Electric tankless water heater greenbuildingadvisor.com gas x extra strength vs ultra strength


The grid infrastructure required to run the very high power intermittent load of a whole-house tankless (or even point of use) are substantial. You may even need to upgrade the transformer serving the house to be able to support a tankless electric. A typical whole-house tankless draws in excess of 25,000 watts and needs a 150A/240 VAC breaker. (Many houses don’t even have 150A service for the whole house.) A typical tank draws less than 5000 watts.

In many locations utilities are struggling to recover their infrastructure costs, particularly on low-use or net zero electricity houses with solar. The concept of applying "demand charges" to the billing has been common for commercial and industrial ratepayers to cover infrastructure costs has become commonplace. Demand charges take the heaviest use half hour or 15 minute interval during a billing period and apply a $/kw (not kwh) multiplier to that period, and it’s often the largest single line item for those customers. This type of billing is being proposed for special classes of residential ratepayers all over the US, the most common special class being low total-use solar ratepayers.

If demand charges for solar customers gets implemented in your area, an electric tankless can easily DOUBLE the demand charges assessed- it’s a liability! A single 8 minute shower can add 10kw+ to a 15 minute interval, whereas a typical electric tank adds less than half that. It might use slightly more energy overall (due to standby losses), but it’s drawn out over time.

At 1.5 gpm with a 70F rise is still over 15kw, and the tankless would "only" need a 75A/240V breaker. A pair of those would add more than 150A to the service drop capacity requirements. That’s already 100% of the capacity of the panel in my house, and could be more than the existing panel capacity at David’s vacation house. The transformer serving my house (and 7 others) does not have sufficient capacity to bump all 8 houses on it to 200A, let alone the 300-350A or so per house it would need to run 8 multi-tankless houses.

It’s also not clear whether the 2kV distribution feeder supplying my neighborhood could manage a whole neighborhood at 300A per house. Even if it has that much raw capacity, the power quality in the neighborhood could spin out of control quickly if tankless water heaters became ubiquitous. The infrastructure needed to manage loads like that costs real money, and YOU (and your neighbors) are ultimately paying for it.

Even the largest capacity Level 2 car charger draws less than 100A/240V. Most Level 2 car chargers are under 50A/240V. Unlike water heaters, it’s fine to interrupt car charging when the local grid load is at or over it’s capacity limits, for a few minutes, and "smart" car chargers that allow the use of that intemittent load to stabilize and smooth out the distribution grid load can even extend grid component lifespans while improving grid stability, and moving more kwh. In Denmark electric car owners with smart chargers are already being compensated with cash for allowing the utility to use it for ancillary grid service, whether it’s just a one-way power flow (as people with electric tank water heaters can be paid for that in some parts of the US), or 2-way flow, allowing intermittent draws from the car battery (but still net-charging), where they get paid even more, on the order of €1300/year. A similar (but much smaller) demonstration program exists for the EV fleet at the Los Angeles Air Force Base with energy flows both to and from the car batteries. There the energy is always free, and they are paid between $1000-2000/car-year for the ancillary services provided.

Over the next decade as EVs and plug-in hybrid numbers grow this sort of thing will become common. It will likely become the go-to method of managing otherwise potentially grid-disruptive loads of EV charging if done in an uncontrolled manner. And in the process it improves both grid stability and grid throughput, and becomes a readily available controllable load to suck up the belly of the PV "duck curve" on sunny cool days with minimal mid-day air conditioning load, and lots of excess PV output.

But you can’t do ANY of that with a tankless water heater- it draws what it needs, when it needs it, and would not provide satisfactory service if it could be interrupted intermittently for grid management. It’s just a very heavy, intermittent load that forces the grid capacity infrastructure requirements higher. With no possibility of using it for improving the power quality on the distribution grid, it’s just a problem that other infrastructure needs to be put in place to solve, the opposite of "green", despite the very low standby losses.

The efficiency of an electric tankless is still pretty good, typically scoring 0.99 or better under EF test protocol (= less than 1% thermal losses to the surroundings). That’s compared to the EF 0.95 minimum for the (US) National Appliance Energy Conservation Act since 2015, for tanks 55 gallons or smaller, or a 5% net standby loss.

The EF test assumes something like 63 gallons of water per day, a storage temp of 140F, a room temp of 65-68F etc (it changes a bit every few years- I’m not keeping close track.) If you’re only using 30-32 gallons/day the standby losses don’t change, but will be more like 10% of the total water heating energy, but that’s 10% of a much smaller number.

There’s really no "payback" in financial terms on an electric tankless, even if the thermal losses were ZERO. A typical 50 gallon EF 0.95 might use ~4500kwh/year at "typical" 63 gallons/day use, 225kwh of which is standby loss. At 20 cents/kwh that standby loss is less than $50/year. If you factor in anything for the higher maintenance cost and the additional cost of the heavier breaker & wiring it won’t break-even over the lifecycle of the unit. Most people who end up going that route are all about the space-savings, not energy or cost savings.