On 31-Mar-2004, wrote:

A tankless water heater uses less energy overall. But it uses that
energy when energy costs (demand) is higher, so it is, in the end, a
disadvantage. It's higher current drawn, and lack of wattage step-up,
means it causes more significant voltage sags when it kicks in.
Electric utilities don't like them for that reason (because neighbors
complain that lights blink).

That is all true. Tankless electric heaters typically use about 12 kW when
they are running which is a 50 amp current draw at 240 volts. That means you
have to run some hefty wires to the heater, and it's going to eat up a
significant portion of the total power capacity of many main breaker panels.

In addition, industry data posted by another person shows that tankless
electric water heaters save only about $2.30/month compared to tank electric
water heaters. That's right in line with the measurements I took which put an
upper limit of $4/month on heat loss costs.