Thank you all your comments.
Now you convinced me and we are going back to gas water heater.
Its just would be nice if we can easily turn the heater off and on, for long or short trip we regularly take.
Turn the thermostat all the way down, then back up to your desired setpoint when you return. With an electronic ignition unit that would cut the standby loss by well over half.
That works for standing-pilots too, but it doesn't turn off the pilot, so it's standby use will have a lower-limit determined by the BTU-rating of the pilot, which is substantial (about 5-7 therms per month), but at a buck a therm that's 17-24 cents/day. While not big money, it's not breaking the bank even for a 3 week vacation.
What Runs with bison said, insulate all of the hot water distribution plumbing, and even the cold-feed within 10' of the tank , and the temperature & pressure outflow piping with 5/8" wall closed cell pipe insulation. It'll save an order of magnitude more money in the course of a year than turning off the HW for a few weeks of vacation time, and save water too, since you'll have higher temps available between successive draws. See:
http://www.leaningpinesoftware.com/hot_water_pipes.shtml
According to in-situ measurements by CA utilities in the past decade, in a typical CA home something like 15-20% of all hot water heating energy gets abandoned in the distribution plumbing. Under IECC 2012 code, hot water distribution plumbing must now be insulated to at least R3 (half-inch wall closed cell foam), but it's still cost effective long term to take it up to R4 (5/8"-wall foam.)