Faulty Inverter

I bought an Ellies inverter at Builders Warehouse in Feb 2017. In August 2017 I had to take it in for repairs, after it switched itself off & would not turn on again. Dec 2017 it did the same thing. I again returned to Builders asking for a refund. They in turn phoned Ellies & was told that a refund was only done within the 1st 7 days of purchase - really? - I had the manager called in & again Ellies rep was telephoned with the same story, it must be booked in for repairs. My argument is that if it has to be repaired twice for the same fault in the guareentee period there is a problem with the inverter &I either want a refund or a replacement - which wasnot offered. The Builders management was as helpful as they could be, but I feel that they should have pressured Ellies more to refund or replace. I'm sure they have insurance for this kind of thing. Now in 3 months time if the inverter does the same thing again, it is then out of warranty & I have to AGAIN return to builders for a repair & pay for it - WHY, since its been in for repair twice already. I sent an email to Ellies & to date no response from them.