Water heater question

Yes you are right, it is a resistive element.
What you need to know depends on your role. If you are designing them, there is a whole load of things to know. If you are selecting one to use for a particular system, likewise.
If you are a householder with one in a solar system, you are possibly interested in how to minimise the electricity it uses for your particular system and to check that your system design is most appropriate for your particular requirements.
This generally means heating the minimum amount of water to the lowest temperature that is acceptable, with the heating system as near to where it will be used and heated "just in time" to meet the demand - unless you can make use of reduced-price electricity at off-peak hours.
The design of the system will thus typically be radically different between a system designed to use single rate electricity and one designed for multiple rates.
So, if you would care to identify what your specific interest in electrical water heaters within a solar hot water system is?

Thank you Sue for your reply. I'm the householder type. When I bought the system I understood it came with a 2.4KW heater. After a couple of cloudy days recently it didn't seem to heat as quickly as I thought it should. Thinking the heating element may have gone I put a meter in the circuit and got 7.5A instead of the 10A I expected. A 1.8KW heater could explain the longer heating time unless there was something else I didn't know about heating elements. Hence the above post. I can live with an 1.8KW heater now that I know what to expect.
Thank you again for your help
Gordon

A quick calculation leads me to suspect that peak value has been used in rating your elements rather than RMS. At a nominal UK voltage of 230V RMS the Peak Value of the sine wave is 230x1.4 = 322 v. Then 322x7.5 = 2415watts. A bit of a con really but is a bit like saying you have a 30 inch tv when you do but only measuring corner to corner.

I think you might be right Bob, isn't marketing great :o(
Gordon