Electric water heaters: the best load to control

Typical connected load

A standard residential electric water heater carries two 4,500-watt elements, one at the top of the tank and one at the bottom. They are wired non-simultaneously, meaning only one can energize at a time, so the real connected load is about 4.5 kW rather than 9 kW. Larger tanks and commercial units run 5.5 kW per element and sometimes higher. Verify the nameplate on the tank rather than assuming, because 3.8 kW and 5.5 kW units are both common.

Heat pump water heaters are a different animal. In heat pump mode they draw roughly 0.5 kW, which is negligible for demand purposes. But most of them include resistive backup elements that engage during high draw or cold ambient conditions, and those elements draw the full 4.5 kW.

Why it spikes demand

The water heater is invisible and unscheduled. It does not care what else is running. A thermostat inside the tank senses that the water has dropped a few degrees below setpoint and closes a contact, and 4.5 kW appears on your service with no warning.

That is 4.5 kW added on top of whatever else was already running. If your air conditioner was drawing 3.5 kW and your base household load was 1.5 kW, you were sitting comfortably at 5 kW. The water heater takes you to 9.5 kW. If that overlap lands inside your billing interval, you just doubled your demand charge for the month.

The reheat cycle is also long. A tank that has been drained by a shower will run its element for 20 to 40 minutes to recover, which means it is very likely to overlap with something.

Thermal storage and deferrability

This is the single most deferrable load in a typical building, and it is not close.

A water heater is a thermal battery. A 50-gallon tank at 130 degrees holds a large amount of usable heat, and it loses that heat slowly through the insulation. Delaying a reheat cycle by ten, twenty or sixty minutes does not empty the tank. It just means the tank recovers later.

That is why water heaters are frequently assigned priority 17 on an Energy Sentry system, a special setting that means the load stays off for the entire on-peak period. On a 5-to-10 p.m. on-peak window, the tank coasts on stored heat through dinner, dishes and evening showers, then reheats after the window closes. Most households never detect it.

What an interruption feels like

Nothing. That is the honest answer for the overwhelming majority of homes.

Water comes out of the tap at the temperature it was already at. The element being off does not cool the water. What eventually happens, if you use enough hot water during the deferred window, is that the tank runs down and the water gets cooler than usual toward the end of a long shower. In practice this takes a lot of consecutive hot water use to reach.

Where it sits in the priority order

Near the bottom, meaning it is among the first loads held back and the last restored. On a typical residential configuration the dryer is priority 1, air conditioners are priority 2, electric heat follows, and the water heater sits at the bottom or at priority 17 for the full on-peak block.

Water heaters also get no minimum on-time or minimum off-time restriction. Resistive elements have nothing to protect. The controller can switch them freely, which gives the demand algorithm the most room to work.

The honest caveat

Priority 17 is aggressive, and it is not right for every household. A family of six with staggered evening showers, a home with an undersized 40-gallon tank, or a house where someone works nights and showers at 7 p.m. may need the water heater at a normal low priority instead, so it can grab short reheat windows whenever demand allows. That is a configuration decision made at commissioning, and it can be changed. If your hot water runs short, the fix is a setting, not a hardware problem.

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