Our PV System Two Year Update

Our DIY PV system has been up and running for two full years, so I thought I would give an update on how things are going.





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.

- Applied some more preservative to the treated wood posts near the ground and at the joints and any other place it looked like it could use some.  Half an hour total.  This is probably an every couple of years job.

The Enphase internet monitoring of the system provides regular reports on the health of the system.  The only reports from the Enphase server was for a couple of hours when it said the utility frequency was not in range and the inverters were shutting down until the utility frequency got back in range.  I think the total down time was an hour or two.  This happened one time only, and I don't really know the cause -- perhaps the utility frequency did get out of range for a bit.



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.


The lifetime output for the system is now reported as:


First Year: Nov 10, 2000 through Nov 9, 2010 was 3291 KWH      (Enphase report in Excel for 2009-2010)...


2nd Year:  Nov 10, 2010 through Nov 9, 2011 was 3051  KWH      (Enphase report in Excel for 2010-2011)...


The system actually started operation on Nov 6, 2009, but the first few days the system was off and on operation, so I've dropped those days.


My own records showed 3320 KWH for the first year, so there is a little disagreement with the 3291 for the Enphase report.  I've decided to go with the Enphase numbers.   The only reason I was initially keeping my own records was that our internet connection was not able to support the Enphase reporting system.


So, the 2nd year is down about 7% from the first year.   I believe that this is mainly due to the near record setting cloudy weather we have been experiencing.  The same La Nina weather pattern that brings the news reports of near record setting droughts to the SW pushes the jet stream further north, and gives us much more cloudy weather and more precipitation.   The production of our solar thermal system was also less than typical last year.  There is also normally a small degradation each year as the PV panels age, but, I think the weather is most of the 7%.


There is a day by day plot for the whole year at the bottom of this page...


The weather man is saying that the same La Nina weather pattern is setting up for this winter, so we may have another lower sun winter -- bummer.


I would say that the first years production is probably the more typical, or even on the low side as it was also a more cloudy than average winter.


This is the PVWatts output for our PV array just for reference:

* * * 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

For our first year on the system, our utility bills averaged around 200 KWH per month (this is with the PV system in operation).


For the 2nd year, our utility bills have been more erratic and higher -- averaging about 330 KWH per month. 


It was my goal after the first year to drop our consumption down some from the 200 KWH per month, and obviously we failed there, since the average bill went up.  I can blame about 20 KWH  per month on the lower PV production, but clearly most of the increase is our fault.

I still have it on the list of things to do a bottoms up analysis of where the electricity goes and see if some improvements can be made. 


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 (thermal and PV) are a lot of fun to build and own!



DIY Install Savings Update

I've done an update on what the typical saving for a DIY installation of a PV system might be based on prices published in Solar Today magazine for typical installed PV prices across the country in September 2011...


In a nutshell, our DIY system cost 52% less than the average US PV installation of the same size -- as saving of about $8000.



Enphase provides a variety of nice reports. 


Here is one example that I like.  It shows the lifetime power production for each of the 10 PV modules of the system.


module production

The nice thing about this is that you can tell at a glance that each of the modules has had the same total production, so they are all working OK.

On non micro-inverter systems, it can be difficult to tell if one module has a problem of some sort and is not producing up to the level of the others.  Some of these problems go undetected for years.


Plot of Daily Output for 2 Years

This shows the day by day output for the first two years.  The blue bars are the first year, and red the 2nd year.  This is one of the regular downloadable Enphase reports.


daily pv output



November 10, 2011