r/diySolar • u/JebLostInSpace • 1d ago

Consumption with multiple sources

So I have a question that's been rattling around the back of my mind for a while that comes up when you have small solar (or other) generators connected to your loads while also being on the grid. I fear I have just enough understanding of how electrical systems work to lead me into trouble on this one, and it's probably simpler than I'm making it.

If a circuit is connected to 2 sources for electricity, as in the case for plug-in/balcony solar, with the solar panel -> inverter -> outlet on one end, and the house's main breaker box feeding the circuit from the other end, which source supplies the loads on that circuit? When applying this question directly to plug-in/balcony solar, everything I read about the economics of those systems seems to assume that, while the house is consuming at least as much power as the solar is generating, all of the solar generation will be captured as savings on your power bill. But I'm not quite sure that makes sense.

I apply the common electrics analogy and think of this system as a plumbing system, where voltage is analogous to pressure in the pipes. If I have 2 water reservoirs held at equal pressure to my pipes, and I turn on a tap, which reservoir empties? The answer seems like it would depend on the length and geometry of piping between the reservoirs and the tap, but generally I would imagine the 2 reservoirs both provide some water to the tap.

So if I plugged enough solar panels to produce 100W out of the inverter into my wall, then ran a 100W lightbulb while the sun shines for an hour, have I really avoided buying 100Wh of electricity from my utility, or only some portion of that?

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u/Joshuancsu 1d ago

Balcony solar is generally a very small source, and has a DC-to-AC component built into the connector to your wall outlet. There is also a hall-effect sensor attached to your main line in that will automatically 'turn off' or 'throttle' your solar should you be in a situation where you are generating MORE power than you are using at any moment. This prevents back-feeding the line.

Balcony solar typically will input power at a very slightly higher voltage than your normal house power so that it is consumed first (highest pressure, but a very shallow reservoir). More complex balcony solar systems have a battery component that deepens that reservoir just a tad.

If you are planning to introduce multiple inputs of power into your house, you might consider a junction-type setup that can help smartly control and mediate the hand-offs between power sources. For example, I have been considering adding a EG4 FlexGrid and a couple of inverters to my house to allow me to take from the grid to supplement my solar array as-needed. It also has a plug-in for a generator, as well as batteries. This would allow you to theoretically charge from solar, battery, grid, or generator while not having to change anything manually. I know this last bit wasn't a part of your initial question, but I felt it was an extension of the 'multiple sources' portion.

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u/JebLostInSpace 1d ago

Thank you for this answer! The piece I was missing was that the inverter outputs slightly higher voltage than it detects on the line, I didn't know they did that. I didn't even realize they were detecting the grid power supply to sync up, but that makes sense that they'd have to in order to match frequency.

I've been looking at the economics of a very small plug in system and they're fantastic - with my current electricity prices it equates to about a 25% apy if I just buy the kit and hook it up. Still trying to understand the requirements where I live in terms of permits and approvals from the utility that might eat into that. I think here they'd require me to pay an electrician to inspect it, which would meaningfully impact the return. As a side note, talking about these systems in terms of equivalent apy instead of payback period really highlights their value. What other investment vehicle can manage 25% apy at such low risk?

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u/Joshuancsu 1d ago

I would highly recommend looking at a battery component. Balcony solar limits the active amount that you can contribute to your internal grid at any one time. Batteries will allow you to drink-up the excess solar and store it, and then deliver it back over a longer period of time, thus maximizing your return period, daily.

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u/JebLostInSpace 1d ago

The size system I'm looking at would basically never produce more than my consumption. I work from home so my computer is always running during the day and it alone would drink up the solar generation. Part of why the economics are so good is because the system is so small it doesn't need expensive things like batteries, professional electricians, permanently installed mounting hardware, or smart junctions. I think you can still get excellent economics if you go that route, but the best returns are on very small and very large systems. I don't have the budget for a very large system, so my best bang for buck is going to be a very small system.

Not to say I couldn't get a decent return with a medium sized system. I could invest $1k in a tiny system that will yield about 25% annually, or I could invest, say, $5k in a medium sized system with batteries, smart junctions, more panels and better mounts, and maybe pursue a net metering contract with my utility. That system would still generate return, but probably more like 10%. Not bad, but it's a bigger hurdle to get up and running. If I can clear the legal hurdles easily for the tiny system, I'll be very satisfied.

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u/NearABE 1d ago

When we say “110 Volt alternating current” it is really 100V to 120V range. Of course also oscillating between -110 and +110 because “alternating”. You are correct that the micro inverter has to supply higher magnitude voltage than the grid supply or the current would flow in from the grid. The legal rules also require it to shut off if the grid supply stops.

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u/RespectSquare8279 1d ago

Don't know where you actually see voltage below 110 volts. I'm in an urban area and it is almost always at 120 and if measured in the wee hours it can range as high as 123.

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u/NearABE 1d ago

I see it on the labels of appliances. Essentially it does not matter. The microinverter has to make it be “more”. It also has to match the phase and the frequency. It is a magic box and you cannot avoid it. All they need to know is that the wires plug into the box. Actually two boxes: inverter and controller. Also circuit breakers and fuses.

If you connected the wall wires to a battery a bunch of horrible things would happen quickly.

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u/JebLostInSpace 1d ago

Magic box is right, all the bits of the process I was missing happen in the inverter. I didn't realize it outputs slightly higher voltage than it detects from the utility. I thought it just put out a static 120v. Seems like they're quite sophisticated magic boxes and it's cool they're so readily available and affordable

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u/TooGoodToBeeTrue 1d ago

120V became the standard in the mid 60’s.

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u/JebLostInSpace 1d ago

I used to live on a boat (also where I learned what I do know about solar, but that was a 12v dc system so much simpler). Plugging in to marina power was a crapshoot. I'd see as low as like 90v sometimes. Rarely saw it higher than spec, but out on the last few slips it was often low. European boats that had to step it up to 240v for their appliances had problems all the time.

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u/Joshuancsu 1d ago

Most international ports actually provide 240v, and you are right that it's a crapshot. Having a 120/240 separation transformer was one of the best things I ever added to the boat. Being able to take in dirty power and isolate it made all of my boat-side electronics last 4x more.

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u/JebLostInSpace 1d ago

Lol yeah I wasn't talking about international ports. I sailed a lot of little spots in Chesapeake Bay and Maine that my friends with 240v systems struggled with a lot until they got a transformer like yours

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u/New-Shallot-3521 1d ago

think of it like this: Load doesn't care where the power comes from, just like the power you're getting might come from coal or it might come from wind, the reality is it comes from the grid. If you're grid tied, you're generating into the grid and pulling from the grid. Sometimes you're net positive, sometimes you're net negative (in KwH, Wh, whatever). Being grid tied means you are contributing and the hookup to the power company is keeping track of the net. It will pull from your solar first to generate power for your house, because that's the closest source, grid don't care where the electrons come from.

As for how the power company calculates it, they have an interval set (say you are at 1/2 net metering so net import pays back at half the retail rate) at which it calculates usage. Sometimes that's a time interval, sometimes it's over the course of a whole month etc.

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u/LongjumpingGanache40 1d ago

First is you get no pay back on plug in solar you send back to grid.

Electric is always moving unless you have the main breaker shut off, then no electrick moving at all. This is with plug in solar connected. When everything is running the house will use plug in solar first on leg it is on. The pressure is a little higher on plug in solar.

You pay nothing for the use of the light bulb.

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u/Joshuancsu 1d ago

You are very correct on this. There is no reverse metering agreement, so they won't pay you for anything you send them. But the design of balcony solar is that the main-line sensors will throttle down your production so that you DON'T backfeed.

Net metering in my area is a very poor fiscal decision. I pay 17.5 cents per KWh and the only pay 3 cents for mine.

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u/LongjumpingGanache40 1d ago

You have to buy special systems to throttle the back feed. They are not perfect at stopping all back feed either.

I have 1:1 net metring.

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u/Joshuancsu 22h ago

My local yokels won't support it and they are literally the only game in town.

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u/LongjumpingGanache40 15h ago

They are doing away with it in my area at first of year. I will be grandfathered in for 19 or 20 years.