Too cheap to meter, as long as you have no SLA. Much like solar-generated heat is "too cheap to meter" in traditional greenhouses.
Desalination is not sensitive to input power fluctuations, as long as you have a large enough reservoir to even out the spikes. The natural desalination cycle, with evaporation of ocean water, clouds, mountains, and rivers, already worrks like that, but probably a more localized setup with electric pumps and reverse-osmosis membranes could bring freshwater more directly where humans need it.
Labor of installation is anywhere from 5 to 25% of the cost, depending on location. Add in any missing parts (wiring, inverters) and permitting costs, and you may find yourself just as well off buying new panels for the added efficiency.
You'll either need less labor and materials, or get more output from the same land space, depending on your needs.
Reusing old cells makes sense in some applications, but for almost anything commercial or not a handyman special, I don't know that the numbers work out very well in their favor.
Desalination is not sensitive to input power fluctuations, as long as you have a large enough reservoir to even out the spikes. The natural desalination cycle, with evaporation of ocean water, clouds, mountains, and rivers, already worrks like that, but probably a more localized setup with electric pumps and reverse-osmosis membranes could bring freshwater more directly where humans need it.