% electrical efficiency is not a useful measure. What matters is how much do I save (measured in money and/or carbon emissions etc.) compared to taking other me

% electrical efficiency is not a useful measure. What matters is how much do I save (measured in money and/or carbon emissions etc.) compared to taking other measures, such as a more conventional heating solution, better insulation or installing solar panels. If you measure the efficiency of solar water heating in the same way, it uses no electricity, so it's efficiency is infinite.