Comparing solar proposals is difficult because the headline price is the least informative number in the document. The figures that determine whether the investment performs are buried in the assumptions, and they are rarely presented in the same format twice.
Check the yield assumption
Every proposal converts system size into annual generation using an assumed yield figure. A proposal using an optimistic yield produces a better payback on paper without changing a single component. Ask which modelling tool produced the figure, what orientation and tilt were assumed, and whether shading was modelled or simply ignored.
Check the self consumption assumption
Savings depend on how much of the generation the building uses rather than exports, because exported units are worth a fraction of imported ones. If a proposal assumes a high self consumption rate without reference to your half hourly data, treat the saving figure with caution.
Check what is excluded
Common exclusions are scaffolding, roof strengthening, asbestos survey work, distribution network operator fees, switchgear upgrades and out of hours working. Any of these can be significant, and a quote that excludes them is not comparable with one that includes them.
Check the warranty terms
Look for the module product warranty as distinct from the performance warranty, the inverter warranty and any extension cost, the workmanship warranty from the installer, and who administers a claim. A twenty five year performance warranty is worth little if the company standing behind the workmanship offers two years.