I get requests for "green" technology almost daily.
Gas tankless's are worthwhile, they save roughly about 30& off the hot water bill, but the installation costs can be inhibiting where gas & venting has to be run as opposed to a simple tank type replacement.
The major savings on gas tankless is the fact that they heat at about 85% efficient as opposed to tank types that heat at 60%.
I have done the math on electric tankless's and in all honesty I feel like a crook if I don't inform customers that they save almost nothing.
The standard for heater tanks standby heat loss efficiency is generally no less than 15 btu per sq ft of surface area for the tank.
(1 therm is about 29.3 KW)
Lets say a standard 40 gal heater is about 2 ft diameter by 4 ft tall...thats about 29 sq ft of surface area.
That means 435 btu/hour, 3.132 therms per month or 7.2036 KW per month.
I'll overestimate the cost of eletricity and call it $0.20 /KW.
My math isn't perfect, but you save $1.50 a month there for an electric tankless.
The average homes service panel for electric is 150 amp, impossible to service a 120 amp appliance along with a full homes demamd for electricity, so this requires a sertvice upgrade on top of the initial installation.
I saw a guestimate at $1500, not likely...trenching runs around $50+ /lineal foot for an excavator alone.
I'm sure my math is fuzzy, buit even if I'm ten percent off, I just don't see how these electric tankless's are even in business.
Gas tanless, on the other hand have proven to save, atop the convenience of not having to wait a half hour because your 16 yr old took her nightly half hour shower.