What’s Best – Leave Hot Water On Permanently Or Turn It Off?

leave hot water on permanently

If you have a hot water tank, you’ve probably wondered which is cheaper – to leave hot water on permanently or to just turn it on when you need it? The results are in…

There are several deep, meaningful questions that we often ask. Is there a god? How did the universe start? Is there any point in me turning off my hot water tank? I know the answer to all three, but today, I’ll be giving you the answer to the most important.

When I last had my boiler serviced, the engineer “Phil” – that’s what we’ll call him, as that was his name, seemed surprised that I turned my hot water off. He told me that it wouldn’t cost me any more to leave it on permanently. I was left a little confused, and I didn’t want to argue with him because he hadn’t given me the bill yet.

So, I went and did some research on the internet and found there were two schools of thought.
1. Leave hot water on permanently. Your boiler doesn’t have to heat the water from cold, so it’s far more efficient.
2. Only heat the tank when needed. Otherwise, you’re burning energy when you don’t need to.

My water tank:

The experiment

I decided to put these two views to the test. Over July and August, using my handy smart meter, I went about discovering which method would save me the most money.

Now, I am a trained scientist. I have a C in double science G.C.S.E. Read A Brief History of Time for five minutes (before it got too heavy) and watched a fair amount of the Big Bang series. As you can see, there aren’t many more qualified people.

Why July and August?

Both have 31 days (hopefully). Also, I wouldn’t be using any heating during the summer months, otherwise the experiment almost impossible to measure. Plus, we aren’t going away for any period during these months. Our holiday is booked for sunny November – thanks for asking.

The variables

These are things that can affect the results.
1. The number of baths. This is where we use the most hot water. We are definitely a bath family and very rarely use the shower. For this experiment, I made sure that we had exactly the same number of baths. This is actually harder than it sounds and ended with me running some unused baths!
2. Washing-up. If we haven’t turned the water on for baths, we use the kettle to boil water and use this to wash-up. So to stop any discrepancies, the washing-up was done using a kettle for both months. And for those interested, on average it costs 2.5 pence to boil your kettle.
3. The hob. The only other gas device we have is the hob. It isn’t used much and not for long. So I haven’t taken this into account for the results.

The results

I was a little surprised by what I found.

July (the month where the water was turned to constant), we used 253 kWh.

August (where water was only turned on when needed), we used 199 kWh.

So leaving the water on constantly meant we used over 25% more gas. Sounds quite a bit doesn’t it? But what does that translate to in cost?

For gas, we pay 3.071p per kWh.

In July that meant we spent £7.78 on heating water and in August £6.11.

My conclusion

As expected, if you leave hot water on permanently, it does cost more. However, £1.67 is quite a small difference. I know this site is about money saving, but I also like convenience. If it costs less than £2 per month to have hot water on tap, then it’s a price I’m willing to pay.

Bear in mind that the better insulated your tank, the lower the cost will be to keep your water heated.

If you’re interested in keeping a closer eye on your energy usage, take a look at smart meters.