Solar power for your shed/garage/cabin (smaller than a house, basically)

I'm looking at providing power to my garages, one of which is used to store the car and has an electric door which requires 230v AC for ~15 seconds to operate, and the other which will be used as a bike workshop and home office, and will require lights, power to charge a laptop, run a wifi AP and potentially other items - a little headroom would be good.

My thinking is to have panels on the roof tilted to the 38 degree angle that I believe is correct for London, then a solar charge controller that feeds power into a/some batteries.

My current thinking is to take 12v power either direct from the batteries (if a 12v system) or through a 24v>12v converter to the lights, and the wifi AP.

The garage door opener and the sockets will be powered from the inverter, which will be in sleep mode when not wanted.

My understanding is that 24v systems are more efficient, but 12v is simpler, and I'm on the fence there.

I think my continuous power draw during the day time will be modest - 3-4 bulbs at around 3.5 watts per bulb, plus whatever the laptop charger pulls, but I'm trying to build in a sensible overhead so a planning assumption is a constant 50 watts per hour, for 8 hours so I'll need 400 watt-hours, which needs a lot of panel to support.

This is the output over the year of a 360 watt panel, so I think I probably need 400 watts for a safety margin verging on serious overkill:

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Or just totally lose my mind and pull the trigger on this lot.

I've decided to go very slowly, and have ordered a Victron Phoenix 12/250 inverter.

I'm going to see if that will run from my existing solar charger that I use for keeping the 996 battery topped up, which (via it's super cheap and crappy charger) can charger two batteries, one of which will be connected to the inverter, the other is in the car.

The Victron Phoenix has an Eco Mode, which promises 0.8w consumption and the ability to turn on when a load is detected. I have to say I don't believe it, but I'm going to find out empirically - what I don't see is how the RF unit is going to be able to detect the signal from the bipper if it's not powered fully, but we shall see.

Inverter turned up today. I have also ordered an MPPT solar charge controller and an LiFePo4 50ah battery. And lots of quite thick wire. Fun experimenting next week.

Can I use the frame of the garage door as a ground? It’s a big chunk of metal.

You’d probably want an earth spike. Structural metalwork as a means of earthing is reliant on that metalwork being fairly embedded in the ground itself.

It’s like, copper has low resistance so it can carry x amount of current, aluminium has a bit more resistance so you need a fatter cable to carry the same current. When you bang an earth spike in to gain a path to earth, you’re just connecting to much more resistant material, but there’s fuck loads of it - as if the earth itself was one massive cable. So the deeper and more firmly the steelwork is buried in the ground, the better quality the earthing will be.

Same way that the resistance of an earth rod decreases when it rains as it lowers the resistance of the material and improves the connection around it. It has been known for electricians to piss on earth rods so that they can get them to meet test requirements.

Thanks for that, so - sounds like I need to get an earth rod. The garage door frame is bolted to the walls of the garage, it's isolated pretty effectively from the earth underneath the concrete slab.

Hmmm.

Would a waterbutt do?

What’s the ground for? Low voltage DC systems don’t need a ground and inverters powering a single device normally run isolated with no earth (IT) since the scenario where where a human puts themselves between the two live AC wires is extremely improbable.

There's a ground on the inverter, so I thought I should use it - if not doing so won't lead to the end of the world then that's good news.

System is going to be (currently all ordered):
120w solar panel>Victron MPPT 75/10 solar charge controller>Renegy 50ah LiFePo4 battery>Victron Smart battery protect>Victron Phoenix 12/250 Inverter>Hormann Promatic garage door operator.

There's a ground on the inverter, so I thought I should use it

According to the manual it's not connected to anything other than the inverter housing. It's there because in some configurations you might connect one of the AC output wires to it to create a neutral, but that's a whole can of RCD-requiring worms.

Thanks for the info- I’m hugely ignorant in this area so this is incredibly helpful

Without checking the manual, I would say it is acting like an isolation transformer because the earth is not tied to ground. It is theoretically safer because the output voltage is floating and not referenced to true earth.

Ooo this thread is interesting.
So, we’re putting up a 6x8 shed on our allotment soon and some minimal power would be nice.
Is there a super basic system that would generate enough for a light and laptop? No power tool or anything else. Ideally a plug n play system as electrical work gives me the fear.

Sorry if this has already been covered.

There are loads of places that do we off-grid kits, I’ve put my own together because I quite enjoyed it. It would certainly be simpler to buy the whole thing at once so you get everything you need.

My setup should be capable of what you describe, in theory anyway.

There are shitloads of one-box systems now that have a lithium battery, mains inverter, USB outlets and solar panel controller. Mostly sold as "camping power packs". This kind of thing:

https://www.amazon.co.uk/Jackery-Portable-Explorer-Generator-Outdoors/dp/B08RNPYLQW/

The downside is that if you later decide you want e.g. a higher rated inverter you're stuffed. You'd also have to use USB lighting rather than being able to setup a hardwired 12V system. Also probably quite thievable.

I have a Goal Zero version of that, with solar panels- it’s handy but really designed for taking in the car for weekends of camping as the panels the kits come with won’t do much more than extend the run time a little.

Stuff has arrived.

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Ok, I have cable, cable ends, heat shrink and so forth and I've ordered a fuse holder and some fuses.

To check my understanding of this, the fuse should go on the live side of the circuit between power>load couplings, so on my system that would be between:

• Charge controller and battery
  
• Battery and inverter
  
• Battery and 12v loads
  
  

I was going to go with 60a fuses, as that seems a decent size for a small system like mine.

Fun stuff! Following.

Does this make sense?

CC is charge controller and BP is the battery protect unit.

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The battery protection thing is unnecessary with most LifePO4 batteries. The battery has a BMS board inside that does exactly the same thing (cutting the power if the voltage drops too low).

You want one big fuse between the battery and everything else with as little wiring as possible on the unfused side. Ideally the unfused wiring should not be long enough to reach any negative points. I don't see why you'd use 60A fuses when your input is 10A max and your output is <25A.

Then smaller fuses for DC loads connected by spindlier wire.

60a is the smallest available in the mega fuse range which seems to be what the solar kit places use, so I got those- minimal thought involved I admit.

I’ll get some smaller fuses, although I should probably check the draw on the inverter because from memory it was surprising.

In terms of the BMS with battery protection thing- I didn’t know that. Was going on reports on solar forums of people who’d totally drained their LiFePo batteries to the point that they had to use a special charger to wake them up again, which sounded annoying.

My 120 watt solar panel was meant to have arrived today- maybe time to start walking around the block looking in the usual places that courier firms throw large packages.

The stands for my panel have arrived, with a little adjustment of the fixings they look like they'll work out very well.

Initially I'm going to want to move the panel around to see what the best position is - therefore I need cheap-but-heavy things to weight the stand down with. I was thinking paving slabs - anyone got a better idea?

Parasol weights or sandbags