OK, I don't like to preach so maybe I was a little subtle before, this is my opinion:
Do you believe those adverts that provide loads of evidence that you can add inches to your middle leg? They have years of medical data, convincing testimonials and real before and after pictures.
Of course they are a scam, but people still pay good money for the products based on hope and ignorance.
So we all hope to save money and many are ignorant of electric stuff, then heres my top tip, don't buy one of these devices. The evidence for them is based on power factor correction, this is a real effect that can be measured and does need to be taken into consideration when designing electrical installations, especially industrial installations.
However domestic installations are billed in kW and do not have an additional power factor element added to their bill. Industrial installations add very expensive equipment to reduce the power factor effect, and may have a pf element added to their bill.
Making comparisons between bills sounds a good way to validate them, but there really are too many variables to give meaningful results. If these devices really save 40%, you should see an immediate effect on your meter, even 10% should be noticeable by eye.
So unless you are being billed a power correction factor these devices are of no use to you, indeed because of how they work they may well actually increase your bill (albeit insignificantly so).
I don't believe my daily horoscope either.
Save money, its good for your wealth!