Changing my lighting to Solar with Transformer

[gbrown100]

Hi,

I would like to get some solar generating going and like to get things done under my own steam however I respect electricity enough to bring in help where I need it. Basically I want to power my lighting circuit (whole house on one circuit) in my 3 bed semi using solar PV with the option to switch it back to mains again. I am working on the basis that I will get a few panels and a bunch of batteries, I will then replace those bulbs I have that are not low energy to bring down the total wattage of the house. I should have around 16 11 Watt 240 Volt bulbs all together (we are sparing on our lighting usage anyway).

So wiring up the panels and the batteries will not be a problem for me and getting them ino a reasonable transformer will be OK. What I cannot get my head round is the way of swapping between solar and mains. I recently had a UPS fitted at my work for my fileservers and have an isolating switch on the wall that allows me to seamlessly transfer between mains and UPS power which is essentially the same principal...

So, really - anyone done this? Advice on the switch I need? Is it legal (of course, I will get someone to part p and do the switching part) Just how much battery storage will I need to cover 6 11 Watt Lightbulbs for 7 hours (based on winter usage 17:00-23:00)

PS - For those other questions:

1. Solar hot water not much good - I do not have hot water tank and only two of us we really don't use that much
2. 12V - no good since do not want cost & complexity of dual circuits and having to worry about voltage drop so much.
3. I can get solar panels for £1.25 per watt (providing I make them myself) so getting 200 Watts or more will still be quite cheap)
4. Cannot be bothered or afford complete grid connected system as it will not cover my usage and the government does nothing to encourage us mere mortals to move to solar / alternative.

 

[electronicsuk]

2. 12V - no good since do not want cost & complexity of dual circuits and having to worry about voltage drop so much.

What's your basis for thinking this will need dual circuits? To me, it would make more sense to run your lighting circuit at 12v (still with fluorescent lamps, of course) direct from battery. You could then have a charge controller that will accept power from the panels and a backup source (i.e. a 12v charger running from mains supply) and this will take care of the switching.

 

[gbrown100]

What's your basis for thinking this will need dual circuits? To me, it would make more sense to run your lighting circuit at 12v (still with fluorescent lamps, of course) direct from battery. You could then have a charge controller that will accept power from the panels and a backup source (i.e. a 12v charger running from mains supply) and this will take care of the switching.

1. I would have to replace every light fitting in the house. (I assume) Thats 16 bulbs, 18 switches (i only replaced with flushmount brushed chrome a year ago), 16 fittings.

2. Can you really get low energy 12v DC bulbs that give decent light?

3. Then there is running everything from DC I would have to rewire too right? Especially because I have 1 circuit, voltage drop at the end would be horrendous!

Another Question - Do you need PARTP for wiring your lighting in 12v DC?

Basically I am serious about this, I guess if what I am thinking is either going to place much greater demands on batteries or is just plain stupid then maybe I should go the DC route. Where do I start though? I guess I would rather change everything to DC than live with an AC transformed system that just doesn't cut it...

 

[Space cat]

1. I would have to replace every light fitting in the house. (I assume) Thats 16 bulbs, 18 switches (i only replaced with flushmount brushed chrome a year ago), 16 fittings.

There's no need to change the switches.

2. Can you really get low energy 12v DC bulbs that give decent light?

Possibly; try a caravan shop - but in my experience of 12 volt caravan lights, they don't have the same light output as mains lights.

3. Then there is running everything from DC I would have to rewire too right? Especially because I have 1 circuit, voltage drop at the end would be horrendous!

It's precisely because of the voltage drop. To get round this problem in our caravan (wired for mains but running on 12 volts) I had to connect the far end of the circuit back to the CU to form a ring. Some might also argue that having mains and low voltages in the same kind of cables is asking for trouble.

So wiring up the panels and the batteries will not be a problem for me and getting them ino a reasonable transformer will be OK. What I cannot get my head round is the way of swapping between solar and mains. I recently had a UPS fitted at my work for my fileservers and have an isolating switch on the wall that allows me to seamlessly transfer between mains and UPS power which is essentially the same principal...

"-- getting them into a reasonable transformer will be OK". Are you sure you understand transformers? They don't work on DC. You'll need an inverter to turn the DC from your batteries into AC before you can connect it to a transformer. Unless you're an electronics wizard I would get a UPS. They have the inverter and all the switching built in.

Calculating the battery requirements isn't particularly difficult. You have 66 watts running for 7 hours which is 452 watt hours. You can get a UPS that size complete with sealed lead-acid battery but I would go for something much bigger. What will you do if the sun doesn't shine for a day - or three?

Your inverter won't be 100% efficient so you'll want about 600 watt hours of battery capacity - that's 50 amp hours - per day. That's a lot of battery! Something else I learnt from caravans is that batteries don't keep their rated capacity for long. A 75 amp hour battery may only be good for 50 amp hours next year and one of mine died completely after two years. Some of my best batteries were HEAVY DUTY car batteries salvaged from scrapped cars. They would typically give me about 18 amp hours for three or four years - and they were free!

 

[gbrown100]

"-- getting them into a reasonable transformer will be OK". Are you sure you understand transformers? They don't work on DC. You'll need an inverter to turn the DC from your batteries into AC before you can connect it to a transformer. Unless you're an electronics wizard I would get a UPS. They have the inverter and all the switching built in.

Indeed you are correct I just used the wrong word - in understand that the panels will either produce 12 or 24 volts DC which I will then charge the batteries with. The Inverter (inc transformer) will step up to mains voltage AC which I then have to get into the lighting circuit somehow. I can buy a unit that will invert / transform.

You mention a UPS however they expect to be charged from 240V AC, which I could use an inverter for BUT I need to be able to switch to mains if the batteries fail. A UPS does not allow me to select two input sources.

Something else I learnt from caravans is that batteries don't keep their rated capacity for long.

Have you seen this?

http://www.reuk.co.uk/Interconnecting-Batteries-for-Battery-Bank.htm

Basically I am looking at something like this: (What are your thoughts on it?)

http://www.reuk.co.uk/Switch-from-Inverter-to-Mains-Automatically.htm

Or since that seems a bit technical to me at the very least I would like to find a way to wire the newly inverted solar power into the mains lighting circuit by means of some sort of manual switch which would isolate the inverted supply and subsequently switch to grid without tripping anything out or being dangerous. (Exactly what the I have for my servers at work)

Your inverter won't be 100% efficient so you'll want about 600 watt hours of battery capacity - that's 50 amp hours - per day. That's a lot of battery! Something else I learnt from caravans is that batteries don't keep their rated capacity for long.

Are you telling me that I could run my 6 lights for 7 hours per day if I managed to fully charge a standard 240V 600VA UPS every day? That can't be right surely? I have to admit I never even considered the idea of using a UPS, If I could use an inverter behind the UPS and then just run off that with a switch to allow me to go from the UPS to mains if there is a problem.

 

[Space cat]

You mention a UPS however they expect to be charged from 240V AC, which I could use an inverter for BUT I need to be able to switch to mains if the batteries fail. A UPS does not allow me to select two input sources.

Oops! The words Space, cat and plonker suddenly come to mind ---

You're absolutely right. The UPS must be plugged into the mains for auto switch-over but how do you also switch it's internal battery charger from mains to solar?

That second link gives the right general idea. You find an inverter with auto shut-down on low battery then use its own output to switch a relay. All of that will work just fine - but why do they show a solid state relay then start talking about coils???

Are you telling me that I could run my 6 lights for 7 hours per day if I managed to fully charge a standard 240V 600VA UPS every day? That can't be right surely?

It isn't right. The 600VA refers to its maximum power output, not its capacity. If it said 600VA hours that would be different. We have 600VA UPS's around here but they can only supply that kind of power for about five minutes.

PS: I had a look at your first link. My caravan batteries were on separate leads of identical length all converging on a pair of common terminal blocks. They sat in a cradle fixed under the floor directly below the CU. The site has mains power now but the battery box is still there. I always intended to paint a big sign on the front: "CHAPELCROSS B DANGER 12,000 mV KEEP OUT" but I never got round to it.

 

[breezer]

I have to say, it sounds a good idea, but is the whole idea really going to save money? I dont think so.

even at £1.25 / watt for solar panels then the batteries, then the light fittings / lamps (if you change them) chage over switch etc how long before you get your money back?

solar stuff is good in principal but to get max current from a solar panel you are going to need a lot of sunshine, which we dont have in the uk (only the odd day or week)

Any chance of a link to these solar panles, i want some for something elese

 

[gbrown100]

Posted: Tue May 20, 2008 7:51 pm Post Subject:
I have to say, it sounds a good idea, but is the whole idea really going to save money? I dont think so.

even at £1.25 / watt for solar panels then the batteries, then the light fittings / lamps (if you change them) chage over switch etc how long before you get your money back?

Well, I intend on simply keeping all 240 so no fittings, cables etc. Will be batteries, panels, switch and inverter. However you are right, for the amount I am saving it will take a while however... A - I am sick of price rises and paying for fat cat salaries. B - I like this sort of stuff and get a kick from messing around with it. (hopefully not a 240V one...)

solar stuff is good in principal but to get max current from a solar panel you are going to need a lot of sunshine, which we dont have in the uk (only the odd day or week)

Indeed, whilst I have a south facing roof and am thinking of messing with a self positioning rig I know that it's not going to be brilliant, that is why I am purely aiming at my lighting circuit right now.

Any chance of a link to these solar panles, i want some for something elese

http://www.siliconsolar.com/diy-solar-panel-kits.php

The price goes down if you spend on over 200 watts. Not sure on shipping / customs so maybe price is not so good... Of course, you have to build them!

 

[breezer]

Thanks for the link, appreciated, but as you say there is import duty and VAT to pay.

also as you said you still have to make them up, and make them weatherproof, i wish you luck, but TBH i think you will take much longer to get your moneys worth than it looks.

please let us know how it goes

 

[Space cat]

Have a look here for your relay:

http://www.maplin.co.uk/Search.aspx?criteria=relay&source=15&SD=Y

The DPDT 10A 240V relay N09AW should do the job nicely. Use it to switch both live and neutral. Have you considered how you'll connect the solar cells to the batteries?