what does this question mean?
'how efficient can this resource be compared to fossil fuels?'

It means:  Compared to coal, oil, and natural gas, would you need to spendmore or less on this resource to heat a house, or to heat 10,000 houses, or to generate the electricity for a city ?The 'efficiency' of a resource boils down to how much energy you can get from it, use for living, and then clean up after, for a dollar.