I'm cold!

Wear warmer clothes, cozy up in a blanket, or turn up the heat - you'll still save energy, compared to warming up in the shower. It takes a lot of energy to heat water, and it's a huge waste to run it off your body and straight down the drain.

Save more, greater comfort

Enclosing your shower can cut drafts and make your shower more comfortable, even with less energy, and you can comfortably turn off the water while soaping. This site shows one way: http://www.instructables.com/id/Cozy-Low-Energy-Shower. I take a simpler approach - I hang a cut-off shower curtain above the door to my shower.

Calculations

The calculations are based on:

2.5 gallons/minute for a normal shower,

3.8 liters/gallon

4187 Joules to heat 1 liter of water 1 degree C

heat water from 50 to 104 degrees Fahrenheit (30 degrees C)

1 Watt = 1 Joule/second = 60 Joules/minute

Hence, the wattage needed to continuously heat water for a normal shower is

2.5 * 3.8 * 4187 * 30 / 60 = 19888.25 watts, almost 20 kilowatts

20 kw corresponds to 2000 10-watt compact fluorescent light bulbs.

That 20 kilowatts is nearly as much as the total electrical capacity of many homes. The typical standard for modern homes is 100 amp, 220 volt service (see this link), or 22 kilowatts.

Installing a low-flow shower head (1.5 gpm) saves 40% of that, 7955 watts or about 800 bulbs, but it still uses 1200 bulbs worth. An older shower head may use over 5 gpm, over 4000 light bulbs.

How many incandescent bulbs? Compact fluorescent?

800 lumens = 60 watt incandescent, 14 watt CFL, or 10 watt LED.

That 20kw shower would power 333 incandescent or 1425 CFL bulbs.

Caveats

At different times of the year, and in different parts of the country, the outside temperature is warmer so you don't need to heat the water 54 degrees. E.g. if the ambient water temperature is 77 degrees, then cut everything in half.

This does not consider conversion losses (when producing electricity), transmission losses, efficiency of a gas or oil hot water heater, or the energy cost for delivering gas or oil to homes. It ignores evaporation - that steam from your shower comes with evaporative cooling, sucking heat out of the air in your home. Now if you're in a hot climate you may think that's good - but if you're running the air conditioner, it uses even more energy to reverse the process to get the moisture out of the air.