Planning grid-tie, 6.6kW (DC), 22 panel, residential system, looking for advice

[scottcannon]

I have a (almost) flat-roof residence in Tucson, AZ (32°N latitude) and want to design and install a high-quality DIY grid-tied system. My annual residential energy usage (reported by utility) is 13,438 kWh. Utility recommends a 6.6kW (DC), 22-panel (423 sq.ft., 39.3 sq.m.) system. Panels point due south, no obstructions. Looking to have adjustable panel pitch (8° June to 55° Dec from horizontal) to maximize output. Assuming Direct Normal Irradience (DNI) of 7.62 kWh/m2/day (www.solarenergylocal.com), annual insolation available to panels assumed to be 109,300 kWh. Looking for overall system recommendations (panels, inverter, system voltage, wiring, etc.). Thanks in advance!

 

[sunshine_eggo]

Please do your research. I'm in Mesa with SRP, and a system that meets 70% of my annual needs would only reduce my monthly bill by 30% due to the fees and unfavorable net metering. I had a DIY system lined out at about $10K for a 10kW system. That would have had an 11 year payback period. I decided against it as we will be living elsewhere completely off-grid by that time.

 

[iamrich]

If my fuzzy math is correct, you are using ~40kwh per day, so I would shoot for closer to 8kw of panels (8kwx5h=40kwh) or maybe more (10kw maybe). The cost to put up panels should be fixed if the number remains the same, so it will just be the cost of the panels themselves that change. x22 365w (or higher) panels should do the trick. Do you plan on running batteries or will this be only grid tied?

 

[sunshine_eggo]

If my fuzzy math is correct, you are using ~40kwh per day, so I would shoot for closer to 8kw of panels (8kwx5h=40kwh) or maybe more (10kw maybe). The cost to put up panels should be fixed if the number remains the same, so it will just be the cost of the panels themselves that change. x22 365w (or higher) panels should do the trick. Do you plan on running batteries or will this be only grid tied?

Those that don't live in AZ might be surprised at our insolation... 5h/day is the winter performance.

32° fixed tilt:





OP, my array is 3 position adjustable: Summer, Spring/Fall, and Winter. I've never moved them off the spring/fall position. Not worth the effort.

 

[scottcannon]

Please do your research. I'm in Mesa with SRP, and a system that meets 70% of my annual needs would only reduce my monthly bill by 30% due to the fees and unfavorable net metering. I had a DIY system lined out at about $10K for a 10kW system. That would have had an 11 year payback period. I decided against it as we will be living elsewhere completely off-grid by that time.

I am with Tucson Electric Power (TEP) and I understand they scrapped their net metering for a new "solar export program" in 2018. The initial TEP solar-energy export rate is 9.64 ¢/kWh compared with net metering at a retail rate of 10.8 ¢/kWh, locked in for each customer for 10 years from the time their systems go online. The rate is then "...reset annually based on a five-year rolling average of utility-scale solar costs", whatever that means. TEP customers are charged a monthly meter fee of $2.23 and I'm not sure what else. Can you give me an idea of the extra fees that involved in selling power back to SRP?

 

[scottcannon]

If my fuzzy math is correct, you are using ~40kwh per day, so I would shoot for closer to 8kw of panels (8kwx5h=40kwh) or maybe more (10kw maybe). The cost to put up panels should be fixed if the number remains the same, so it will just be the cost of the panels themselves that change. x22 365w (or higher) panels should do the trick. Do you plan on running batteries or will this be only grid tied?

Yes, my daily usage averages 36.8 kWh/day. I'm not sure how TEP calculates their recommended solar system capacity, but more capacity would be better as long as the pay-back period is short. This system will be grid-tied only because TEP's reliability is good and outages are infrequent. According to UNS Energy Corporation, "TEP consistently benchmarked in the top quartile among our peers for reliability performance over the last several years, and achieved performance records in 2020". Their 2020 SAIDI score (sum of all customer interruption durations divided by the total number of customers served) was 48 minutes.

 

[scottcannon]

I am with Tucson Electric Power (TEP) and I understand they scrapped their net metering for a new "solar export program" in 2018. The initial TEP solar-energy export rate is 9.64 ¢/kWh compared with net metering at a retail rate of 10.8 ¢/kWh, locked in for each customer for 10 years from the time their systems go online. The rate is then "...reset annually based on a five-year rolling average of utility-scale solar costs", whatever that means. TEP customers are charged a monthly meter fee of $2.23 and I'm not sure what else. Can you give me an idea of the extra fees that involved in selling power back to SRP?

My main motivation for going solar is our utility's residential rates have been skyrocketing since 2008:

 

[iamrich]

more capacity would be better as long as the pay-back period is short.

Figure $150-180 a panel for ~400 watt panels, $1000 for racking (ground or roof), $1000 for misc wiring/combiner boxes, shutoffs, breakers, etc., $1500 for a 10kw inverter (Growatt 10000MTLP-US would probably work).

So $7500 DIY? Assuming you are paying $100 a month on electricity, 6 years to pay for it?

Of course, if you want this installed, figure $30k+

 

[DThames]

The difference gained by adjusting the tilt of your panels 2 or 4 times per year is on the order of 3% compared to a fixed tilt. Normally 1 extra panel will make that difference on a single home sized system. Depending on your grid credit rate, you might pick a tilt that is not best for average production but is best for offsetting monthly demand. For example if your cooling load is much more per year than your heating load, a tilt for better summer power might be best. My grid credits can be carried over without them being dropped after X months, so I can do a perfect average tilt. But new customer in my state have their unused credits roll off after they are 4 months old, so a different logic would make better sense for them.

 

[sunshine_eggo]

I am with Tucson Electric Power (TEP) and I understand they scrapped their net metering for a new "solar export program" in 2018. The initial TEP solar-energy export rate is 9.64 ¢/kWh compared with net metering at a retail rate of 10.8 ¢/kWh, locked in for each customer for 10 years from the time their systems go online. The rate is then "...reset annually based on a five-year rolling average of utility-scale solar costs", whatever that means. TEP customers are charged a monthly meter fee of $2.23 and I'm not sure what else. Can you give me an idea of the extra fees that involved in selling power back to SRP?

That sounds pretty favorable, but this page says $.0781/kWh.

Have you run their solar analysis?

Compare Solar Options

TEP provides safe and reliable power to Tucson residents. We offer customers a wide range of incentivized programs to help reduce energy costs and build a better future.

www.tep.com

It was SRP's tool that computed my 30% bill savings based on actual usage and PV system size. Unfortunately, SRP's fee is based on the single highest use 30 minute period throughout the year, and it's generally pretty large.

 

[scottcannon]

That sounds pretty favorable, but this page says $.0781/kWh.

Have you run their solar analysis?

Compare Solar Options

TEP provides safe and reliable power to Tucson residents. We offer customers a wide range of incentivized programs to help reduce energy costs and build a better future.

www.tep.com

It was SRP's tool that computed my 30% bill savings based on actual usage and PV system size. Unfortunately, SRP's fee is based on the single highest use 30 minute period throughout the year, and it's generally pretty large.

Wow, you are right! Thanks for the catch! Yes, TEP's rooftop solar analysis assumes a $21,450 system cost, $6457 tax incentive, optimal fixed panel tilt of 26° (adjusting tilt throughout the year helps slightly), 20-year maintenance cost of $3960, breakeven after year 10, and a 20-year net return on the investment of only $9063. I could invest that same $21,450 at a fixed interest rate (compounded annually) of only 1.78%, get the same return after 20 years, and still get my initial investment back! You've convinced me that solar (in my circumstance) is not a good investment. Thanks sunshine_eggo!

 

[sunshine_eggo]

Wow, you are right! Thanks for the catch! Yes, TEP's rooftop solar analysis assumes a $21,450 system cost, $6457 tax incentive, optimal fixed panel tilt of 26° (adjusting tilt throughout the year helps slightly), 20-year maintenance cost of $3960, breakeven after year 10, and a 20-year net return on the investment of only $9063. I could invest that same $21,450 at a fixed interest rate (compounded annually) of only 1.78%, get the same return after 20 years, and still get my initial investment back! You've convinced me that solar (in my circumstance) is not a good investment. Thanks sunshine_eggo!

That wasn't my intent, but I hoped you would have the tools needed to get to the right answer. It's a shame that AZ utilities are so anti-solar given the choice sun we get year round, but I suspect they saw what was coming for California utilities, and they wanted a soft landing.

I'm contemplating a smaller DIY zero export grid-tie system. I could probably save 20-30% by installing a system half the original size. Might even go with a small battery system for a few hours of peak shaving. What SRP don't know won't hurt them.