Paying for your solar panels. The most cost-effective way to pay for the installation of solar PV panels is upfront and in full. However, if you don't have the cash to pay upfront, you may want to consider remortgaging or a loan.xpected net profit over 20 years: £11,180 (-£7,000 installation cost = £4,180). Expected payback time: 12-13 years for a £7,000 3.5kWp system. Find out more about how the FIT works and what the rates are in our dedicated guide to the Feed-in Tariff.