Water heaters – efficiency, insulation, costs…oh my!


After my last try at this long post, I’m going to break this one down into a few parts. There’s lots of material here, so bear with me.

So recently a friend asked me about water heaters – how much energy do they use and is adding insulation a good idea. Let’s start with energy use…

Energy Consumption

Just how much energy does a water heater use? Is it as significant as people say?

In a word – “YES!” – water heaters are a major consumer of household energy.

No matter how you slice it, this is a numbers question, and I’m a numbers guy. So get out your spreadsheet and prepare to think about BTUs and Energy Factors!

Let’s pull out a famous pie chart showing a comparison of average energy use for different things around the house. I grabbed this from the Energy Star website because they should know, right?

Well, as it turns out, I don’t know what fantasy world they’re getting their numbers from, but by my numbers, water heaters are responsible for 20%-50% of your utility bills, and I’ll show you how.

Here’s the problem – the answer to this question depends upon whether you’re a single person, living alone, who only showers once a week in tepid water, or if your household is multi-generational, with grandparents, parents, and seven children making loads of laundry every day. The occupants are the biggest factor in hot water use, so let’s look at them and how they affect the equation.

Hot water use based on occupancy

On average, people use 25-50 gallons of hot water per day, per person, so right there, you have a wide range. Think about it for a moment…If you shower every day for 10 minutes, that’s 22 gallons of warm water per day. Wash your hands a couple times and you’re using another gallon or two. A load of laundry? 10-30 gallons, and so on.

I emphasized “warm” water because most of the calculations I see count every use of water as all-out hot water, which is stupid. Nobody just runs the water on full hot unless they have their water heater set to a very low temperature. So when you take a shower, you might use 2/3 hot, 1/3 cold water. A 10 minute shower with a 2.5 gallon per minute shower head is 25 gallons of water but about 17 gallons of hot water.

However, there are other complications. For example, every time you run the hot water, you’re filling up the pipes between your water heater and your spigot with hot water. When you turn it off, all the heat in that water is lost. So you might run the faucet for 30 seconds to get warm water in the first place, then use it for 30 seconds and turn it off. The heat in the water in the pipes is “lost”. Yes – lost.

Some would argue that the heat goes into heating your home. Yes, true enough, you capture that waste heat during the winter. But during the summer? Oh…now that waste heat, usually 130 degree hot water, means your air conditioner has to work harder. It’s not so simple, is it? So for my calculations, I say that’s lost energy.

These are all reasons why the spread of usage is so wide. But let’s be conservative and use 25 gallons of hot water per person per day. How much energy is that?

Not so fast. How many people are in your household? Is it just you or do you have a family? For this example, I’m going to talk about a typical family of four. You can just multiply or divide the numbers based upon your own household size.

As it turns out, if you run the numbers, and I have, you’ll find that using 100 gallons of hot water per day costs you between $50 and $100 per month depending on your fuel costs and a few other factors like the efficiency and temperature of your water heater. Without turning this into a thesis, let’s just say it costs $70/month for hot water.

Depending upon the season and energy costs, the total monthly utility costs for a family of four will be $200-$400. Much more if it’s the dead of winter with an old oil boiler, less if you have a state-of-the-art geothermal heat pump. But this gives you an idea. So, if you’re on the low end of this, then the water heater accounts for 35% of the utility costs. On the other hand, it might be 10%-15% during the winter. But that gives you an idea of the typical ranges.

What does this all mean?

When you cut through all the verbiage, you discover that hot water is a big deal. Typically it’s 20%-30% of your total utility costs when taken over a full year – maybe around $750. This means that you should look at water heating as a “high leverage” area for improving your home’s energy efficiency.

I’ll touch on how to reduce your water heating bills in the next installment.

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