Mr Sandman wrote:

It sounds like you can get useful heat from that size of lake, i have
just installed a GSHP on my place which is 170sqm of floor space. Its a
2kw/12kw Heat Pump, but we use 600m of ground loop, some of it in a well.

600m seems a lot. Is that the normal sort of length for a ground loop
and (without the well) how much land would be needed?