Off Grid Solar

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moizeau
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Off Grid Solar

Post by moizeau »

I'm looking at putting an off grid solar system in a knackered house we're buying. It will be used for lighting, power tools occasionally and possibly a bit of music. It won't be used all the time or everyday. It's primarily as a workshop for him and a workshop for her. Think big garage, repairs on cars and bikes for him and seeding vegetables and making stuff for the garden, house for her.

I was thinking of 1.5kw panel array at 48v into batteries via the charge controller / inverter and stored in batteries.

Has anyone done something similar that can give me dos, don'ts and makes and methods to go for or avoid. There is so much conflicting information out there, the longer you read, the more hazy it becomes!

Storage methods seem to be more important than the panels.

Cheers

Peter.N.
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Re: Off Grid Solar

Post by Peter.N. »

We have solar panels but they are grid connected so no batteries although we did have a wind turbine at one time working a 24v system plus inverter which was purely an emergency supply for the house, we only used it a couple of times as we rarely get power cuts in spite of being out in the wilds. Eventually the bearings went on the alternator and the batteries started failing so I scrapped it.

Solar panels are certainly the way to go, little or no maintenance, easy installation and now probably cheaper than a wind turbine.

As you say, storage is the most important and most expensive part of the installation. How much you need depends primarily on how often you are going to use it and what sort of loads you are imposing. If you are not using it every day 4 x 12v deep cycle batteries would probably suffice, if you are using it regularly and drawing a lot of current are secondhand fork lift batteries are about the cheapest mass storage solution.

If you can give me more information including how often you will be using it and for how long at a time, plus what sort of load you will be putting on it I can probably give you some more info.

Peter

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moizeau
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Re: Off Grid Solar

Post by moizeau »

Cheers Peter, mainly lights, which these days use next to nothing. My bit will be for me motorbikes so a compressor very occasionally along which grinders, drills. So I may be pulling 3kw at times, but not very often. I was thinking in the lines of a 3kw inverter with 1 to 1.5 kw of panels feeding the battery bank (48v) of around 400ah? This is the bit I'm unsure of because depending on the batteries you can't use the full ah. Heavy loads, if I understand it correctly, can be used with a relatively small solar array if they're not used often and the battery bank is large enough.
Any advice and light you could shed would be much appreciated. Thanks

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Re: Off Grid Solar

Post by Peter.N. »

Yes true enough but 3kw at 230 volts is around 60 amps at 48 volts so you will need some pretty hefty cable cable. Don't buy cheap batteries, traction batteries are probably the best. Check the specification for number of cycles the will last. The more capacity you have the better as they will only be part cycled which will make the last a lot longer, something that will last 1000 cycles would be ideal but get the best you can afford.

It would be more economical to run lights from 12 v which many LED fittings are, saves running the inverter when you only have lights on but if you are running the inverter anyway it won't make to much difference. You may be able to series connect 4 x 12 light fittings and run them directly from 48v.

There are a few firms offering secondhand batteries, there are quite a few ex computer batteries that have had very little use - but you have to find them. I have some 12v 100ah as back up for my amateur radio gear and they must have been on there for 10 or 15 years and are still going strong, I think they are Hawker Siddly. Have a look on ebay.

Peter

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moizeau
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Re: Off Grid Solar

Post by moizeau »

If you could verify my maths please?
4 x 12v / 100ah batteries in series = 4.8kwh
But batteries should only be discharged to 50%, so in effect I have 2400 wh (144000 wmins) of use less whats eaten by the inverter etc.
I've converted to wmins to try and work out usage, e.g. a 2kw kettle takes 3 minutes to boil = 6000 wmins, if you boil it 5 times = 30000.
A laptop running / charging with a 60w transformer for 2 hours uses 7200 wmins
100w lamps for 4 hours uses 24000 wmins
So far I've used 61200 wmins of my usuage.

A 50 litre air compressor (750w) running for a hour = 45000 wmins
A 2000w drill for 15 mins = 30000 wmins

So now at 136200 I'm about out, but is quite a lot of use and I'm not taking into consideration recharging.
I read that the solar array should be at least double the daily draw to be on the safe side, so a 1kw array should give 8kw in decent weather in a day, so that seems fine.
Am I being a muppet somewhere in my calculations? Most of the time there would be minimum usage, a lot of days none.

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Re: Off Grid Solar

Post by Peter.N. »

I can't remember the ohms law equations - although I was a TV engineer for 50 years. :oops: I do my calculations on the basis of it taking one volt to push one amp through ohm. Watts are a bit easier, I reckon a 12 v 100ah battery to have a capacity of about 1kwh (100x12 = 1.2 kwh) including charge/discharge and inverter losses, but that is of course using all its capacity. You can probably run the batteries down to 30% or even lower providing you don't leave them like that. The main thing is to not leave the batteries in anything like a fully discharged condition, that will kill them quicker than anything.

Your calculations sound about right, but the bigger the batteries the less the worry. 8-[ Thes people seem to have been going for some time.

https://www.ebay.co.uk/itm/2-POWERSAFE- ... ondition=4

Peter

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white exec
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Re: Off Grid Solar

Post by white exec »

V = IR
I = V/R
R = V/I

W = IV
W = I²R
W = V²/R

For heating things (kettles etc, and even welding...!) low voltage has a distinct advantage, because the heating effect is a function of the square of the current (W = I²R).
So, if you double the current, you produce 4x the heat; triple it, and it's 9x, etc.

A microwave (700-800W) is one way of avoiding the need for 2∼3kW kettles.

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mickthemaverick
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Re: Off Grid Solar

Post by mickthemaverick »

Nicely done Chris, I hadn't realised you were an estate agent but you clearly have a terrific grasp of "Homes Law"! OK back to my cave :oops:

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moizeau
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Re: Off Grid Solar

Post by moizeau »

Cheers Peter and Chris. That chap in the link seems to have some good prices. Not sure about the descriptions though. In the link he quotes 61ah and also 3kw but the battery states 170ah, so that would be 4kw? I'll contact him to ask about postage to France also.
Why would I be looking at ohms apart from maybe cable sizing?

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Re: Off Grid Solar

Post by Peter.N. »

Thanks Chris, I still won't remember, 1v-1a-1ohm got me through my RAE exam, and you can work it out in your head - at least I used to be able to. :?

Peter

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mickthemaverick
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Re: Off Grid Solar

Post by mickthemaverick »

As an apprentice Peter I was taught that:
Women Are Virgins and Virgins Are Rare
which equates to
Watts = Amps x Volts and
Volts = Amps x Resitance (ohms).

Not a perfect mnemonic but it has stayed with me for 55 years and does the job!! :-D

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white exec
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Re: Off Grid Solar

Post by white exec »

moizeau wrote:
21 Mar 2020, 15:43
Why would I be looking at ohms apart from maybe cable sizing?
Because there's no place like ohm.
(Rather apt, at the moment...)

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mickthemaverick
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Re: Off Grid Solar

Post by mickthemaverick »

There is a spare room in my cave if you feel you need it Chris! :-D