Our PV System One Year Update

Our DIY PV system has been up and running for a full year, so I thought I would give a rundown on how the system has done over the first year.

Maintenance

The system has been completely trouble free.

The only maintenance has been:

- Mowing the grass around array once in a while.

- Cleaning off the PV panels once in a while (garden hose).

- Clearing snow off the panels after snow storms (this is a quick job with a broom, and is not really critical as the snow usually melts off fairly quickly)

- Checking all the PV panel hold down bolts -- did not find any loose ones, and probably won't need to do this again. Also looked the whole thing over for any anomalies -- none found.

- I expect to have to apply some additional wood treatment to the timber supports once a year, but did not do it this year.

- We put in a few shrubs behind the array to improve the appearance from the north side.

The Enphase internet monitoring of the system provides regular reports on the health of the system, and has not found any problems over the year.

Performance

Before building the system, I used PVWatts to estimate what the performance would be. The results are pasted in below.

I used Billings, MT as Bozeman was not available, and I adjusted the derate factor from 0.77 up to 0.82 as recommended by Enphase.

The PVWatts predicted output was 3073 KWH for the year.

Our actual energy produced for the year is 3320 KWH.

So, the system did about 8% better than the PVWatts estimate for this year. This will of course vary from year to year with weather, but it has been a near record rainfall year with quite a bit of cloudy weather -- so, I would guess an average year might produce more output than this last year. The system is brand new, and can be expected to drop in output somewhat over the years, so that accounts for some of the difference.

Anyway, its nice to see the system doing better than the PVWatts estimate.

* * *

AC Energy
&
Cost Savings

Station Identification
City:	Billings
State:	Montana
Latitude:	45.80° N
Longitude:	108.53° W
Elevation:	1088 m
PV System Specifications
DC Rating:	2.2 kW
DC to AC Derate Factor:	0.820
AC Rating:	1.8 kW
Array Type:	Fixed Tilt
Array Tilt:	45.0°
Array Azimuth:	180.0°
Energy Specifications
Cost of Electricity:	7.9 ¢/kWh

Results
Month	Solar Radiation (kWh/m²/day)	AC Energy (kWh)	Energy Value ($)
1	3.23	187	14.77
2	4.16	213	16.83
3	5.21	287	22.67
4	5.29	269	21.25
5	5.79	297	23.46
6	6.02	286	22.59
7	6.60	318	25.12
8	6.33	308	24.33
9	5.88	285	22.52
10	4.77	253	19.99
11	3.61	197	15.56
12	3.02	174	13.75
Year	5.00	3073	242.77

Our Electricity Consumption

Our net electricity consumption has dropped to an average of 200 KWH per month.

We started at about 1000 KWH per month and got this down to right around 500 KWH per month through conservation measures -- we spent about $1,200 implementing the various conservation projects.

The PV system reduced our net consumption by about another 275 KWH per month down to around 200 KWH per month. The PV system cost about $10,000 before rebates and about $6,500 after rebates.

So, one clear message here is that even though PV systems have come down in price, doing conservation and efficiency changes is FAR more cost effective than PV -- at least in our case. I'd say do the conservation measures first, and then think about doing a PV array.

Impressions

I've built several solar thermal (heating) systems, but this is the first solar electric system I've built -- these are some impressions on how the two types of systems compare:

The high reliability and low maintenance of the PV system is impressive. The solar PV equipment appears to have gotten to the point where you can just about forget it.
Solar thermal systems seem to require a bit more attention -- its not they they are high maintenance, but there are a few things you need to check once in a while.
While the solar electric systems are not nearly as efficient under sunny conditions as solar thermal system (it takes 3 or 4 times the collector area of PV collectors compared to solar thermal collectors to make the same amount of energy), it is impressive how PV panels continue to generate some power (albeit not much) even under difficult conditions. Long after the solar thermal panels have stopped producing any energy the solar electric panels are still trickling out some power.
In spite of the fact that PV has come down in price, the price per KWH of energy produced for DIY PV systems is much greater than the cost per KWH of energy for DIY solar thermal systems.
As an example, my Solar Shed project cost about $5,000 and generates about 9000 KWH per year of of energy --compared to the PV system that cost $10,000 and generates only 3300 KWH of energy per year.
When you compare the PV to the $1000 Solar Water Heating system, the comparison is even more lopsided -- the two systems produce about the same amount of energy, but the $1K Solar Water Heating System only costs about $1,000 BEFORE rebates.

Both kinds of systems are a lot of fun to build and own!

Gary

November 5, 2010