Solar Effect

When we got our Solar Photovoltaic (PV) array installed I hoped that it’d do two things:  

  • Produce enough electricity to cover our personal electrical needs. 
  • Offset most of the increased electrical consumption due to having a relatively large amount of computer equipment always running due to my business.  

The array was installed in August of last year and to-date it has produced over 3.8 MWh of electricity which offsets about 3.15 tons of CO2. This is awesome and I’m extremely glad that we decided to have it installed.  However, what is perhaps more interesting is how having a PV array effects our actual usage of electricity. I’ve been meaning to look more closely at this for a while, but only now have finally put together enough data to really make it interesting:

Solar Usage & Production

(Data Gathering Note: The problem I ran into is that while our PV array has a nice little monitoring utility, it doesn’t really have the ability to sync easily with my electric bill, e.g. there’s no reliable way to say, “how much did I produce over the billing period of mm/dd/yy - mm/dd/yy?”  My solution has been to put the meter reading dates (ours are planned out & available a year at a time) into iCal and set an alarm go off at noon (generally when I’ve seen the meter reader in the past). Later when the alarm goes off, I check the monitoring software from my iPhone for the current total production (a much more accurate number than daily totals), and jot it down in my notes on the phone. When I get my bill I can simply check the total on my bill against my notes to find the amount I produced over the same period and from there my total usage.  The only caveat is that sometimes due to bad weather meter reading dates are changed and I have no easy way to deal with this.)

This graph shows a bunch of things, but the most important are the red and orange bars.  They show our actual electrical usage over the past three months and the same three months of 2009.  Each of the past three months has seen a reduction of over 100kWh compared to the same month a year ago.  This fact is even more interesting when you notice that this February had an average temperature that was 6.5°F lower than February of 2009.  An average temperature lower by 6.5° is a pretty big difference (e.g. it was a really cold February).  Our house was built in 1920 and is heated by natural gas.  The heating is uneven at best and we use small electric space heaters in several parts of the house to make them habitable.  A cold winter month generally means more electric heater use and therefore higher electric usage—however even with an extra cold February we were able to reduce our usage by 385 kWh!

I’m hoping we can continue this trend.  The initial estimate for our PV array’s annual production (7.2 MWh) came in about 2400 kWh under what I calculated our annual usage was at the time (9.6 MWh).  If we can continue to cut our usage by an average of 600 kWh per quarter, we should be close to producing 100% of our annual electrical needs.  How are we doing? Well, winter is generally the time when you go negative in your usage to production balance, yet after roughly 7 months (beginning of September through yesterday) we’re less than 950 kWh in the red.  I’m pretty confident we can make up most, if not all of that difference before September.

Lastly, we’ve done some improvements in the past few months that should decrease our electrical usage going forward:

  • New Energy Star dishwasher (should save ~180kWh annually).
  • New Energy Star clothes washer (should save ~200 kWh annually).
  • New 93% efficient gas furnace installed (should mean less gas and electric space heater usage).
  • Home weatherized (the process dropped our air leakage from somewhere in the area of >10,000CFM to <3,500CFM @-50 Pa.  This translates into going from somewhere north of 1.6 natural air changes per hour down to less than 0.6—this should drastically decrease our heating/cooling needs [1 air change is equal to all the air in the sealed envelope of the house being exchanged with new air from outside]).
  • Last summer we put a new, high reflectivity, metal roof on the house, I expect it to reduce our A/C usage this summer (last summer was very cool so we didn’t use the A/C much at all).

Edit: Tweaked layout and statistics a bit. Also, just to note, to write this post I pulled info from a bunch of sources:   

Notes