No question about the benefits of the electric heater..........the problem is the huge cost increase.

BTW, a 1/2" line will flow far more water than the furnace, or the electric water heater, could possibly make.

That maybe so with an unrestricted outlet, but that's why we have a 40 gallon tank of hot water. Pressure drop caused by more than one consumer is the problem and increasing flow minimizes that.
I still think the increase in your rental electricity use is due to something other than a new water heater.

Unless you've got a four bathroom house, there is no possibility of suffering from insufficient hot water flow with 1/2" piping. The reason for the 40 gallon tank has nothing to do with quantity..........it's there to maintain temperature for 20 minutes at a "reasonable" flow rate...........maybe 2 gallons per minute.........max.

Take a look at the supplies for the sinks, the DW, and the toilet. Tell me what you find.........

I know taking a shower when the DW or clothes washer was running was unwise. Re-plumbing changed that. And I don't know how the number of bedrooms would affect hot water use or flow. On the otherhand, I don't use 500 kwh a month of electricity to keep my hot water hot.

Agreed. With the tankless coil, you absolutely cannot demand more water than what a shower requires (about 1.5 gallons per minute of 130°F. water). The coil can't deliver the hot water in excess of this rate. However, it's got nothing to do with the size of the pipes............and everything to do with the capacity of the coil and the furnace.

The supply lines to the DW, toilet, and sinks are all 3/8". The only 1/2" line is the washing machine and it doesn't flow at that rate through the very limiting stop valve.

Bedrooms don't affect hot water use.............bathrooms do.

Or water pressure.

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Agreed.

I'm assuming typical city water with 60-80 psi. If you're down around 30 psi, all bets are off.

If that's so, how does a tankless heater make sufficient hot water?

The tankless is limited by volume. It cannot make unlimited hot water............and if you tried to run 3/4" lines without restriction, the water would barely be warm.

Tankless water heaters make 'sufficient' hot water as long as the flow rate demanded does not exceed their rated flow rate.

A Brian points out for a shower, 1.5 - 2 gpm and 130 degrees. If you try to put a shower and washer on the same heater, and run them both at the same time, your water temperature will drop.

You must install an individual heater at each device, or a really big honking heater sized for the combination of devices you want to operate simultaneously, such as dishwasher, shower and a sink or two.

Or, you can put in a tank to store the total gallons demanded by one cycle, which is how the typical "hot water heater" came about in the first place.

there's this thing called Kilowatt Hours....typically how they rate electrical device consumption. If you look carefully at your electronic devices you will, most likely, see a sticker that outlines the consumption of electricity.

so, to put it simple terms, say you have 10 80 watt bulbs lit up
right now in your house and kept them on 24 hours a day/night.

what you have here is 800 Kwh times 24 at roughly $.27 a Kwh
and I'd reckon it'd cost around 52 cents a day to keep 10 80
watt light bulbs on for 24 hours a day in your house. And
I'm paying high here......could be less...by a margain

There's these fellows called "engineers" who will call you out when you put up a condescending post with factually incorrect information.

In your scenario, you're using 800 watts for 24 hours..........equating to 19.2 kwH which will cost you $5.18 per day.