discrepancy, then the 7,120 kwh/year is BS, and not based on your actual site parameters.

Also, when you do get an explanation, pass it along. Lots of people here can help augment your BS detector. It's practically a contact sport in this forum.

[slopoke]

Ok, the system he sized is 5880w DC, est. 7120kwh/yr. My TSRF is 80% on average. Panels are 24x245w Trina panels. When it's all said and done, after rebates and incentives, he figures I'd be positive $78.85/mo the first year. Based on today's electricity rates (which by the way are going up 30% in two months, and they just went up 37% in Nov), for the life of the system (25 years, but will most likely be more), I'll have a total positive value of $60,506 The cost of the system is reduced 50% by tax credits, incentives, & rebates. That's good right?

I'm not an accountant, but using your numbers of 1088 kW hours average per month and that comes to paying your POCO $196.00 per month. If your proposed system does produce 7120 kW hours per year/ 593 kW hours per month average. So now your monthly bill is $89.00 for electricity, saving you $107.00 per month, or $1284 per year. I don't agree with the solar company that you will go positive by $78.85 per month.

I'm assuming this is a purchase not a lease. So now take the after incentives cost and divide that by $1284, that will give you how many years to break even. You'll have to figure out how much ROI is right for you. When I did my system, I did not factor in any increases to what my POCO charges because I don't know what the actual increases might be if any. My system should take just under seven years to pay for itself and if there are any increases to our electricity rates, that will just help to break even sooner. For me 7 years to break even is a ok ROI, good luckk.

[Naptown]

Ok, went to PVwatts, assuming I filled it out right, came out to 5963 kwh/yr.... way less than what he said (7120kwh/yr). How accurate is PVwatts?

What was the derate factor that you used?
Default will always come in very low
Average derate is about .83 Default is .77

[jlatenight]

Sounds like a lot of money. What's it going to cost up front ? Depending on your situation what you describe could be a great deal, but I'd dig into the details and verify. I think I may be seeing some smoke and mirrors. As I suggested, not everyone tells the truth or the whole truth all the time. Trust but verify on your own, especially about (projected) utility rate increases and things like "total positive value" - what is that exactly ? SREC's in there ? Cost of funds ? tax considerations? assumptions ? And how is it calculated ? Many vendors in San Diego shouted about 30% - 40% rate increase last Sept. Published rate tariffs proved that mathematically impossible. A little digging and Socratic questioning showed that most of that was hype, word play or just plain B.S. believed by people all too eager to repeat what they read/hear.

Caveat Emptor.

I'd get more quotes and more education on the subject, and take some time.

BTW : What's TSRF ?

Cost to me is nothing up front. I guess what they mean by "total positive value" is the benefit to me over the life of the system given it's output, it's cost, all the discounts, my annual electrical grid use, etc. Basically everything calculated together. What I'm worried about is if all those 'variables' are a)correct, and b)accounted for. TSRF stands for Total Solar Resource Fraction. It's the ratio of insolation available accounting for both shading and TOF (Tilt Orientation Factor)....I guess it's basically how much sun hits your panels or something like that. Maybe he was "overstating" the rate increases. I can find the 38% increase from Nov, but not the one that supposedly happening to two months. Plus I think the 38% increase stops in April. Boy....this is some confusing stuff.

[jlatenight]

What was the derate factor that you used?
Default will always come in very low
Average derate is about .83 Default is .77

Yeah I used .77. I reran it again w/ .83 and got 6447...better but still 673 less than he said. Should I be concerned or is that just the margin of error?

[Ian S]

Yeah I used .77. I reran it again w/ .83 and got 6447...better but still 673 less than he said. Should I be concerned or is that just the margin of error?

My PV Watts calculation seems to work better with 0.85 derate but YMMV. Since you have no cost upfront, this must be a lease right? If so, then they usually guarantee some level of production. Is that the production number he quoted?

[pleppik]

Yeah I used .77. I reran it again w/ .83 and got 6447...better but still 673 less than he said. Should I be concerned or is that just the margin of error?

In my case, I managed to get within 1% or so. My installer gave me all the parameters he used, and stepped me through the whole thing and explained why he used the values he did.

The thing is, there are a ton of different ways your actual production is likely to be below the optimum. PVWatts generally defaults to a nearly optimal configuration, so if your installer is telling you to expect 10% more power than PVWatts, there's some assumption your installer is using which is significantly higher than the PVWatts assumption. Until you get a good explanation for this, it's safer to assume that PVWatts is closer to the truth.

Your installer is (or should be) using base data for available solar resource which is nearly identical to the base data in PVWatts, so there's no room for differences in the basic amount of sun available.

Any differences between the two have to be attributable to:

• Orientation of the array (I sure hope you got the right azimuth and elevation from the installer!)
• Shading (lots of room for differences here)
• Efficiency of the electronics, wiring, etc.

So assuming that you are using the same numbers for the orientation, a 10% difference between your estimates can only be explained by either (a) not using a realistic shading factor, or (b) assuming a lot more efficient wiring and electronics.

You should ask for these numbers. Specifically, ask what he used for a shading factor, and what the assumptions for the system efficiency are. And tell us what he tells you. People here can tell you if those numbers make sense.