FWIW, I have a small 67-litre/18 gallon, 1.5 kW/120-volt electric
water heater that pre-heats the supply feed to my indirect oil-fired
tank and my watt meter tells me the standby losses are a little less
than a 1.0 kWh per day. The exterior surface area of this tank is
roughly half that of a standard size model so, in theory, its losses
would be likewise about half. Then again, it's not as well insulated
and I keep the thermostat set at 70°C/160°F so, on that basis, I'd
expect the losses of a conventional tank to be more-or-less similar.

Why keep the tank set so high? Basically, to reduce the likelihood
that this water will be contaminated with legionella bacteria. To
whit:

"The optimal temperature for Legionella proliferation in water varies
between 32°C and 35°C [90° - 95°F], but it can easily proliferate at
temperatures of up to 45°C. Usually, there is no growth above 55°C,
and a temperature of over 60°C has a bactericidal effect. Thus, the
WHO recommends that water be heated and stored at 60°C. However,
studies in Quebec have shown, even when the thermostat is set at 60°C,
a high percentage (approximately 40%) of electric water heaters remain
contaminated because of the lower temperature, about 30°C to 40°C at
the bottom of the tank. The probability of contamination will increase
considerably if the temperature setting is lowered to 49°C."

Source:
http://www.pubmedcentral.nih.gov/art...?artid=2094925

Once established, legionella is, practically speaking, impossible to
eradicate.

Cheers,
Paul