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Fred BraxtonHi Richard, Norman and Scott, thanks for your contributions.
We’re using PV to heat most of our hot water — via 240V AC rather than DC.
When our old (15 years), inefficient solar HW died we took a gamble.
We don’t use a lot of hot water anyway so instead of replacing the SWH we spent the equivalent money (and then some!) on beefing up our small 15-year-old 0.75kW PV system to 3.6kW along with a new off-the-shelf mains electric HWC. (Interestingly the additional 2.9kW cost less than the original 0.75kW!) We are grid-tied.
Heart of the WH system is a Chinese SS4 SWH controller. This clever device has a range of user-adjustable timers and temperature differential settings. I’ve configured it to do an electric boost between 11am and 3pm (when PVoutput is normally at its maximum) but only if the HWC temperature is below 45C. On low-PV days of course it sucks mains.
The NZ-designed and -made Solastat and Ecostat can probably do a similar job.
In addition we program the dishwasher and washing machine to operate in that time window also.
The controller also manages the wetback pump (it thinks the wetback is a solar WH). The wetback has its own heat-exchanger in the cylinder. In addition it has a legionella booster function and there’s a manual boost with adjustable cut-off temp.
Critical to the system working well was super-insulating the HWC, valving and pipework, to minimise standing loss. There’s no need for the whole system to sit at 65C all day long when tap temperatures need only be 50C or less.
In case of extra HW demand it’s easy to change settings or just push the manual boost. The keypad is in the kitchen, so very easy to do.
I see this has turned into a rant about efficient waterheating systems! A small investment of about $500 in a decent controller and a bit of user education should have a quick payback. It has certainly given me a feeling of being in the driving seat of this major energy user.
Best to all, Fred
