Environmental
Find the appliances that actually cost money, work out whether a solar array would cover them, and stop paying to fight your own thermostat.
Intuition is bad at this. A 1,000 W microwave used for fifteen minutes a day costs far less than a 55 W fridge that never switches off — because energy is power multiplied by hours, and the fridge has all of them.
A 4 kWp array is a typical domestic size. What it yields depends on where you are, which way the roof faces and how steep it is — the tool carries real irradiance data per city rather than a single national average.
A lot of energy goes into fighting humidity rather than temperature. If the air is damp, drying it makes a room feel cooler at the same thermostat setting — and costs less than dropping the setting.
Put the install cost and your real tariff into the solar tool and read the payback rather than the brochure number. Then compare it against simply using less — which has an install cost of zero.
The chain runs in the right order deliberately: find the real consumers, cut what you can for free, and only then size generation against what is left. Put your own tariff in — it changes every number on the page.