Posting this late because this is the first severe winter that I've had a chance to use the heater. Not sure what my electric bill will be but that aside, this heater works great. We've had near 0 degree (F) weather for weeks and my pond has no ice at all. The pond is about 10' by 8' by 4' deep and the water temperature has not gone below 44 degrees. My expectation was that it would only keep a small hole in the ice but so far there has been no freezing at all. On one day we lost power and the pond froze over within a couple of hours with 2" of ice. As soon as the power went back on the ice thawed and my KOI and gold fish are again swimming around like it was late fall, early spring. I hope my electric bill is not too high but if it is not, this product is perfect.

Ordered this product after reading reviews. I saw a cold spell approaching for Indianapolis, IN and i was concerned about my small backyard pond freezing over. Thanks to Amazon Prime and two day delivery, I received just in time, Dec. 8th, 2016. Today is December 15th, 2016 and we've had a week of below freezing temps with a few days only in the teens and overnight lows down to 3°F. My little pond has done well with the pond heater keeping a circular hole open. The ice is around 3" thick everywhere. You may notice in my photos that the small waterfall feature is still running. That has helped keep the pond "open" also, but I'm sure within the next day or two, that also will freeze over, leaving only the hole created by the heater. My one small Koi and few goldfish will do fine thanks to this neat little product. I have the 200W model.
1 year later and still works. Here in Indianapolis, we just broke a record for the longest stretch of temperatures never going above 20°F at 11 days and counting. Incredible cold and lasting a long time. Cold like would be normal for Canada. This little heater continues to keep my koi pond open despite being completely frozen over with 6" ice. Highly recommend. Also added new photos from this year.
2 years later… Another cold spell arriving next week with temps forecast to go as low as -20°f. I placed the heater in my pond as it skimmed over with ice this evening and checked on it one hour later. It wasn't working. I'll have to investigate why it quit later, but for now… I just order another one! It works fine even if only one or two seasons, well worth the money.

Disappointed to find out after this arrived that it needed the OPTIONAL temperature controller… so it is on all of the time that it is plugged in. Local one at retail HAS the temperature controller.

Editing review: after contacting the manufacturer they did educate me on a thermostat reset and I think it worked!
It’s called a dry test. Bring it inside for an hour, let it stay dry at room temperature, do not plug it in, take it back outside for an hour or put in a freezer if it’s not cold enough outside, still unplugged, then bring it back in the house and holding it by the cord only, as it could burn you, plug it in. Hover your hand near it, It should fire back up immediately. Mine did! We’ll see how it goes!
They do advise to store the product inside over the summer, btw. And don’t forget to check your electric hook up! My extension cord lit up but was not working compounding the problem.
Buyer beware. Worked great for about four months last season, which is all I needed it for, but does not work at all now. Also I contacted the seller and they shifted me off to the manufacturer, so not much help there.
It is awkward but not terribly heavy. It scales up very quickly and it does not come off. It’s not hot to touch if you tap at it, but dont leave your hands on it too long. No on off indication lights. It worked great for one season, but I wouldn’t buy this again. It’s kind of an expensive throw away product. I definitively expected to last longer. A little disappointed with the seller, but they offered the only advice they could. We’ll see if manufacturer is any more help. I’ll try to update this if they are.