1) The alternative use that the land would be put to if a solar farm wasn’t built on it
2) The approach to reversibility of the solar farm development (eg whether the land will be
restored back to a sustainable natural state when it is eventually decommissioned)
3) The positive impacts of the substitution effect - ie the reduction in the environmental impacts associated with the marginal primary energy source - basically, the carbon-emitting (probably gas-fired) alternative new energy power plant that would otherwise need to be built (or kept in operation longer) to produce each unit of energy produced by the new Solar Farm being proposed
On the first point, if the alternative use being contemplated by the landowner is, for example, car park, housing, commercial or industrial, then this strengthens the case for the solar farm. If the alternative is agricultural use, then less so. However, even then, low-grade agricultural land use might be a poorer alternative to solar farm use.
On the second point, reversibility to a sustainable natural state is a strong positive. It also has a protective angle – if there’s a solar farm on the land, for which there is this intention to restore it to a natural state at some point, then this is preferable to many other alternatives that would permanently alter the land to a less sustainable state.
The above is a rational analysis, not an emotional one, and so doesn’t address some of the objections like visual impact (which is often a matter of personal taste).