I bought a 30-year old house that had hydronic panels/tanks installed around the same time as Carter's. I replaced them with photovoltaics in 2003 and did not notice any change at all in my gas bill (used for water/house heating). So the design was not efficient at all, but it did qualify for the tax incentive, which is why everybody was installing them then.

I removed the photovoltaics after a little over 10 years for the same reason that Reagan did, roof repairs. I did not re-install them.