I have one of these beauties in 500w guise.
I went to do a 200L water change yesterday, and as I was heavily hungover I neglected to shut off the temperature controller (which powers the heater) while I drained the sump.
There was a strange smell, and once I'd figured out what it was, I shut off the temp controller. The heater was hotter than an iron or hair straighteners. These things are thick titanium and are very heavy, so hold a lot of heat. 15 minutes later the thing still had enough temperature that drops of water I dripped on it bounced off with a sizzle.
Once cooled, I put it back in the sump (now full of new water) and switched on the temp controller which went to the heat position. 5 minutes later I felt the heater and it was still cold! :cry: I was gutted, assumed I'd burnt it out. It does have a "Do not run dry" sticker on it, although the specs say it's got overheat and run dry protection on it...
Luckily 10 minutes later it was hot and working perfectly again, so a win for those heaters.
If you've ever run a glass heater dry, they get hot, but not even approaching the heat a 500W titanium heater belts out when run dry! So be careful you don't make the same mistake.
(in future, if I ever build another big/bigger setup, when doing the sockets, I'll run a bank of them on a separate master switch which can be "water change" sockets, for all the things that need to be turned off during a water change. )