Compare the system size, location, installation date and STC quantity shown in each quote, rather than relying only on the advertised discount. Small-scale technology certificates are generally calculated using the system’s expected renewable output, postcode zone and remaining deeming period, while their market value can change. Check whether each installer has applied the same STC price and whether the quote is showing an upfront discount or passing through a later certificate payment. Ask for the certificate calculation in writing, and confirm current scheme details through the Clean Energy Regulator or an accredited solar professional.