There's actually a number of issues with such a setup. Firstly, the heating element has a fixed resistance, and is designed to run from a fixed voltage so it runs at a particular power rating. For instance, a 2400 watt element draws 10 amps at 240 volts, so it has a resistance of 240/10 or 24 ohms.
Now, if you attached a couple of 300 watt panels connected in series to this and they produce their maximum power at, say, 40 volts each panel, or 80 volts total, that's 7.5 amps from the panels at 80 volts (maximum). But applied across a 24 ohm element you won't get the full output.
Assume the panels produce 50 volts each open circuit, or 100 volts total. Across a 24 ohm element you can see that you will get a maximum of 4 amps from the panels 100/24 but in fact it will be less than that, as as the panel current goes up the voltage at which they produce it goes down, until it hits a point on the panel's IV curve where the voltage and current 'balance' (not really the right term, but you get the idea).
So, in order to get the full output, you have to match the voltage and current of the panel to the element's resistance.
The other issue, apart from the dodgy wiring mentioned, is the thermostat on the tank, which is designed to open when the water reaches a set temperature. These thermostats are designed for AC power, not DC. When the thermostat opens (not likely in the example given by the OP, but for others thinking of doing this), the DC will arc considerably and burn the contacts. In this situation, the thermostat won't last long as it is being used well outside its rating.
There's probably other issues as well, such as the inefficiency of such a system, as PVs are around 16-18% efficient at best, so it's not a good use of roof space, or the electricity being generated for that matter.
Posted Wednesday 23 Mar 2011 @ 3:13:58 am from IP
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