They quote $2,500 10-year savings vs oil. I have my hot water piggybacked on the oil-fired condensing boiler unit that's also used for my central heating, and I doubt I burn more than $250/year total on the hot water side of the equation (in a 4-bed, 4-bath house).
That places my central heating on the order of 5-10x higher energy expenditure than my domestic hot water - this does not seem atypical in my area, and apparently data for the entire EU is similar (60% heating, 15% hot water): https://ec.europa.eu/eurostat/web/products-eurostat-news/w/d...
With the heavy use of direct combustion for home heating in the US, heating wouldn't be hugely interesting in that context. I consume more natural gas for heating each month in the winter than I do for hot water annually.
"Space heating" is also on the same graph, and it's nearly 1-to-1 with water heating. Which seems odd