.. i dont work for envion. but i would guess they just use regular old electricity at night.
also solar is a small part of the business right. It will be a cost benefit analysis using their new system they have in place that locates cheapest prices overall, which may or may not be solar. they are also providing heat for locations using the miner and being paid for that to offset energy cost.  Also remember that electricity being found cheaply is just icing on the cake. its very profitable without doing that.