My house uses natural gas for space heat and domestic hot water. When analyzing the benefit of my solar collectors, the obvious thing to do is to examine the actual change in my gas consumption. To answer this, I've plotted the amount of gas I burned per heating degree day (HDD) per month. Using HDDs adjusts for the variability of the weather and allows an apples-to-apples comparison between different years.
In the above graph there are three lines: The blue line is our gas consumption from when we had a typical forced air furnace and gas-fired water heater. The orange line is the average of the two years were we used a high-efficiency condensing gas boiler without any solar collectors. The yellow line is the combination of the boiler with solar collectors.
At the risk of stating the obvious, the graph shows how the solar collectors don't collect much energy in the depths of winter – in December and January in particular – when the amount of sunlight is at it's lowest level. Based on the previous years consumption, it appears that in September and April, about two-thirds of my heat is supplied by the solar collectors. In October and March, the solar supplies about one-third of my needed heat, in November I got about one-fifth and February I got about one-tenth of my heat from solar.
This highlights the big advantage of photovoltaics over solar thermal: In the summer, PV can make your power meter run backwards and offset the consumption from the winter but solar thermal is limited by how much heat you can store.
Here are two graphs showing how much heat I've collected and the outside temperature. The first graph is the raw data, the second is a one-week moving average where I try to smooth out the data to make it easier to see trends.
The final question is how much money am I saving. Clearly my gas consumption has decreased but calculating the savings is complicated by changes to the price of gas. Overall, my gas bill is $191 less than the same eight month period a year earlier and $151 less than the average over the past 3 years. Offsetting this is an additional 102 kwh of electricity consumption over the same period a year ago. Cost of this is $8.
I haven't supplied a graph of my power consumption as there does not appear to be a direct correlation with my electricity consumption and installation of my solar collectors. Since installing my collectors the power consumption for some months are higher than average and other months are lower than average. I believe that the driving factor for electricity consumption is weather and lifestyle choices like how many electronic gadgets you have plugged in around your house. The pumps that run the solar do not appear to make a significant change to power consumption.