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.





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.



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

Solar Radiation
AC Energy
Energy Value
3.23      187    14.77   
4.16      213    16.83   
5.21      287    22.67   
5.29      269    21.25   
5.79      297    23.46   
6.02      286    22.59   
6.60      318    25.12   
6.33      308    24.33   
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.



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:


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




November 5, 2010