You'd only have a claim if you can show that it is somehow "defective" for an energy-saving box like this to not have an error message or alarm, or an automatic byass--basically, you'd have to show that the normal or standard accepted one does this, and your box was therefore defective in not having those features. And even if that were the case, under the rules of court, you'd need some electrical expert (an electrician or electrical engineer experienced with boxes like this) to testify that it's defective (your testimony, as laypersons, would not be accepted as evidence); since you'd (presumably) have to pay that person to examine your box, write a report, and appear, you could spend more on the lawsuit than the $900 you want to get back. It does not appear, based on what you have written, that this is likely to be  worthile or viable case.