Normally when I look at a product on Amazon, the first thing I notice is the number of sales and 5-star reviews, which is what I've done this time. Some reviews can be fake though, so I keep scrolling to look for the ones which sound genuine. Then I look at the features and what makes one product stand out against the others. This helps me decide my number one choice and the ones that follow. When you haven't tried a product yourself other people's feedback is invaluable.
You are getting 4 units which cover up to 1000 sq feet with this offer - quite impressive. Like all the others on my list, they are safe for humans and larger animals like cats and dogs. I don't know about birds - you'll need to ask a question about it, and if you keep hamsters and guinea pigs at home, then these things are not for you.
QWESEN 4 Pack Indoor Ultrasonic Pest Repeller for Insects and Rodents
This other set of 2 units is rather powerful covering up to 1200 sq feet and can be used both indoors and outdoors. It combines the electromagnetic and ultrasonic technology with a very economic power consumption. The number of 5-star reviews makes it my second favourite.
Plug In Outdoor and Indoor Ultrasonic Pest Repeller for Insects and Rodents by QWESEN
This is number three on my list because it involves a new bionic technology to add to the ultrasonic and electromagnetic action, and I still don't know anything about it. The number of purchases is modest, but the review score is high and the reviews are genuine, so it's worth a try.
Three Way Ultrasonic Pest Repeller for Insects and Rodents for Indoors and Outdoors
This single unit covers a very large area indoors and outdoors, so is powerful and economical at the same time with the consumption of only 1W of energy per day. If you don't want to have devices plugged all over the house then this is the option for you.
Powerful Wide Coverage Indoor and Outdoor Ultrasonic Pest Repeller for Insects and Rodents by Akra
This device uses a range of different frequencies to ensure that insects and rodents don't get used to them. It also combines an ultrasonic and electromagnetic action for a stronger effect. The coverage area of the device is pretty amazing at 155,000 sq feet, so it can be used in warehouses as well as in a large home. For smaller homes like mine, I'd suggest going for a smaller coverage unit.
Smart Variable Frequency Indoor Plug In ULtrasound Repeller for Mice, Ants, Mosquitoes, Roaches and Flies