Basically it comes down to amperage.
Are you using a 110 or 220v outlet? And once you figure that out, how many amps do you get at that outlet? Once you have your volts and amps, you can decide how many watts your element(s) can be.
If you are running off of standard 110v outlets, you'll likely want to keep the load at or under 80% of your maximum amperage rating for that outlet. Most of the time, you'll have multiple 110v outlets "linked" together under a single breaker. This is why you dont want to load a single outlet up to 100%.
However if you're using a dryer outlet (220v), those typically have a dedicated breaker for each outlet. You can load this up to 100% of the rating.
For example, in my HERMS system, I have two elements (one for BK, one for HLT), but can only run one at a time. They are connected to a 30A 220v dryer outlet. Each of my elements (5500w) pulls about 23W if I remember correctly. Then I have to factor in my pumps, panel lights, fans, etc. The lights and fans are minimal, but they're still a factor to consider.
**I am not an electrician, and you should have someone verify my advice. I am not responsible if you burn your house down or worse. I have just done a crapload of research on this as I built my own HERMS system about 2 years ago, and consulted with many electrical engineers and an electrician.**