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Posted By Topic: Solar installation - timer for HWC circuit

Feb 13 2020 09:03

I've been asked to install a timer for the HWC circuit so that during daylight hours only the solar is delivering it's power. At first i though easy but now i'm not so sure. Does anyone have experience with this. The solar install doesn't have battery storage, just instantaneous power. Would the cylinder require a 2nd feed to it with the normal supply being on timer and contactor?

Feb 13 2020 12:55

Unless they've got a special device to manage the available power with a dynamic voltage delivery then any regular PV inverter is going to puke with a fixed resistive load on it. Anytime it can't maintain the required 230 Volts it'll shut down.


Feb 13 2020 12:58

ie: If there isn't 3 kW coming off the panels then it won't heat a 3 kW element.


Feb 13 2020 13:02

Are they wanting direct DC on the element maybe? Efficiency is pretty crap that way too but since it's a cheap solution they probably don't care much about that.


Feb 13 2020 13:17

Just install a timeclock so that the HWC is only powered from say 10am-4pm. Not sure what's difficult about this? There's no point trying to monitor sunlight/solar output, as you'll have no hot water if there is none.

Feb 13 2020 18:43

I have solar with excess exported to the grid. I use a time clock to stop the HWC from heating until 10am. By that time if the suns coming up, it’s up and I’m away to work. My 12 panels max out at 2.8kW so even on a good day I’m not quite covering the 3kW HWC

Feb 13 2020 19:05

I have zero experience with solar so i'm not sure how the system works. I will have to go back to have a more thorough look. Her request was that while the sun is up the cylinder is being powered via the panels or more specifically that while the sun is up the cylinder doesn't have the normal supply available. I'll have to check the capacity of the system too. I imagine they have a 3kW cylinder. Supplementary question: how do installations with PV arrays decide where to draw current from? I.e either the grid or the solar system. Any relevant links would be handy.

Feb 13 2020 20:06

Need a dual element - one for regular mains and one for PV. Mine has 2 x 1500W elements (which includes a thermostat) and the PV elemnent is connected directly to the array output, no regulators or any other complication (obviously MCB'd). For most of the day, with bright light it sits at around 200v at 5A.

The mains side is on a timeclock so only comes on 11pm-7am and only if the thermostat says heat is required.

The solar has never boiled the tank but if it did, the thing is vented to the roof so couldn't go bang.

Been running 100% since 2012. KISS applies.


Feb 14 2020 02:16

Oops, apologies. I was dreaming that it wasn't grid connected for some bizarre reason.

Of course if the PV doesn't provide enough then the rest comes from the grid. So, yep a timer will help optimise PV use by only heating during the day time.


Feb 14 2020 16:16

PaleAle, this is a common question when people can't wrap their heads around grid tied systems. Nothing in the installation "decides" whether to use the PV generated power or grid power, there's no differentiation between them.
If the PV is pumping out 2kW, and the household appliances are combined drawing 3kW worth of power, 1kW will be drawn from the grid to make it up. Similarly, if 2kW is being generated by the PV, and the household appliances are only consuming 1kW, the remaining power will flow back into the grid and over to your neighbour's.


Feb 15 2020 12:51

Medistat has a non-grid-tied solution. It's a direct DC solution so is cheaper on parts but less efficient use of power from the PV array.

But the big win is, you get to keep the much cheaper night/day electricity plan. The moment you're grid-tied this option is taken away by the lines company. The retailers don't get a choice.


Feb 15 2020 14:17

If you want solar hot water, the most efficient way is solar heating of the water (either directly or using a heat-transfer system- direct is generally best). PV to drive electric heating of the water doesn't provide most efficient use of the insolation, doesn't allow the array to reduce the effective consumption of the rest of the house, and is difficult to get compliant. (Does avoid the need for building consent).

"KISS" may apply; but so do ESRs, and I don't believe the system as described by medistat complies with those in a number of areas. But since that's not related to OP; should be discussed elsewhere.

Given that this appears to be a grid-connected system (PV + inverter); OwenK's is the appropriate option