"Spannung gegen Erde" and "Spannung gegenüber Erde" mean exactly the same thing, that is, voltage against ground, which in turn is equivalent to the (also more or less common) expressions
- voltage to ground / earth
- voltage measured against ground, voltage relative to ground
Thus, any of the three answers suggested so far could be used; nevertheless, "voltage to ground" would probably be the preferred term in most cases and would also fit in this context (equivalent to "Spannung gegen Erde").
The DC voltage generated by solar modules is usually (if not always) referenced to ground (i.e., the voltage of one or *both* poles is measured against the local ground potential) when used in connection with inverters or other kinds of DC-AC conversion circuits that adapt the photovoltaic (PV) power station to the grid. In some cases (depending on the kind of DC-AC conversion circuit), one pole of the PV generator (Plus or Minus, again system dependent) must be even grounded (connected to ground via the neutral conductor on the AC side) for technical reasons.
Cf.
http://www.sunways.eu/static/sites/default/downloads/de/prod...
The translation workplace 
