Friday, 11 September 2015

How much heating does the house need? Part 2: like, how much money does it cost?

The question of how much the heating costs is more straightforward. The electricity bill is the only payment for heating, and the heating only comes on at night time, so I don't have to worry about the cost of my solar electricity.
The electricity bill is all thrown in together so I can't just isolate the cost of the heat source, and even if I could, the same system is used for hot water and heating, so it would be difficult to separate those two costs.

We need to look at how much the electricity costs when the heating is off, look at how much it costs when the heating is on, and the difference is the heating cost. Simple enough, but there are a couple of questions.

First, the hot water is generated by an atmospheric heat pump, which takes heat from the nighttime air. It will use a lot more energy to get heat out of the winter air, when it drops ten below zero for a few days, than to get heat out of the summer air, which sometimes stays over 25. In other words, part of the extra cost of electricity in winter is for the hot water rather than the heating. Also the incoming water will be colder in winter, so it will need to be heated up from a lower temperature, and use more energy still. How much of the extra cost in winter should be attributed to the extra cost of hot water?

On the other hand, the hot water tank and pipes are in the house, so some of the hot water heat is going to be helping heat the house. Should that be taken into consideration?

Another thing that may make a difference is that fact that we pay different amounts for electricity in the day time, at off peak time and at "at home" time, and we pay nothing for electricity if it comes from our solar panels. In the winter it may be more overcast in the day time, so while that electricity would have cost us nothing on a sunny summer day, in the winter they may have charged us 24 yen per kWh. In the summer, when the heating is off, we only buy one or two kWh of day time electricity, but in the winter it's more like eight or nine. Is this going to matter or will it just be a few hundred yen a year?

We use about ten times more electricity in the morning and evening "at home" times, and three times that off peak, so imported day time electricity is only one or two percent of our total.

The second question is easier, so I'll answer that first.

No. Heating cost should just be the energy used in the heating equipment. 

Here are three estimates with different answers to the first question, starting high and pessimistic, and finishing low and optimistic. Each estimate is based on two or three years of data and rounded to the nearest 100 yen. 

1. The no-heating cost is estimated from the months of June to September, when heating is not needed. This is multiplied by twelve and then subtracted from the annual electricity cost.
20,800 yen per year
This will not take any account of extra winter costs for hot water, and will include the incidental heating costs that I just said should not be included.

2. The no-heating cost is the average of the months of April to November, when the heating is off. This is also multiplied be twelve and subtracted from the annual electricity cost.
12,500 yen per year
This is not as pessimistic as the first figure, but it is still treating some of the incidental costs as heating costs.

3. The no-heating cost is the average of April, May, October and November, the colder months when the heating is off. This is subtracted from the bill for each month when the heating was on, then an annual heating cost was added up and averaged.
3,600 yen per year

Our monthly electricity bill is the lines at the bottom. Can you see the heating on it?
I could probably try a bit harder and get an estimate that would be paying me to heat the house. Or, I could round it to one significant figure and call it around 10,000 yen.