Using a Solar Panel to Backfill power on a single AC circuit

Labs
1

Apologies in advance for any ignorance expressed in the following. Also, this is very much a thought exercise and I am no way one soldering iron stroke away from burning down my house.

With that out of the way, would someone mind helping me think through the following?

I have been knocking together super simple 12v devices for my kids during the pandemic. (think a water-pump that drives a waterfall in the bathtub. A motor to cause a cat picture to wave its arm. As series of muffin fans to cool the patio.) I have been using 12v power supplies to do it.

The other week, I hooked the fans up to a 12v solar panel in the yard to see what would happen.
And this got me thinking.

Toys aren’t used very often throughout the day, but the panel is always generating power when it’s in the sun. I understand that you could use a grid-tie inverter to convert power generated the AC power which could be used in a typical house.

Wouldn’t it be cool, since the amperage is so low coming out of a single panel converted to AC to simply connect it to a three pronged plug and plug it into the electrical outlet?

(Insert sound of electrical engineers grinding their teeth)

But there are two reasons this is a bad (terrible) idea:

  1. You can’t just hook a grid tie inverter to an AC service as the cycles of the inverter AC and line AC aren’t synched which could cause problems for any sensitive device that is pulling current from that circuit.

2)The much bigger issue: you aren’t prepared for a short. If a short happens on that circuit somewhere, it could be fed by amps from both the solar inverter as well as the house AC service. The house circuit breaker could supply 15 amps (or whatever) while the inverter could provide more. That would mean that you are now feeding more than 15 amps through wires in your wall that are not rated for that amount of amps and could start a fire.

The first issue is something that I would need to be solved of course, but I’m especially interested in how the second could be potentially solved with through Emon devices and programing.

As I understand it, you could install a CT clip and base on the line which you would want to “plug” your panel into. You could then monitor how much power was passing out of the panel. You could then program your inverter to limit itself to produce only the amps needed to fill the gap between the amps being drawn from the panel and what the circuit is rated for (in the 15 amp circuit example, if the CT sees 10 amps passing by, the inverter would know to only produce 5 amps.)

So, I’ll pause here and again apologies for my ignorance.
But am I at least thinking about this right?

Thank you so much in advance,
Andrew

2

Welcome, Andrew, to the OEM forum.

I think with some of those ideas, you need to have a serious talk with your insurers. :exploding_head:

The point about a grid-tie inverter is it will synchronise to the mains - it needs the mains to work, and it won’t work without. And that’s entirely deliberate and for a very good reason: Let’s just imagine you did connect it via a plug and socket, and it carries on working - you immediately have a plug with live pins, and a very good chance of screaming if not dead kids. The consequences don’t bear thinking about.

The bigger issue isn’t an issue, because you’ve forgotten that electrical engineers - all engineers really - are taught to think through all likely scenarios, and the inverter will doubtless be protected internally. As soon as it starts to feed too much current into the fault, its protection will trip and it’ll shut down, most likely far too quickly to heat the cables, and most probably far quicker than the 13 A fuse in your plug will blow, even if it ever sees enough current, which is most unlikely. Fuses take time to ‘blow’ - a 13 A fuse will carry about 30 A for 5 minutes - you work out how much solar panel you need to generate that much current.

If you want my advice, as a Chartered Electrical Engineer, don’t go down this route. If you really want to do it yourself, you must install your solar panels and inverter properly with all the necessary isolation and protection, and observing all the safety requirements. Failing that, get the professionals in.

I’m sorry to put a dampener on your ideas, but you stand to get yourself and your family into big trouble if you don’t fully understand the dangers.

1 Like
3

Robert,
First off, thank you so much for your patience and kindness.
To be sure: I am NOT looking to do this with my Romex or anyone elses’.

But when I first showed the panel driving the muffin fans, my seven year old daughter ask if the electricity could power the reading light in her room. I said yes, but not really.

Her question sent me down the rabbit hole of plug-and-play solar. The overwhelming consensus I found was that they are extremely dangerous because they must be installed on a dedicated circuit. The issue is that, even if the circuit is dedicated during installation, you run the risk of someone plugging in something else to the same circuit at a later time and overloading the wiring on the house.

This made sense to me, but had me wondering why the companies weren’t solving this admittedly critical safety issue. It seems like some folks had developed safety gates that would sense other devices on the same circuit and disconnect the plugged solar, but that seemed crude.

I was wondering why no one had solved the issue at the inverter-level. If the inverter could know how much load was already on a circuit and adapt accordingly. After coming across Emon, it seemed that it would be possible to develop an inverter that could know what was going on and adapt accordingly.

And at the risk of sounding like an even bigger fool, my take away is this:
Emon isn’t even necessary.
The first problem isn’t a problem as any microinverter would adapt itself to the existing network.
The second problem also isn’t a problem as any microinverter worth its salt should know to not input more amps into a circuit than the circuit can handle.
The real problem is that it might be attempted by a fool (myself) who doesn’t know what they’re doing and burns his house down. :slight_smile:

Is that a fair assessment?

Thank you again for your patients and time!

4

You answered that yourself above:

It means exactly that - NOTHING but nothing else on the same circuit. Hard wired, no plugs and sockets.

Actually, you’ve got that the wrong way round too, the cable and its protection is sized to suit the output of the inverter.

At this point, I’ve got to tell you that there’s a British Standard covering the electrical installation in buildings - BS 7671, and it’s invoked by Part 5 of the Building Regulations, and those are law.

Unfortunately, the law doesn’t protect fools, ignorant or not, from themselves. At least you’ve asked first, so not that much of a fool :smiley:. I’ll grant you that.

1 Like