Calculation of Electric Water Heater Capacity

1 BTU will raise the temperature of one pound of water 1°F. You can take it from here, right?

At 100% efficiency, the conversion from heat to electricity is at a rate of 3412 Btu per kWh. A well-insulated tank and lines approach this.

Standard recovery differential is 100 F (water temp rises 100 F) in gallons per hour.

1 kwh will heat 34.12 lbs (~4 gallons) of water 100 F.

A typical 220v heating element is 3000 watts, so you'll get 12 gallons of pretty hot water per hour. A dual element tank will give about 25 gallons an hour.

An 80 gallon tank will hold three 25 gallon showers before showing cooler water.

Plus, it takes an hour to run three showers that big, so add another 25 gallons, for a fourth shower.

You REALLY should look into solar water heating. In an apartment building, it can pay for itself in two years if the system is sized at 85% of maximum demand.

Natural gas is much cheaper than electric. Solar is cheaper still. A 120 gallon system with two 4'x10' panels will give 90,000 btu per day. Use electric backup elements.