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 spend

more 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.