While listening to the SelfHosted podcast, one of the hosts brought up their peculiar living arrangement multiple times. Essentially, he and his family are living in an RV that runs off of batteries, has solar panels, utilizes Home Assistant for automation, supports multiple mobile networking providers and also has some Raspberry Pi-s running in a homelab. In such an environment, low power usage is critical.

That got me thinking: how much electricity am I currently using and how low can I go?

Could lowering my overall power usage help transition to an off-grid setup in the future?

Initial measurements

Before making any changes, it is important to make some measurements. After all, if you don’t get an overview of the current situation, then how are you going to do comparisons? Your local power company is measuring the total power usage for you, so you might as well utilize that to get an overview of daily and monthly power usage.

In my situation, the monthly power use would be between 200-250 kWh and daily power usage would jump between 6 kWh and 13 kWh, depending on the day. I am not sure if that is normal power usage or not, but at least it’s a starting point.

Example daily power usage graph. Blue parts indicate nighttime pricing (cheaper).
Example daily power usage graph. Blue parts indicate nighttime pricing (cheaper).

What’s using all this power?

There are many candidates for appliances that use the most power: water heater, refrigerator, electrical oven, electrical kettle etc. With some of them, it’s quite easy to reduce the power usage: just use it less! With others, it might not be as straightforward.

For example, with a refrigerator, you cannot really use it less. Sure, you can try to open the door fewer times so that it has to do less work, but you really shouldn’t try to limit its operating window, as that can result in food going bad. It will probably help if you got yourself a refrigerator that has a better efficiency rating, but that’s it.

However, with a water heater, you are free to limit its use significantly. After all, the worst thing that can happen is that you might run out of hot water while showering or washing the dishes. Today we will thus focus on an old water heater and try to see how low we can go.

Reducing the total power usage of a water heater

The way that a water heater works is quite simple: if the water temperature is below a threshold, heat it. This is usually controlled with a dial on the water heater itself. After you have used hot water for a while, you will notice the water heater kicking in and doing its thing.

A water heater can use quite a lot of power. In my case, it uses 2000W while heating the water. This result was measured by using an electronic power usage meter that you plug between the appliance and the wall socket. From the data provided by the power company, the days with showers or other activities that used hot water spiked up to 13 kWh, which is quite a lot when compared to the daily average of 8 kWh.

Water heaters also have an insulation layer to prevent the water from losing heat. Once the water is heated up to the desired temperature, it should be able to keep it at that temperature for quite some time without having to heat it up again.

With that in mind, I tried a simple approach: buy a mechanical timer, set it to run at certain times throughout the day, test out the results and find the sweet spot. After all, why would I want it to run during the night, when nobody is using hot water anyway?

Initial testing

I picked a sensible starting point: run for about 1-2 hours in the morning and again some time in the afternoon (16-18). By starting with that, I should be able to avoid situations where I run out of hot water at the most inconvenient time. This worked well, but due to the water heater potentially running for up to 4 hours, it would still mean a maximum of 8 kWh power use for the water heater alone.

During the next week, I experimented with shorter values. I limited the evening heat cycle to 30 minutes and went as low as 45 minutes in the morning heat cycle. At that point the water didn’t heat up enough in the mornings and the boost in the afternoon didn’t really resolve the problem, either, which meant that I found the low point.

With more recent tests, I have found that for our use case it is perfectly viable to set the timer to run for 90 minutes in the morning and have hot (or warm) water for the rest of the day, while also allowing for comfortable showers in the morning and afternoon. Just make sure to set the temperature setting to the highest value so that it does not stop heating the water while the water heater is allowed to run.

Results

While looking at daily power reports, a couple of things become clear:

  • the spikes have now been replaced with a more stable usage
  • it is possible to limit the water heater usage down to 3 kWh per day without giving up too much convenience
  • a mechanical timer allows me to use power while it is cheap (nighttime)
  • with water heater use concentrated on one specific time slot, it is easier to identify other appliances with high power usage
  • you will suddenly find motivation to shower as fast as possible, because who knows how much hot water you have left?
Not very hard to guess when the water heater is doing its job.
Not very hard to guess when the water heater is doing its job.

Is it worth it?

Not from a financial standpoint, definitely. However, it does give a good feeling when you are living in a country that is relying on one of the most polluting energy sources.

To be quite fair, at this point it has become more of a game for me than anything else. It’s fun to find out the limits and try to push them as far as you can go.

Are there better solutions?

Probably. My solution here is cheap (the mechanical timer only cost 4 euros), but there could be smarter solutions that learned your daily habits and tried to optimize the water heating process using that. However, predicting when you need the hot water becomes much more difficult if you deviate from your usual routine. Your children found the wonders of playing in a puddle and they need to shower at an unusual time? Well, there goes all that data collection and machine learning effort.

Alternatively, you could risk getting electrocuted and get an attachment that will heat up water on-demand. The downside with that option is the hassle and upfront investment of having your kitchen sink and shower set up like that.

What next?

Now that one of the big ticket items is handled, I can shift focus to my computing setup.

What can you do to have a more efficient homelab?

Do you need to sacrifice anything in your attempt to balance data hoarding and low power use cravings?

How much power is your desktop PC using while doing nothing?

Stay tuned for future installments of my quest towards energy efficiency!

2022-01-18 update

At some point I stopped using this trick because while I was paying a bit less on my electricity bills, I still consumed a lot of power. On a 35 EUR power bill, that would have meant about a 5 EUR reduction in cost. You still pay to have those electrons moved to your home. Using the water heater on a lower temperature setting throughout the day used less power in total due to having less water to heat up and the hot water usage being a bit unpredictable.

However, with the current state of the energy market, the prices have been multiplied. I’ve gone back to this setup to take advantage of lower rates during nighttime, which tend to be 2-4x cheaper. At these prices, there’s a real benefit to trying to use the off-peak hours. Let’s see how well this works out.