Powering a garden building with solar, batteries plus grid backup?

[John Smith]

I’d like to purchase the hardware for a basic system that I could use to power (or part power) a garden building I have. I have been taking an interest in solar power over the past year and have a basic idea of how it works. I also have a friend who is a qualified electrician to help with the install. This is what I’d like to do, can anyone advise on if such a system is viable?

I had an office in garden, it has been taken over by my kids for gaming and recreation. I’d like to build a system that can power, or more realistically part-power, this building. I’ll need to power a couple of XBox’s, two screens, an amp, various other bits and bobs and crucially an electric heater. I have ordered a power meter in order to find out how much power the building draws when every device is running.

What I would like to know before I start is this. Is it possible to get a device that will automatically switch to use mains power from solar when the batteries need to shut down? And vice-versa? Obviously I’d like to do this in a legal manner but without having to get permission from my utility company if possible.

I am in the south east of England but will have to place the panels in a less than ideal position due to the layout of my garden so I will probably need the back up of the grid. This is about learning as much as powering the building though so am not 100% bothered about a long ROI.

Can anyone offer me advice about such a setup?

 

[SeaGal]

Welcome! 3 questions spring to mind:-

1) why do you say "... but without having to get permission from my utility company if possible."?
2) what power is the heater and would I be correct in assuming that you will only need to use the heater when the sun isn't shining?
3) how long will the heater need to be on for typically?

 

[John Smith]

SeaGal, thanks for replying!
In retrospect point 1 does look a bit suspicious doesn’t it? Nothing nefarious, I just fancy getting something set up and seeing what can be done with it Without getting anyone else involved. Certainly don’t want to be doing anything illegal!
The heater is a little 1500w fan heater. The building itself is well insulated and I have measured it losing about 1c an hour when the outside temp is 5c But I do anticipate it being used more and more as a kind of sleepover/party/all night gaming session venue for the kids so the heater s definitely going to be on for longer in the winter months. I don’t have to have this heater of course, I can get one that uses less power.
The main thing is that I’m fairly certain that I won’t be able to generate enough electricity to go totally off grid with it so would like to know if there’s a safe way to automatically switch to mains when the batteries are getting low on energy.

 

[SeaGal]

OK - makes sense. Two options are a grid-tied system or a 'pass-through' system. Grid-tie will need you to inform (not permission) from your DNO once it is up and running, up to a 3.6kW size inverter. Pass through doesn't need that, though IIRC on second thoughts, you may now need to inform DNO depending on battery size. No big deal though, just a form to send off.

Really depends on whether you may want to expand the system to cover (partial) house usage in the future and what budget you are looking at?

In my mind, if you are deploying panels, batteries and a hybrid inverter why not benefit from (some of) that power powering the house, assuming that the kids won't be in the shed all the time ?

 

[Rednecktek]

I'm going to defer to SeaGal here on the paperwork end of things, but as for the functional end of the system it's pretty do-able. The biggest hurdle I see is that heater. For reference, a 1500w electric heater will drain a 100Ah 48v battery completely in about 3.5 hours. The general consensus for heating is "Anything But Solar!!" because it requires a LOT of money in batteries to get it to run long enough to warm a place up. Common alternates are propane or (my favorite) the parking diesel heaters.

Most AIO units on the market can take input from your grid power/generator/giant-hamster-wheel and change over to feed from that when the batteries hit a low point. That will continue to feed the loads (can't stop pwning n00bs now dad!) and anything left over goes towards recharging the battery.

As for the solar panels, due to location and you mentioning you can't get good angle, I would just grab a measuring tape and figure out how many panels you can physically stick out there, because you're going to need all of it and more.

Outside the heater a 3Kw unit and a 100Ah 48v battery would be the simplest option. With the heater you're looking at tripling your battery capacity at the very least. I recommend looking into the Diesel Heaters or another heat source. A heat pump mini-split would also be a decent option if finances allow. While a pretty hefty up-front cost for the mini-split and a 6Kw inverter, they are very efficient and don't suck nearly as much battery power. If you think you want to expand the capabilities, buy the larger equipment now so you don't have to throw it away later OR ensure you get a make/model that is stackable so you could buy a 3Kw MPP or Growatt now and strap on another 3Kw to it later when you install the heat pump.

Batteries are going to be the largest single expense of the system as they start about $1500 for a 100Ah 48v pre-built wherever you go. You can save about 30% if you decide to DIY a battery but for a starter system I wouldn't recommend it. If space is available you can save a chunk of money using leisure batteries as you can usually find a 100Ah rated battery (and lead acid is only good for 50% usage) for about 100 quid anywhere you go. 4 of those gets you about 2400Wh of battery for about $400. Takes up a chunk of space though and won't last forever, but I like to tell people "If you think you might break something, break a CHEAP something." ?

There is also the option of using individual components which is great for learning but will be more expensive than an AIO. Outside of batteries you'll need a charger/inverter and a charge controller. There are quite a few charger/inverters out there that will take from the grid when the batteries get too low and you'll need a good charge controller to charge up the batteries. Add in all the extra wire, fuses, breakers, etc and it does get quite a bit more expensive, but since most charger/inverters are larger units you'll have more capacity for loads. There aren't really any small charger/inverters out there, especially for 48v.

Now, let me go grab my blurb to get the rest of the project pointed in the right direction.

 

[Rednecktek]

Blurb time!

Well, I'll start the default answer to these questions and we can work from there. Here's you To-Do list:

1: Power audit! This will give you some important information on how big your inverter needs to be as well as how much battery capacity you'll need. There is a link in the FAQ section (I think, or someone here will post it shortly) so fill in the blanks and see what it comes up with. You'll probably need some sort of Kill-A-Watt to get accurate measurements. Are you going to be running a 12v system? 24v system? 48v system? What are the specs on your solar panels? VoC? Vmp? Being as this is a new build, throw together a wish list of what you want and estimate on the high side.

1a: Where do you live? Speccing out a system for Scotland is a LOT different numbers than Arizona due to the amount of light you actually get. Someone here can post the link to the PVwatts.com or JCR Solar Uber-Sun-Hours calculator sites to help figure out how much you'll have to work with. That will be a box in the Power Audit form.

2: Parts list: You don't need a make & model list, just a parts list to start from for reference. You'll need an inverter, a MPPT charge controller, fuses, shunt, buck converter, batteries, wire, etc. Once you have a basic list it can be fine tuned to make & models after that. If you're looking at the All-In-Ones check for correct voltage outputs (120v or 240v Split Phase for North America, 220v Single Phase for European type areas) and make sure it has enough capacity for a little bit of growth and fudge factor.

3: Budget!: Steak is great but doesn't mean anything if your wallet says hamburger. Figure out what you're able to spend now vs what you'll have to cheap out on now and upgrade later.

4: Tape measure! Figure out where you're going to stick all the stuff you'll need. A dozen 3000AH batteries sounds great until you're sleeping on the floor because there's no room left for a bed. Is there a compartment that can house all this stuff? Will the server rack batteries fit? Are you going to have to make space? Physics can be pretty unforgiving.

5: Pencil out what you think you need and throw it at us so we can tell you what you've missed (because we ALL miss stuff the first go-round ) and help figure out which parts and pieces you're going to want to get.

Well that's the thing about solar systems, there is no 1-Size-Fits-All answer. Your system will need to be designed to fit YOUR needs. When you design and built the system, it's not going to be the perfect system for me, or Will or 12vInstall or anyone else, but it Will be the right system for You and that's the goal.

As for where to get started, let me throw my standard blurb in here to help point you in the right direction. There's going to be a lot of math and research involved, but that's going to be a LOT cheaper than just buying parts off of someone's list and finding out that it doesn't do what you need.

Don't panic on the Power Audit, you'll actually be doing that a few times. When you do the first pass put in ALL the Things that you might want. AirCon? Sure. Jacuzzi? Why not. MargaritaMaster-9000? Go for it.

The second pass will be the "I Absolutely Need This To Survive" list that isn't going to have much on there.

The third pass will be the "This is what is realistic" audit that you'll use to design the rest of the system.

The Power Audit is going to tell you 3 primary things: 1: How big does your inverter need to be to power your loads? 2: How much battery bank do you need to last $N number of days with krappy weather? and 3: How much solar panel will I need to install to refill those batteries in a 4 hour day (the average usable sun hours rule-of-thumb).

Once you know what you Want and what you Need and what your budget can Afford there will be somewhere in that Venn diagram where those three things meet.

After that, THEN you can start looking at parts.

Yes, it's a long drawn out process, but it's worth it in the end. Not every house has the exact same floorplan, not every vehicle is the same make & model, and not every solar system is designed the same.

 

[Pennyovert]

Sounds like you've got a great plan in mind! With solar power gaining popularity, your idea of powering your garden building with solar, batteries, and grid backup is definitely viable. Having a qualified electrician friend on board for the install is a big plus.
To address your concerns, yes, there are devices available that can automatically switch between solar and mains power based on battery levels. This setup ensures a seamless transition and can be done legally without involving your utility company.
If you need further guidance, https://www.mklibrary.com/how-to-build-metal-raised-garden-bed/ has some useful resources on solar setups and DIY projects. Good luck with your project, and happy gaming and recreation for your kids in the revamped garden office!