Reading at waterheatertimer.org, they say a water heater runs on average 3 hours per day.
3 hours a day @ 12¢ per Kwh
5500 Watt tank = $59.40 per month
4500 Watt tank = $48.60 per month
3500 Watt tank = $37.80 per month
1500 Watt tank = $16.20 per month
I have no idea for how long per day my water heater actually kicks on. The chart is in line with what I was thinking, if it is only on for an hour a day, it is going to cost me $16.50. I turn it on every other day, sometimes 3 days, so, $8.25, it would seem switching off the water heater will result significant savings. Maybe I should unplug the refrigerator at night also to really add up some savings. I will continue shutting off the breaker to the water heater this month and see what the bill is. Also, can I wire a switch to the water heater to avoid wearing out the breaker?
Ok, I think this approach to estimating costs is not reliable, and overstates the savings of a timer (no surprise from a place that wants to sell you timers).
Each gallon of hot water, assuming it needs to be heated by 70°F, will require 70BTU/lb*8lbs/gal = 560 BTU to heat. Since 1 kWh of electrical energy contains 3414 BTUs of energy, each gallon of hot water you use costs 560/3414 = 0.164 kWh, or about $0.02 of electricity.
First let's estimate your actual usage of water: You say the two of you can get 2 days of usage from a 50 gal tank. Let's say you use 40 gallons in two days, 20 gal a day. This is 600 gal/mo, or about 96 kWh/mo or $12. About of a third of your total elec usage.
The unknown is how much of this energy usage is due to standby loss in the tank, and how much is required to heat the water you actually use. Given that it is still warmish after 3 days, it sounds like the fraction from standby loss is rather small, maybe 1/3rd or less, which is consistent with insulation levels of tanks made in the last 10 years. So, even if you perfectly insulated the tank, you might save 35 kWh/mo, or $4. And the timer will **do nothing** since the tank will stay warm when the timer is off, the heat will still be leaking out, and you will need to run the heater later to replace what is lost (when you turn it back on to take a shower.
So, it sounds like you already use so little hot water you can't save much by reducing usage. Standby losses are $4/mo, might knock it in half with a tank blanket, for $2/mo of savings.