The promise of Solar PV Power is not really worth the effort due to such low payback rates here in NZ - the best is a measly 6c a kw. So our proposal is this, to use a 36v 450w / 900w DC water heating element which also has a 2.1kw Dual AC element (trademe, $70 ).
We are a household of 6, and our main drain on the energy we use is Electric How water with a solar tube array. Our bills and usage (night time rates for water heating) average about $220 a month, using 1100kW our current provider is Ecotricity (previously Electric Kiwi - but they just bumped up their rates by 15%).
The plan A - use solar pv to offset what little electricity is used during the day ( about 0.4kw an hour 8am to 6pm ) with 2 x 270w panels. This will save the 0.27c a kW cost at this time , with 2x solar pv panels @ 270w costing $550 for both, and small inverter $300.
The potential saving : $350 a year - with -0.06c factored in.
The plan B- we are going to test a single 270w panel with the DC element and see how long it takes under real conditions to heat up a 42 Litre Rheem tank fitted with this element. Total outlay $330.
Should this succeed then our potential savings for this tank alone are (based on night rates and 1 hour heat at 3kw ) : $150 a year. If it works we will add / replace an element on our main tank.
Will keep you posted with pictures.