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Off grid house lighting

Ian Wallis

Nov 13, 2017
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Hi, I want the lighting, off grid in my house, max 7 bulbs 12 volt 9watt each, using two new deep cycle 75amp batteries. Do you think 150 amp will be enough for 4 to 7 bulbs for six or seven hours?

The idea is to take the lighting cables out of the consumer unit, and run them to the garage where the batteries are. Will a 100 watt solar panel be enough to charge them at rate of 5 amps? Or should I go 200 watt and 10 amps? Obviously I'd like more batteries but they are so expensive.

Any input welcome. :)
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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Do you mean 75 amphour batteries?

at 12V a 9W bulb draws 0.75 A. 7 of them will draw 5.25 A. A single 12V 75Ah battery will power these for about 14 hours, however that would leave the battery dead flat, and I would not recommend that. two of those batteries will use about a quarter of their capacity over 7 hours. That's nice and conservative.

Steve
 

Bluejets

Oct 5, 2014
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Will a 100 watt solar panel be enough to charge them at rate of 5 amps?
So you charge at 5 Amp but drain at 5.25A for 6 or 7 hours.
Sooner or later, even at idea conditions, you loose.

In the UK especially, I would be surprised if you get 4 to 5 hours peak usable output every day from the solar panel so you will have to work out a percentage there also.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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A 100W panel should be able to charge the batteries at more than 5A if you use an MPPT charger. These will also (typically) remove the load if the battery voltage falls too far.

At a minimum you want to arrange for 150% of the energy taken from the batteries to be replaced each day. Practically you should allow for significantly more so that you can cope with a few days of less than optimal sunshine. Also remember that both battery capacity and solar panel efficiency will drop over time.
 

Bluejets

Oct 5, 2014
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A 100W panel should be able to charge the batteries at more than 5A if you use an MPPT charger.

As I said before, op is in the UK where it rains more often than not and one can only get out what goes in less any losses or inefficiency.
 

dave9

Mar 5, 2017
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Go with the 200W panel and if it's not enough you then need to calculate through real use and worst case estimations, whether you need another 200W or can get by with adding only an additional 100W in parallel.

You might even need far more than that if you need to put a heater on them to melt snow and ice in winter, or will you have grid power available still?
 

Ian Wallis

Nov 13, 2017
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Thank you all for your input, I can see that I will need to beef it all up.

Unfortunately I have run out of room for more panels on my garage roof, so I may have to fit two 200w panels (or more I'm guessing) vertically on the back of my east facing house.

Lol, I just thought, are there any UK regulations about using the domestic 240 volt lighting cables for 12 volt?
 

duke37

Jan 9, 2011
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I doubt if there are regulations for running cables at 12V but the wires must be thick enough to keep voltage drop low and the cables should be labelled so that they are never connected to the mains. Remember that a battery can supply sufficient current to start a fire so use a suitable fuse.

A separate installation would help safety.
 

Ian Wallis

Nov 13, 2017
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Thanks.
Just out of interest, is it possible to use two 100 watt panels, with two 5 amp controllers charging the same batteries?
I'm guessing that if this is possible the panels have to be the same make and same out put (and use in line diodes)?

Just found out the two 5amp controllers that I have are rated at only 100watt input
 

Alec_t

Jul 7, 2015
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Lol, I just thought, are there any UK regulations about using the domestic 240 volt lighting cables for 12 volt?
The lighting cables (typically 1.5mm2, rated for 5A) may get a tad warm, especially if enclosed at any part of their run. As per Oz, there are UK regs re running low voltage cables in the vicinity of mains cables.
 

NMNeil

Oct 3, 2014
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I have a similar system to your plans but have the distinct advantage of living in New Mexico with about 6.5 hours of sunlight each day.
2, 100watt Renogy panels wired in series to give about 45 volts, through a cutoff switch and into a EPever MPPT 30 amp charge controller then via a 30 amp DC breaker into 2, GC2, 6 volt 201 Ah golf cart batteries in series. From there through a 600 watt pure sine inverter to a regular 120 volt breaker box.
The system operates 3, 36" ceiling fans each having 3 LED bulbs and is also hooked up to the shower exhaust fan and vanity lights. The fans are each 100 watt at maximum speed but I run them all at the lowest speed all day and the lights are rarely needed. I'm guessing that each fan on low speed uses about 20 watts, but it's just a guess. I turn them all on in the morning and all off before I go to bed and so far at sundown the batteries normally show 12.8 volts and the lowest ever morning reading was 12.1 volts after I left one of the fans on all night.
The golf cart batteries are about 6 years old so will be replaced with LiFePo. Ordered a few Headway cells to make an experimental battery, but am having the devil of a job finding out which BMS to use. So much contradictory advice out there.
 
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