How can I easily measure (or determine) the actual electrical usage necessary to keep a constant temperature for my water heater?
The easiest way I can think of to get some idea of this is to sit by your water heater and listen to it. Assuming you (and no one else) has used any hot water recently (like, within the last hour or so), and that no one uses any more hot water for the duration of this experiment, then the water heater will turn on only to maintain a relatively constant temperature of the water. So, if you can determine how long the water heater runs, and how often it runs, you should be able to determine the approximate energy consumption to maintain the temperature.
For instance, let's say you sit down next to your water heater when it happens to be running. As soon as you hear it turn off, start your stopwatch. As soon as you hear it turn on again (which might be hours later), note the time on the stopwatch, but keep the timer going until you finally hear the water heater turn off again. Now, determine how long it ran, and the "period" of the complete cycle (time elapsed from when it first turned off until it turns off again). Next, if you look around on the water heater, you should be able to find some information about the power consumption in watts or kW (either J/s or kJ/s). The energy consumption per cycle will be that power rating times the amount of time it ran during the cycle. Or, if you want the average power required to keep the water warm, you could take the power rating of the water heater times the amount of time it was on during one cycle divided by the period of the entire cycle. That should give a decent estimate of what you want.
Again, note that it is very important that no one uses hot water during this experiment. And it might take quite a while to do this. Here's some information on water heaters that you might find useful if you want to just do some estimating instead of experimenting: https://en.wikipedia.org/wiki/Water_heating http://homeguides.sfgate.com/many-hours-day-hot-water-heater-run-89124.html