Grid tied PV system in Tempe, AZ

System · February 2010

I'm trying to figure out how to size a grid tied residential solar system located in Tempe, AZ. I've got a graph of the last years energy usage here:

How do I take this information and use it to size my solar install?

I'd like to put together a system that can scale to that 2500 kwh peak, with the idea that I'll probably start with fewer panels and scale up later.

I'm working on increasing my efficiency with a solar water heater and insulating my attic. However I am an IT consultant and I run several servers in my home office so my usage will always be a little higher than most.

What major equipment do I need? Power inverter and panels?

Anyone here have experience with an APS grid tied system? One thing I noticed was that my bill was only $70 electrical usage and $100 fees and taxes. Am I only going to save on that $70? I put in an email to APS, but they don't seem to be in a hurry to reply.

BB. · February 2010

Re: Grid tied PV system in Tempe, AZ

Sherman,

For a pure Grid Tied system... You really do not worry about peak loads, but average kWhr used for your particular time period...

Say your utility use is

1,700 kWhrs per month average * 12 months per year = 20,400 kWhrs per year

So, using the PV Watts website and Phoenix AZ, use 1kW of solar panels (even number) and use the rest as defaults:

"Station Identification"
"City:","Phoenix"
"State:","Arizona"
"Lat (deg N):", 33.43
"Long (deg W):", 112.02
"Elev (m): ", 339
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.770"
"AC Rating:"," 0.8 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 33.5"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 8.5 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 5.09, 113, 9.61
2, 6.06, 122, 10.37
3, 6.61, 142, 12.07
4, 7.54, 153, 13.01
5, 7.53, 155, 13.18
6, 7.28, 140, 11.90
7, 7.13, 142, 12.07
8, 7.17, 144, 12.24
9, 7.15, 139, 11.81
10, 6.75, 141, 11.98
11, 5.59, 117, 9.95
12, 4.88, 109, 9.27
"Year", 6.57, 1617, 137.44

So, 1kW of solar panels will give you (on average) 1,617 kWhrs per year:

20,400 kWH per year load * 1/1,617 kWHhrs per year per 1kW of panels = 12.6 kWatts of solar panels.

I don't know your power rates--But I would guess you are near $0.09 per kWhr -- Very cheap as rates go... In California, our peak rates can be as high as $0.45-$0.60 or more per kWhr depending on plan, residential/commercial, how much power per month, etc...

That is a ~10.8 kW rated system... Some utilities/states change the rules for systems larger than 10kWatts--so you may run into some issues if you try for >10kW.

I don't know if you have Time of Use pricing (higher for summer peaks, lower for off peak usage)... In California, that can reduce the size of the system by 1/2 in some cases (if you can time shift your power use to off-peak power).

As for saving money--may be difficult. Assume $7,000 per 1kW of rated capacity installed (I am not in Biz--may be higher or lower):

10.8 kW * $7,000 per kW * 70% (30% fed rebate) = $53,000

Assume 20 year useful life of system (simple cost of power, no present value / future worth type stuff):

$53,000 costs / (20 year * 20,400 kWhr per year) = $0.13 per kWhr

You may also have other rebates/credits avialable in your area--See here:

www.dsireusa.org

Our host Northern Arizona Wind and Sun (their webstore is here) is out of Flagstaff--So, I would guess that they could talk with you about the hardware for your location (I am not connected with NAWS in any way).

A pair of Xanrex 5kW inverters will cost ~$3,210 each.

62x Evergreen ES-A-205 205 Watt Solar Electric Panel will cost around $42,000...

Permits, racking, wiring, possible main power panel upgrade, installation, etc. will all add to the costs...

Is this what you where looking for?

-Bill

BB. · February 2010

Re: Grid tied PV system in Tempe, AZ

Regarding your billing--Every utility is different. I don't know any details outside of my utility (Northern California's PG&E)...

The "best plans" generally are 1 year net metering... You have a minimum bill of $5-$25 or so a month. If you use more power than you generate, you get the Normal Bill added to your account. If you generate more power than you use, you get the retail credit to your bill for that month. At the end of 1 year, you either pay for the excess amount of power you used (retail rate), or if you have a credit--the account balance is set back to zero...

There are a lot of details and your local installer is usually the person to work out the details with.

-Bill

System2 · February 2010

Re: Grid tied PV system in Tempe, AZ

Thanks BB. That's definitely helpful information. I did look at the PV Watts tool, but wasn't quite sure how to use it. Thanks for the breakdown!

I found the APS rates, there are several plans, but the one we are on has a peak rate 9AM - 9PM of $0.17866 per kWh and $0.05774 for off peak during the summer and $0.14 / $0.05 for the winter. The real cost is laid out in this byzantine document:

http://www.twocell.com/files/et-1.pdf

There are two tables, one for "Energy Charge" and one for "Generation Charge". I'm not sure if I add these two together ... also everything is subject to 8 different "adjustments". Working backwards from my bill I think I'm paying $0.30 kWh peak and about $0.08 off-peak.

Lot to mull over. Thanks for the help!

BB. · February 2010

Re: Grid tied PV system in Tempe, AZ

Sherman,

I am guessing -- but the Energy charge is the entire kWH billing rate--and the others (including the generation charge) is just all of components added together for the Energy Charge...

However, you $0.30/$0.08 charges don't quite agree with your rate sheet (assuming it is the correct one)...

You also have $0.556 per day Service/Connection fee for ~$16.68 per month--Perhaps that is rolled into the apparent $0.30/$0.08 charges too?

I don't know--I am not in Arizona... It is hard enough to understand my own rate plans (I have not figured out the gas one yet).

-Bill

Grid tied PV system in Tempe, AZ

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