Martin's corner on the web

Hot water usage analysis

I thought it would be interesting to analyze our hot water usage, at this stage this is just for fun, but maybe in the future to make more it energy efficient. During the winter, I use my heat pump to also heat a 120l hot water cylinder as I have described here. I have no hot water usage meter, so I chose to use indirect method instead – by using my tool for calculating the gained/lost energy based on the change of temperature of a fixed water volume over time. Here it is visualized for a period over 24hrs


Blue is the temperature of the hot water in the cylinder, red is the calculated momentary energy gain/loss and orange is the heat pump electricity consumption. The heat pump heats the house and the hot water cylinder, so once the cylinder’s temperature reaches the temperature of the refrigerant that passes through it.

So in my experiment I am interested in the loss of energy i.e. the use. Normal faucet use doesn’t really affect heavily the temperature, these are visible as small dips, while taking showers is obvious. There are also variations as the heat pump revs up or down – the tank follows the refrigerant temperature too and acts a a buffer. I created a modified report based on my pchart reporting tool to only take the losses and reverse the sign so I can analyze the usage:


…then modified the report a bit further to average it over three months period but by hour:


So it is clear that our top hot water usage is around 8pm and also some in the morning, in-between just normal daily usage by my wife and youngest kid. Makes perfect sense, nothing unexpected.  I ran another report across day of week for 90 day period and found some funny statistics that we use most hot water on Thursdays and least on Wednesdays :).. go figure.

I needed this information for further energy use optimizations in the spring, when the heat pump is not constantly on and my solar water heater is still not sufficient. At some point I thought of using electricity at night because it is cheaper, but by the time I need water most the water cylinder would have lost due to normal heat loss roughly 10 degrees or the advantage is lost.