'ceptin' you need to multiply the therms by the operational efficiency of the heater to get the DELIVERED BTUs. Typically 90% of the kwh used in tank heater end up as hot water entering your plumbing, whereas with gas fired tanks 60% is more the norm. For every therm used, figure you get only 600000BTUs (0.6 therms) out of the tank, and for every kwh used you get 0.9kwh out of the tank. So if you're paying $1/therm you're getting 60,000 BTUs/$. The same 60000BTUs would be delivered using 60000/(3413 x 0.9)= 19.5kwh So it takes 5.1cent's kwh to be cost-competitive with $1/therm gas.
Do your own math with your own utility rates, but rare is the market where NG is a cheaper way to heat hot water. If you live in 5cents/kwh land maybe... (In my neighborhood heating water with electricity vs. gas in a bottom-of-the-line efficiency gas tank is a 3x cost multiplier!)
Putting an electric hot water tank on a timer has an almost vanishingly small effect on how much you pay, unless it's a big tank and your billing arrangement with the utility is peak-demand or time-of-day metered, and you heat it up only with cheap off-peak kwh, coasting through peak demand periods. Standby losses add up to about 0.5-1kwh over a 10 hour period and doesn't drop appreciably until the temperature of the tank is 10s of degrees cooler, which doesn't happen overnight. The vast majority of that small bit of power you didn't use in the off period gets spent bringing the tank back up to temp. The bulk of the energy spent in electric tanks is getting the 45-60F water up to 120F+. If you saved more than 0.1kwh/day (35kwh/year) you'd be lucky. That's less than $10/year except in extreme electric rate situtaions (tiny oil-fired isolated-grid island utilities, etc.)
Still, nickel kwhs are rare, $1 therms common... What it is in your neighborhood is on the utility bills.