Rheem Marathon 100 Gallon Water Heater Usage

We just purchased a home with a 100 Gallon Rheem Marathon Electric water heater. I was excited about this water heater as I had read about how efficient it was supposed to be and long lasting. After getting my first month electric bill, >$350/month, I was pretty alarmed as we live in a cold climate with no A/C contributing to this. I installed my sense, and need some help doing some analysis. Here is what I am seeing for my water heater to date. Any reaction to these numbers?

$1340 Est. cost/year
Based on last 30 days
40.0**% of monthly use

Estimated kWh/year

Based on last 30 days
12178.1 kWh
Average usage 3766 w
Average times on/month 2065x
Average run time 5m 29s
Average cost/month $98

I’m also in a cold climate (MN), but I have a gas tank water heater. My first impression is that the estimates aren’t unfathomable, but suggestive that your household goes through a lot of hot water. How many occupants? Are you always doing laundry with hot water? Showers have lo-flow faucet heads?

Using this calculator, I typed in a # of gallons until I got close to the annual projection, it suggests that you’re consuming about 164 gallons of hot water a day. The costs, of course are harder to comment on since we all have different rates. But I’d suggest using this calculator a bit and seeing if you can get comparable numbers and then think about hot water usage and determine if it all seems reasonable.

Our forced-air furnace is our only other gas appliance, so I can look at my summer gas consumption (only about 8 therms/month) and get a good idea from the calculator about what our hot water usage is (about 25/gallons per day). 25 gallons per day may be low to some, but for our household I think it sounds pretty reasonable. We only wash laundry on cold, and so 25 gallons is about what we would use for just the two adults in our house.

Based on your description, I’m guessing this is your model: https://www.rheem.com/product/marathon-100-gallon-electric-grid-enabled-water-heater-with-limited-lifetime-warranty-mrg105245c/ which sadly I don’t see having earned an EnergyStar rating: https://www.energystar.gov/productfinder/product/certified-water-heaters/results unless you can find it. No clue how up-to-date the Fed Gov’t keeps this list. And sometimes there are good reasons for not getting EnergyStar rating, e.g. if the criteria aren’t keeping pace with technological changes and/or consumer preferences.

Indeed, that seems like some frequent cycling: (5.5 x 2065) / (30 x 24 x 60) = 0.26
i.e. your tank is on quite a lot, at least every 21min!

1 Like

Here is an example of what yesterday’s usage looks like. Even during the middle of the night hours! How would I diagnose this? This definitely seems like an issue?

@chadfetter if your water heater has storage (a “tank”), then it will cycle on/off 24/7 regardless of whether you’re using water or not… just to maintain the set temperature so that hot water is always available.

Usually don’t see electric tank heaters (unless natural gas isn’t available) because they are less efficient

From Consumer Reports Online:

What temp is the tank set to?

It’s not impossible to calculate what the standby usage should be and then work out whether you maybe have a leak or perhaps some crossover.

“Efficiency” is dependent upon how you define it.

  • (Electric resistive) raw watts-to-water-heating is >92% these days in the minimum Energy Code requirements if you ignore losses through the tanks imperfect insulation.

  • (Gas) BTU[potential]-to-water-heating is around 60-70% at best.

  • (Electric heat pump) is greater than 200%.

1 Like

Yes, I’m aware of their strict efficiency values, I’m a Certified Energy Manager. I purposefully provided the info from Consumer Reports since they digest information which is relative for most consumers’ real-life-use-cases… which would include load factor and as with the previous calculator which includes the actual water usage, all of these things matter. In this example, I’m just trying to show that what was “sold” as efficient, might just be relative to other electric heaters, and not relative to all heater types. I’m in a similar climate, electric tank heaters are not at all common, I think he was lied to.

If, for example, the water heater was replaced by the seller just before the time of sale, the seller went with the cheaper (sale) option knowing that they wouldn’t be on the hook for ongoing (energy) costs.

I would discourage the installation of any gas appliances these days unless the source is methane (biogas).

I went from a gas-fired WH to electric (and pay the penalty!) to pre-empt and encourage solar install and not having to pay for gas service. With grid usage you can also encourage the Utility to migrate to renewables. As you would know, the calculus isn’t straightforward.

Meanwhile an old 100 gallon tank could be a good “solar battery”.


I’m all for discouraging gas also, what with climate change and all. A gas-fired tank WH is just what my 1970s house had when sold to me a couple years ago, and I’ve not changed it yet. Aside from my furnace and WH, I’m all electric, and my intent is to transition to full-electric over time and add some solar panels. However, given my house age and the fact that we already have one PHEV on 120V, its becoming clear I’ll soon need more than my 100A service, especially if I ever expect to heat my house electrically without constantly breaking circuits.

1 Like

I work with the assumption that the modification of copper (more! fatter!) in the system implies potentially excessive complication. Look at lighting migrating to LED and so allowing more lumens/gauge!

I’m on 100A as well so, for example, instant electric WH was out of the question … and then along came heat pump WH.

You’d be surprised (or not) how efficient and low-current the top-of-the-line mini-split systems are these days in heat mode. [15 HSPF!]

My favorite product in this regard (short of a slow toaster) is a low temperature and wattage, insulated kettle/pot. What’s the hurry people?

Model # is: MR105245

In looking at my EnergyGuide sticker on the WH, The average yearly operating cost is $514, and the estimated yearly electricity use is 4825kWh.

Ignoring the details, I think you’re looking at a maximum of about 1kWhr/day standby on that tank, which has very good insulation … meaning: overnight when the tank is “keeping warm” and you aren’t using water you should only be seeing Sense clock something like 50W/hr.

A couple of things come to mind:

  • Do you have a recirculation pump?? Trying switching that OFF and see what happens for a couple of days.

  • Check for leaks … either direct hot water loss or that cold water is leaking back into the hot water system even when not in use.

Do you use hot water for house heating? How many occupants?

There are four people in my house with a basic 40 gallon electric and it’s 38% of usage.

If it was me I’d swap it out for a Rheem HPWH. I put one in last year and during the summer with the intake ducted to my attic it used $9-$12 a month. Now with the intake ducted to behind my fridge my highest month was $25 . My lowest consumption was 73 kWh a month my highest was 190 kWh. Three people in the house.

It’s running a marathon alright! All over your electric bill


I too have the exact same water heater and I’ve been working with it over the past month. I’m getting about the same estimate ($1500 per year) but I believe my problem (or situation rather) is that I have a circulation system setup where the system is keeping the lines full of hot water. In other words, there’s a return line that feeds into the drain of my tank and as the water in the lines cools with no use, the cold water seeps back into the bottom of the tank. I have a valve to shut that off but haven’t done that yet. We really enjoy having hot water almost instantly but I think I’m going to turn off that valve to see how often it cycles without it on.