Saving energy with electric hot water heater?

Although I currently have my Sense disconnected, because I recently added an EV charge circuit on that 240v breaker, i can not wait to get it reconnected to try a new experiment.
The experiment will be to swap the hot and cold water connections for my kitchen sink and bathroom sinks.
For this reason: I have found myself frequently rinsing my hands, especially in the kitchen, where I do not really need hot or even warm water. But, since I am right handed, inevitably, use my left hand, turning on the hot water. I don’t wait for it to be hot, but hot water does come out of the HW tank, basically unnecessary, but probably causing the HW heater to use significantly more energy.
What do others think of this?
Anyone ever experimented with something like this?

Regardless of whether you change the hot/cold sides the interesting aspect of your experiment would be, of course, to quantify the difference between the activities.

I would put a Flex DCM on your HW heater to get ground-truth data because I suspect the difference is going to be nominal and inside the likely Sense native detection error.

I would also maybe use discipline and/or something red and something blue to designate which is which. Or wash hands behind your back.

What I am expecting to see is a longer frequency between the HW heater on/off cycles.
Yes, with a mirror on the opposite wall!

Wait until the day you need hot water for something & find yourself running the water wondering why it’s not getting hot.

My first thought was you must be single and live alone to even think about attempting such an experiment. lol

Not to hijack this thread, but we just had our pilot light go out and needed the whole gas valve assembly replaced. Part was under warranty as the unit is only 20 months old, but the plumber cost $408. Besides the sacrificial rod that is also in a gas WH, is there a common component that can die in an electric water heater? There’s obviously no pilot, but I’m wondering if an electric WH is more “solid state” than a gas. Of course we went with a gas replacement for our 14 y/o gas unit because it was cheaper and easier to replace. Tack on this $406 and the heat pump water heater I wanted to get would now have been cheaper and would move lowered our $460 natural gas bill last month.


Well a heat pump water heater is quite a complicated device by comparison because it has the heat pump electro-mechanical stuff, refrigerant piping, heat exchanger and fans and a lot of fail points.

A better system would separate the heat pump from the tank but that’s easier said than done when the real motivation for doing that would be to use the heat pump for house heating as well. MUCH bigger tanks would be way better and you need a high temperature heat pump in many cases to do the house heating through existing radiators.

There are reasons to think in the grand scheme of things that, if you have rooftop solar, forgoing the heat pump in favor of direct DC water heating into a tank as a place to “dump” energy (otherwise sent to the grid) could be better but that seems like short term and somewhat selfish thinking.

In parts of Australia where they have “too much solar” they PAY YOU to use energy. In that case a regular electric tank (or 3) seems like a no-brainer. :wink:

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I would look at a tankless hot water heater that is 90%+ eff. The price difference in the exhaust piping will make up the difference. The 90% ones vent thru PVC which is dirt cheap compared to some of the required stainless concentric piping required by the others. I went from a 55 gallon tank to a 97% Navien in 2011 and it cut my natural gas bill by 2/3’s.

No idea how it quoted ixu and taged brian5

We do, and that is the exact reason I wanted to go with a hybrid, even before we had the solar. Now that we have it, especially in the fall and spring, I foresee massive over generation, and unfortunately some of that will be right before our annual expiration. If I could run an efficient electric water heater using my excess, it’d be great. Eventually (5 years, 10 years, 20?) we’ll have an electric car and then be under generating. I’ve already been planning where at least 2, then at least 3 more, and even an additional 2 after that could easily go. I think 7 panels is what we’d need for our current driving needs. For a while coming up, I think there’ll be a game of over generation, then under, then over, under, etc. until all devices are replaced and all necessary panels are installed. We’re talking 20 years I think here.

Well the truth is that efficiency is what you make it.

Even the most basic resistive-only hot water heater has 95%+ (more like 97-98%) thermal efficiency these days. But as @ccook suggests, a tankless system (gas or electric) is more efficient by certain metrics because you aren’t storing hot water and heating unnecessarily and losing some of that energy as the tank water cools down.

The gotcha with a tankless system is that you need high instantaneous energy output. With gas that’s usually not an issue but with electric it usually is. Electric tankless heaters are cheap because they are so simple but expensive to install in most situations, and especially to install safely. But actually the real argument for tank/tankless from an efficiency standpoint is when you look at the whole system and how it’s used. Sporadically used vacation homes in cold climates are possibly more efficient with tankless; always-occupied primary homes with large families (and constant hot water use) are almost certainly more efficient with a tank.

When you add solar to the equation (even if it’s only eventually) I would say tankless (electric) is out of the question and adding as large a tank as possible is the most efficient way of using excess solar in a typical family house in certain locations. Why?

  • Lithium and SLA battery systems have very large embodied energy (difficult to manufacture and they don’t last forever). Although the cost of such a system doesn’t (theoretically) have anything to do with the efficiency I would argue that it absolutely has everything to do with efficiency: the most efficient system is usually the one you can afford to install sooner rather than later!

  • Like battery charging, water can be heated with a “trickle” of energy when there is excess solar. Especially in the case of heat pumped systems, the wiring is also minimized.


  • Size matters as does how the hot water is used: If having excess solar leads to excess hot water leads to much longer showers then are you being more efficient?

  • Location matters: If you are in the North with lower solar potential your metrics are going to be very different than if you are in sunny South location.

If space weren’t an issue and I was in a homebrew mood I would be tempted to get a tank and go bonkers with insulating it waaaay beyond what you can buy. How does an R-200+ tank sound?

For a given overall system volume V with water capacity W and insulation volume I where V=W+I perhaps there are good arguments for reducing W and increasing I far beyond what systems like Mixergy are doing.


One possible option is to install a small tank electric water heater, 6 gallon or so, in the bathroom under the sink. It plugs into 120vac and would use that over generation.