Thermal Energy Storage heat loss coefficient
The standby losses you see quoted should conform to a testing standard that would allow deriving the heat loss coefficient.

For hot water storage, I would expect typical tanks to have between 2 and 4 inches of foam insulation, so aged R-Values R10-R12 (hr*ft^2*F/btu)/RSI 1.76-2.11(K*m^2/W) on the low end and double that on the high end, with 2 inches being by far the most common.