Can I use PV to heat water or for space heating?

No. Photovoltaic cells convert the sun's energy into DC electricity at a relatively low efficiency level. (14-18%), Therefore trying to operate a high power electric heating element on PV’s would be very inefficient and expensive. Solar thermal (or passive solar) is the direct heating of air or water from the heat of the sun and is much more efficient for heating applications than photovoltaic. (A black barrel full of water kept in the sun).