Ask HN: advice on the customer conversion rate needed

Hi community, I am looking for some advice on the approach to market analysis. We are planning to enter the energy saving appliances market in Europe. The idea is that customer purchases an appliance that monitors his energy consumption and allows to save some money on electricity bills. Appliance (and our analytics service) is not free, so there should be an incentive for the customer to start using it. Say, if he spends X USD on electricity every month without an appliance, and appliance costs Y USD while allows saving of Z USD, then what customer conversion should we expect based on those numbers? It's clear that if an $5 appliance allows saving $50 of monthly bill of $100, then few people will hesitate to buy it. What about $20 appliance that allows saving $50 of yearly bill of $1000? Any advices on the conversion expectations here? Maybe something similar to statistics of 1% paying customers in freemium model? Thanks in advance.