How much energy/electricity does a Tankless OR tank-based water heater consume?

Question

Incoming cold water temperature: 11 Celsius (constant for the example).

Indoor temperature: 20 Celsius (constant for the example).

Let us have two choices:

1. Tank-based water heater solution: has to heat up daily 120 L water from 11C to 60C. But although it has a little insulation, it losses heat at a rate of 20% per day (I have no precise info about this), so called "standby-loss"

2. Tankless water heater solution: has to heat up daily 120 L water from 11C to 40C. There is no other heat waste.

The big question: how much energy and electricity does the two consume daily?

Didn't wanted to make it more complex, since the tank based solution is having electricity 30% cheaper vs. the tankless

And the tankless needs a very BIG circuit breaker.

Answer 1

Real-world "standby loss" example with measured data:

Tanked Electric water heater (made in 2021) - 38 gallon nominal size, 35 actual gallons, (132.5 liters) set to 140°F/60°C (tempering valve on output but does not matter for standby losses.) Came with an external fiberglass wrap from the factory as well as the internal foamed insulation. Pipes are insulated.

Presently on but with no use due to work travel, so the standby is right there to be seen on the power use graph. (Why not off? well, we just had a lovely ice storm and power outage and if we get another, then not freezing the water heater beats saving less than 20 cents per day on standby losses. Allows much more time to get the problem resolved if the water starts from hot.)

Kicks on very noticeably in the usage graph about once every 20-24 hours and consumes about 0.8kWh (by comparison to the hours before and after it kicks on - heat pump heating and refrigerator are the other background loads, and neither has the solid bump of a 4.5 KW element engaging for 10-15 minutes that "spikes the graph" as it does.) Updated to include the "15-minute" version of the power use. The first and last ones are clearly spread across two intervals. Second might be. Variation in background is primarily outside temperature influenced.

Spike times (above 0.5kWh/15 minutes)

• 2-09-17:15
• 2-09-17:30
• 2-10-15:15 (21:45)
• 2-11-14:00 (22:45)
• 2-12-12:00 (22:00)

How that compares to use, "percentage wise" depends how much is used, percentage-wise. But that's a hard number for the often wildly estimated standby loss on a well-built (but not fanatically superinsulated) modern tanked heater.

For your use-case, if the tanked heater is 30% cheaper power cost, it will be a win unless you hardly ever use hot water at all. The kWh for resistance-heating the water is exactly the same for the same water use at the same temperature (regardless of storage temperature, since showering in 60°C water will send you to the hospital with second-degree burns, so you won't be doing that, actually.)

Answer 2

It takes 4.18kJ to raise 1kg (or litre) of water by 1 degree Celsius.

From this, we can see that:

• The tank system uses 4180Jx120kgx49C=24.5MJ, or 6.8kWh, to heat the water.

• The tankless system uses 4180Jx120kgx29C=14.5MJ or 4kWh.

Estimating loss is harder. If you mean that it loses 20% of the heat stored within per day, we would need to know the tank size. Assuming 10C loss per day, and a 180C tank, it's 2kWh per day.

However, your maths is unlikely to be correct. When you use hot water, the 60C supply temperature instead of 40C doesn't actually change energy usage much - you use the mixer tap to reduce it down to whatever is necessary, so you use less hot water with a higher supply temperature.

Answer 3

The danger of legionella is a very important point to consider. And there is the possibility to install a counterflow heat exchanger, e.g. DIY -ones as proposed by Rob the plumber on YouTube.

From real experience with both types: A tankless heater with heat exchanger needs ca. 35% less energy (compared to tank heater with heat exchanger), is always ready for use (coming back from business travels, holidays etc.), has lower danger of growing germs, but may need rewiring/new breakers. A tank heater can be adapted to the wiring system (heating during night time) and to special lower tariffs (night), but needs a regular heating up to ca. 65 degree Celsius to destroy legionella. The tank insulation can nearly always be improved by old clothes, mineral wool etc.

Answer 4

The ruling issue is standby heat losses.

For instance, our water heater at the lodge is on a timer. 36 hours after the timer ran out, I had occasion to check it. It was tepid. So that means it fell from 60°C to 30°C in 36 hours, or 0.8°C per hour.

*Except remember: thermal losses are proportional to the difference in temperature, and ambient was 25C. So, skipping a bunch of math, let's just say our losses at operating temperature were 1°C/hr. (closer to 1.75°C according to comments below).

That 30 gallon tank is about 120 kg, so that means we were wasting 4180 J/deg/kg x 120kg x 1°/hr.

Multiply All that and cancel the units and we get 501,600 joules/hour. Or 139 joules/second, which is watts. (or by the other math, more like 850,000 j/hr or over 200 j/s=watts).

The difference is that (our roached old) heater wastes 139 watts when it is in standby. That makes sense; that is simply a matter of insulation losses. A tankless heater would waste 0 watts while in standby.

Our rule of thumb is that a 1-watt load costs 1 USD per year. Probably works for Euros and quid, too. So about $139/yr or about $12/mo. (by the better math: over $200).

Another factor: Piping distance

There will be some length of piping between the water heater and the spigot. After some minutes of disuse, the water in that pipe will have cooled down to ambient. So the common practice is to flow the hot water valve until the water actually warms up, then set to your shower, washing, whatever. When one is finished, one abandons the hot water in the pipe. This cools down within minutes even if insulated. This is a total energy loss. Worse if someone enlarged the pipes for better flow, since more volume of water is abandoned.

This problem invites a wasteful workaround: "recirculation systems" which pre-load the pipes with hot water. That means fast hot water but it also means much greater heat losses.

What does this have to do with the tankless question? Real simple: Electric tankless units are very compact and do not have flues - which means they can fit in all sorts of places a 30-gallon tank would not. That means the tankless(es) can be moved much closer to the points-of-use, and that means less abandoned water and more convenience. At extremes, the British put their "electric shower" heaters right at the showerhead - and those units only take 8500-10,500 watts (35-45 amps).

Answer 5

Electric tank heaters commonly require a 240V 30A dedicated circuit. Electric tankless need much more power -- I've seen some that need 60A or more. That's not feasible in many homes.

If you have space for a tank (sounds like it since you're considering an electric tank), a better bet would be hybrid heat-pump water heaters. Existing ones still need 240V 30A because they have a backup electric resistance heating element, but newer ones are about to hit the market that dispense with that and only need a 120v circuit. As you may guess, that means recovery times will be slower which isn't great for large families.