Estimating Savings From Our Home Insulation Improvements

We got our home air sealed and added insulation back in August. My hope at the time was that it would could our home energy costs by 20-30% and give us a good return on the expense. But I really had no way to know how much it would save us until we did it. Unfortunately I couldn't find or think of any good way to accurately predict the energy savings based on proposed insulation improvements before hand.

I'll watch my electric bills for the rest of the winter and then compare them to last year to see how much the insulation has saved us.

We've had our heat on since the start of October and so we're starting into the winter heating season. I just got the electric bill that covers the month of October.

October 2009 : $113.77
October 2008 : $149.30

So it seems we might have saved about $35 in electric heat this year. If we assume that our base electric bill is $80 then our heat costs went from $69.30 last year to $33.77 this year. Thats a 52% drop! But before I get too excited about my savings we need to look at why our bill might vary from one year to the other.


Was Oct. of 2009 warmer or colder than Oct. 2008?

Its quite possible that we used less heat this past Oct. as compared to Oct. 2008 simply because it wasn't as cold this year as last year. One way to measure the relative coldness for a time period is called a degree day. The degree day is found by taking the average temperature for a day and subtracting from a fixed figure, usually 60 or 65 F. You can find the degree days for a month by adding all the daily figures. So for example if the average daily temperature is 45F every day of the month then the degree days for a given day might be 65F - 45F = 20F and the degree day total for the month would be 30 days x 20 degrees = 600 degree days.

I used WeatherUnderground to find the average daily tempatures and then calculated the degree days myself. For my area the degree days for Oct. of this year and last were:

October 2009 : 362 Degree Days
October 2008 : 374 Degree Days

Thats only about 3% difference between the two years. So I'd say that Oct. 2009 wasn't significantly warmer than Oct. 2008.


Did other electric usage differ much?

Part of our electric bill is for other things besides heat. I don't have any way to know exactly how much our other electric usage might have changed from Oct. 2008 to Oct. 2009. Its possible that we had our lights on more, watched the TV more, ran our washer/dryer more, ran the stove more, etc in 2008 than we did in 2009.

I could guesstimate the variation in non heat electric usage from month to month by looking at the usage variation in the summer months. The heat is off in the summer months so our electric bill for those months is entirely from non heating usage. From June to Sept. of 2009 our electric bill ranged from $73 up to $80. Looking back at 2008 our summer months bills varied from $70 to $81. So, at most our summer months electric bills vary up to $11. I think we can use this figure as a margin of error based on non-heat electric usage of up to +/-$11.

Did our heat usage differ from 2008 to 2009?

Its possible we didn't have the heater on as much in Oct. 2009 as we did in Oct. 2008. I had the heat furnace turned on the full month. But we might have been away from home more in 2009 or otherwise not had the heat up as much. Looking at 2008 and going from month to month I can see that the kWh used per degree day varied pretty drastically. In some months it was as low as 2.2 kWh / degreeday and in other months it was as high as 3.6 kWh/degreeday. That is a 39% difference from high to low. So worst case scenario we might see up to 39% difference between months based on heat usage. However Oct. 2008 was the minimum usage last year. So I think its pretty unlikely that we'll see Oct. 2009 be 39% lower than Oct. 2008 due to usage.

So lets add it all up and find the range of % differences that the insulation probably had...

We start with a 52% difference from Oct. 2008 to Oct. 2009

Lets add in the variation in our basic non-energy related electric usage ranging from $70 to $80:

Oct 2008
Total bill = $113.77
Non-heat usage = $70-$80
Energy usage = $33.77 to $43.77

Oct 2009
Total bill = $149.30
Non-heat usage = $70-$80
Energy usage = $69.30 to $79.30

Now we see the range is up to 37% to 58% improvement.

If we add in the fact that bills could vary as much as 40% from month to month then that would expand the widest possible range to -4% to 70% improvement.

Its pretty likely that the bills will fluctuate $5-10 from month to month based on non heat electric usage. But its not very likely that the bill from Oct. 2008 would be as much as 40% different than the bill in Oct. 2009 due to heat usage differences. We keep the heat at set values with a programmable thermostat and our activity really hasn't changed much. I would think up to a 20% difference in heat usage might be more feasible.

If we assume a $5-$10 monthly fluctuation in electric use and as much as 20% difference in heat usage levels month to month then our Oct. 2009 bill is 26% to 67% reduction from our Oct. 2008 bill.

That seems fairly realistic to me. If this is true then we've got at least a 26% reduction in our home heating costs due to the insulation.

This is just one months data so I'll have to watch it going forward and see what our bills are like for the rest of the winter. Hopefully we'll continue to see similar cheaper bills for the rest of the winter.