I would like advice on a potential new set up I am looking at installing in my home.

I have a small water heating system that runs @ 150w 24v DC, I intend to use the solar to power the heating system in the day (or sunny) but at night would like a standard AC/DC psu to power the heating so essentially a constant feed one way another with the solar taking priority when it can.

1) How would I achieve this?

2) Could I use a standard solar regulator and plug the AC/DC psu into the battery terminals?

I'm not positive from your question, but I think you might be over-engineering a typical solar setup. All you have to do is connect the solar panels through your charge controller to the batteries. Then run your DC equipment straight off the batteries. The DC equipment will draw from the batteries as priority, but as soon as they start to drop in voltage, the charge controller will kick on and allow the solar panels to charge them -- which means that you will effectively be pulling the net power from the solar panels so long as their is power available from them.

The only reason you might try to skip the batteries is if you wanted to minimize the load you put on them, but honestly, you should just get a good quality desulfator to put on your batteries and then you'll never have a problem with keeping them in the equation.

As a side note, what a lot of people don't realize is that your solar panels could supply 36 volts to charge the batteries and the batteries will automatically act as a "soak" for the power, which results in 24 volts (if that's what your bank is rated at) available to any appliances you connect. Some engineers argue with this because its not mathematical enough for their minds, but in practice, it works perfectly. Real practice > the model.

To further back my point, most commercial setups will claim you need diodes in your system to prevent reverse current leaking back through your panels when they aren't doing anything. Try measuring the reverse leak current and you will laugh your butt off. It's tiny. The diodes are overkill in a lot of systems. Keep it simple.

  • There are no batteries, or should I say I don't want batteries. All I would like is solar powering the system when it can and when it can't I would like the DC power supply (ac/dc 24v adaptor) to take over. – DVNT May 27 '13 at 23:00
  • Ah. Well the problem you're going to run into then is that you don't have a soak to drop the voltage. Panels are overrated by 50% voltage to make sure they always have enough to charge the bank. If you've got a "24 volt solar panel", check the voltage in full sun and it should show 36 volts. You can't connect those 36 volts directly to your 24V DC device or you will have major problems. You're going to need a relay and some kind of sensor mechanism to completely isolate the circuits. – JamesHoux May 27 '13 at 23:03
  • Like a solar regulator? – DVNT May 27 '13 at 23:04
  • Well, when I used to be into solar, they often called them "charge controllers", but I think a solar regulator is the same thing. Just poke around on google and make sure the terms mean the same thing. When I say "charge controller", what I'm referring to is the device that senses the voltage in your batteries and kicks on a relay to let current flow from the panels when the battery voltage gets low. But you don't have batteries, so you're looking at this a bit different. A "regulator" is usually more of something that limits voltage to a specific value, which might be what you want. – JamesHoux May 27 '13 at 23:12
  • I've googled to no avail, the search continues... – DVNT May 27 '13 at 23:28

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