Keeping a solar system off-grid in a connected house

I'm wanting to install a system of maybe 3kW peak panel output, location is northern Spain.

We are grid-connected, but the solar system will be off-grid because it will be too expensive and complicated to connect it.

In summer, the power will be used to drive the air-con system, which has an electrical load of 1.5kW max, and heat domestic hot water, also 1.5kW. Air-con usage can be manually controlled, dependent on battery state.: if it's hot or humid, and we have power, we can use it The DHW would be best to be automatic, probably on a timer to limit load to mornings, when the air-con will not be on.

In winter, the power, what there is of it, will be dumped in a hot water buffer forming part of the wood-fired central heating system, thus it is not wasted. At present, the buffer has a 3kW immersion heater in it, but this can be changed to 1.5kW if needs be.

I'm thinking to cable each of these devices to a solar control panel where battery state and panel output is monitored. Each load (the two immersion heaters and the air-con) will have a flying lead at this point with a plug on the end of it, such that they can be physically plugged into the solar system or into nearby (grid-connected) wall sockets; thereby ensuring physical separation of the solar electrical system from that connected to the grid. A less elegant solution than a transfer switch or a zero-export grid-connected inverter but one which requires no proof of competence to specify and connect.

Does this sound practicable?

I should mention that I am an engineer and have wired the house myself, to local code, but have not worked with solar or other uncontrolled electrical energy sources before, so I'm a bit hesitant.

Although possible to plug in and plug out loads I think you would be better served with a manual transfer control panel for the loads you intend to power. There are ready made units or you can build your own. You might look into Off grid with GAB (grid as backup) AIO. This setup does not back feed grid and it is not zero export. Basically it is a UPS system if you are familiar with that. It has a AC input as one supply source and the other source is PV. It can be setup to choose which is the preferable supply based on certain conditions. Indeed some people do not bother with a desperate transfer switch since it is built in. I am including a simple diagram of a Off grid Critical loads panels setup below so you can see how that works.

As you are aware any alternative energy source has to be sufficient to handle the loads you will put on it. PV requires panels, SCC (solar charge controller), batteries and inverter. Batteries are storage and also a load. Panel ratings can be confusing to a newcomer at first so I suggest you do some reading. Understand that every component of your system must work with the other components.

Also keep in mind that it takes power to make power. Any supply you design must cover the overhead of operation.

Matt, I wasn't aware of the AIO, although familiar with smaller UPSs. A first look shows them to be quite expensive per kWh, probably because they all contain lithium batteries rather than lead-acid. I'd be nervous of cheaper Chinese imports on risk of battery fire, but otherwise seems to be an option worth more exploration. Thank you.

Don't confuse power packs or Solar generators with AIO's. AIO's do no contain their own batteries. The owner of this Forum has many videos that you can look at to gain some information from. See the link at the top of the Forum.

LouiseSJPP said:

Does this [a manual plug to move from grid to off grid] sound practicable?

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It is practical and I do something similar to the RV. Must be done safely.

I hard wired my 3 kW inverter to a 30 amp plug. If I want to use the solar, I plug in the inverter. If I want to use city power, I use the shore power connection.

I would not want to be going outside several times a day to switch back and forth. I do plug and unplug a dozen times a year.

LouiseSJPP said:

Each load (the two immersion heaters and the air-con) will have a flying lead at this point with a plug on the end of it, such that they can be physically plugged into the solar system or into nearby (grid-connected) wall sockets; thereby ensuring physical separation of the solar electrical system from that connected to the grid.

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This sounds like it would work, but US code requires these larger wattage items to have a dedicated circuit breaker and not plugged in to a wall. A manual plug and outlet would work, but would not meet a code here in the US if there were could be things on that circuit breaker, like a second outlet.

I was an electronic technician decades ago and just realized why this rule was after I heard of tapping into a dedicated circuit with other items leading to a 30 amp circuit having two high wattage items turned on. Bad things happened, not necessirly right away, but eventually.

chrisski said:

I was an electronic technician decades ago and just realized why this rule was after I heard of tapping into a dedicated circuit with other items leading to a 30 amp circuit having two high wattage items turned on. Bad things happened, not necessirly right away, but eventually.

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Chrissky is making a very important point here. If there's the possibility that both your 1.5kW air-conditioner, and your 1.5kW water heater switch on at the same time, then 3000W of solar is NOT going to be adequate for powering both of them. Just to be clear, 3000W of solar panels is NOT going to make 3000W of power.

There are a couple of reasons for this. First, the panel wattage rating is determined in a test chamber with exactly 1000W/square meter of artificial light with panels held at exactly 25C. In the real-world, panels sitting out in the hot sun will have somewhat lower output. I like to de-rate the specified output to 85% of nameplate.

Secondly, if you have stationary South-facing panels, they will only be close to full output from maybe 10:30am till maybe 2:30pm. Outside of that window, output will be less than 50%, and at say 8am, or 4pm, only 10-20%. 2X is a good rule of thumb; that is if you max load is likely to reach 3000W, then have at least 6000W of panels to support that 3000W load. Having your panel arrays on rotating mounts helps, though I wouldn't expect more than 50% output at 8am even if you can rotate the panels Eastwards toward the sun.

You also want to scale your battery such that it can support that kind of load. 3000W/48V = 62.5A, and let's scale that up slightly to 65-70A for safety's sake. Scale your battery that it will tolerate a 70A draw without ill effect.

Mattb4 said:

Although possible to plug in and plug out loads I think you would be better served with a manual transfer control panel for the loads you intend to power. There are ready made units or you can build your own. You might look into Off grid with GAB (grid as backup) AIO. This setup does not back feed grid and it is not zero export. Basically it is a UPS system if you are familiar with that. It has a AC input as one supply source and the other source is PV. It can be setup to choose which is the preferable supply based on certain conditions. Indeed some people do not bother with a desperate transfer switch since it is built in. I am including a simple diagram of a Off grid Critical loads panels setup below so you can see how that works.

As you are aware any alternative energy source has to be sufficient to handle the loads you will put on it. PV requires panels, SCC (solar charge controller), batteries and inverter. Batteries are storage and also a load. Panel ratings can be confusing to a newcomer at first so I suggest you do some reading. Understand that every component of your system must work with the other components.

Also keep in mind that it takes power to make power. Any supply you design must cover the overhead of operation.

View attachment 207666

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MichaelK said:

Chrissky is making a very important point here. If there's the possibility that both your 1.5kW air-conditioner, and your 1.5kW water heater switch on at the same time, then 3000W of solar is NOT going to be adequate for powering both of them. Just to be clear, 3000W of solar panels is NOT going to make 3000W of power.

There are a couple of reasons for this. First, the panel wattage rating is determined in a test chamber with exactly 1000W/square meter of artificial light with panels held at exactly 25C. In the real-world, panels sitting out in the hot sun will have somewhat lower output. I like to de-rate the specified output to 85% of nameplate.

Secondly, if you have stationary South-facing panels, they will only be close to full output from maybe 10:30am till maybe 2:30pm. Outside of that window, output will be less than 50%, and at say 8am, or 4pm, only 10-20%. 2X is a good rule of thumb; that is if you max load is likely to reach 3000W, then have at least 6000W of panels to support that 3000W load. Having your panel arrays on rotating mounts helps, though I wouldn't expect more than 50% output at 8am even if you can rotate the panels Eastwards toward the sun.

You also want to scale your battery such that it can support that kind of load. 3000W/48V = 62.5A, and let's scale that up slightly to 65-70A for safety's sake. Scale your battery that it will tolerate a 70A draw without ill effect.

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Good to see the figures, thank you. I was planning on only ever having 1.5kW at a time, either one of the water heaters, the summer one or the winter one, and the air-con with neither heater on. I see now that the easiest way to achieve this would be one socket outlet only in service, and three flying leads: only one can be plugged in. A bit messy, but using solar power effectively on a small one-off installation is going to be.

Still needs more thought on what exactly our aims are with this system. In part it is driven by a desire to be ecological, in part because we need to build a roof over our log store and solar panels are not much more expensive now than any other roof covering we could use (in an area where the only other roof type is of heavy half-round tile).

Mattb4 said:

Don't confuse power packs or Solar generators with AIO's. AIO's do no contain their own batteries. The owner of this Forum has many videos that you can look at to gain some information from. See the link at the top of the Forum.

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I may be getting caught here between American and European terminologies. AIO is googling for me: can you tell me what they are or drop a link or two, please?

AIO = All In One. Inverter/Charger and MPPT combined into one box. Can use grid to charge batteries and to replace battery / solar power if there isn’t enough

Hybrid inverters are the same but adds the capability of the inverter/charger to send out to grid

FWIW I find a lot of people in the U.S. grossly overestimate the difficulty to do grid tie. A lot of people here have done it with research + outsourcing annoying bits like drafting code compliant plans. Though I know one or two of the folks already on this thread have the opposite opinion.

LouiseSJPP said:

I may be getting caught here between American and European terminologies. AIO is googling for me: can you tell me what they are or drop a link or two, please?

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You might need to Google Solar inverters. Below is examples of a Off grid AIO and a Solar generator power pack.

Mattb4 said:

Although possible to plug in and plug out loads I think you would be better served with a manual transfer control panel for the loads you intend to power. There are ready made units or you can build your own. You might look into Off grid with GAB (grid as backup) AIO. This setup does not back feed grid and it is not zero export. Basically it is a UPS system if you are familiar with that. It has a AC input as one supply source and the other source is PV. It can be setup to choose which is the preferable supply based on certain conditions. Indeed some people do not bother with a desperate transfer switch since it is built in. I am including a simple diagram of a Off grid Critical loads panels setup below so you can see how that works.

As you are aware any alternative energy source has to be sufficient to handle the loads you will put on it. PV requires panels, SCC (solar charge controller), batteries and inverter. Batteries are storage and also a load. Panel ratings can be confusing to a newcomer at first so I suggest you do some reading. Understand that every component of your system must work with the other components.

Also keep in mind that it takes power to make power. Any supply you design must cover the overhead of operation.

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Nice simplified description. When I looked at battery backup grid-tied systems a few years ago all the solutions were very complex in evolving 7-10+ components and all vendor-specific. Please don't, if I add some more specific comments for general; consumption.

This has all become much simpler with the "solar AIO" where one prime example is the HYybrid Invertor like LVX6048WP. Along with the PV and battery interfaces shown, the LVX6048WP also has a separate Generator input and is also capable of pushing to the grid (in Grid-Tired mode) . It also has an off-grid mode where it treats the AC In as equivalent to a generator and does not try to push current back.

For an existing installation (where you might not want to wire a subpanel), the transfer switch can be accomplished with an external box like the
A510C Pro/Tran2 50-Amp 10-Circuit 2 Manual Transfer Switch. This gives you the flexibility of selectively moving (load) circuits between grid-tied and off-grid. The subpanel is all grid-tied or all off grid.

Ideally, you would want your critical loads to be allocated to the off-grid portion of your system. The problem is that you would now have to size your solar/battery system to the peak critical demand. Here in Southern Arizona, the swing in monthly cooling heating demands is high. In a recent grid-tied energy audit we had an average demand of 54 KW-h per month but a peak of 104 KW-h (worst in through July-Sept). So the point is that being able to accept grid power for these peak months (but in a AIO off-grid mode), but also be grid-tied (in my case another grid-tied invertor) to push some of your excess power during the low demand months to offset these higher demand months.

One of the other important aspects addressed here is to not have the batteries in the house/garage but is a separate detached structure.

As an example he is the system I'm working on which has a grid-tied invertor in the garage, with an off-grid AIO in the shop. Teh house and shop are connected via a subpanel, but that is for the shop not the critical switched loads in the house. The Hybrid Inverter provides growth for PV and all the flexibility for off-grid, battery backup and generator power inpts.


You could do it all with an AIO like the LVX6048WP, but it only handles about 6KW so you would have to parallel it. We are in the the position that we already have a 7.7KW Sunnyboy Grid tied inverter. It is probably easier to get grid-tied approval for the Sunnyboy than the LVX6048WP.

zanydroid said:

AIO = All In One. Inverter/Charger and MPPT combined into one box. Can use grid to charge batteries and to replace battery / solar power if there isn’t enough

Hybrid inverters are the same but adds the capability of the inverter/charger to send out to grid

FWIW I find a lot of people in the U.S. grossly overestimate the difficulty to do grid tie. A lot of people here have done it with research + outsourcing annoying bits like drafting code compliant plans. Though I know one or two of the folks already on this thread have the opposite opinion.

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I second this. I've DIY'ed my grid-tie battery-less system with the only professional electrician involvement being him checking my work and putting his signature on a piece of paper. This is Europe (Poland to be exact)(3~4 years ago).

The hardest part was ensuring the Chinese inverter has the right EU certification and getting official datasheets for everything.

Drawing plans wasn't difficult at all after I saw examples.

Regarding, the off grid system discussed in the thread. I'm not sure how is it in Spain, but I imagine not that different than here in Poland considering same EU regulations are the basis for both countries. Here any device that is capable of generating power has to be certified (there is a list of certified inverters), requires a bi-directional metering, a special contract and permitting, but AIO inverters that use the grid as backup have no such capability so IMO (IANAL) such grid connected systems don't require any special permitting. However, you have to approach the problem in a smart way. Most customer services people working for grid providers have very little idea about pv systems. If you just ring and ask they may well tell you "no, it needs a permit" as they may confuse an AIO with a grid-tie zero export system which is very different and does require bi-directional metering, all the paperwork etc.

So, if I wanted solar power and I didn't want to deal with the complexities and the extra cost of the certified grid-tie inverters I'd use an AIO (in fact I'm in process of setting one up now).

We're talking about devices like the SMH-II series (I'm getting smh-ii-7k, for you a 4.2kv unit would make sense I think). They have a built in transfer switch which essentially bypasses the inverter when the battery is not charged. This way you could have an extra subpanel fed by such inverter just for your air-con and DWH. If your battery was too low, it would switch to grid power automatically.

Luk88 said:

So, if I wanted solar power and I didn't want to deal with the complexities and the extra cost of the certified grid-tie inverters I'd use an AIO (in fact I'm in process of setting one up now).

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I think it’s worth keeping an open mind for both scenarios.

And at some point grid tie eliminating batteries and allowing grid assist (so you don’t need to size to surge) / simplifying moving circuits around can overcome the bureaucratic complexity. Hot take: grid tie can have much simpler engineering design and rewiring complexity. As an example, if you’re in a country that allows you to backfeed a grid tie inverter under a certain size, into a branch circuit

I’d recommend for OP to find a local forum to ask about Spanish grid tie rules. I thought Spain was a pretty crunchy country so it’s probably lenient about grid tie. Unless the distribution infra is really weak and they need to protect it