Trying to work out how to charge 12v battery from 2x160w panels

barney

Hi, I'm new here, and new to solar power and could really use some help!

I have acquired a couple of solar panels, each is 160w, 4.55a, 35.1v

I also have a pretty new 12v / 78ah battery

I only need to power a few led flood lamps (12v /10w X ~5-10 depending on what I can do 😆)

The part I am struggling with is the charge controller.  

I've seen loads of cheap PWM controllers around, but they seem to max out at ~24v max input, whereas my panels each put out ~36v

There is also the question of whether the panels should be in series or in parallel?

I'm in the UK if that makes any difference to the expected panel.performance?  I think given the since of the panels I should be fine, even during winter months, but it would be good to confirm.

I know I'm approaching this backwards, but the panels were given to me by someone who bought a pile and had a couple spare, so that's where I am starting from!

Any advice would be fantastic, along with suggestings that don't cost too much if possible (I know - asking a lot here!)

BB.

Welcome to the forum Barney,

Just to clarify about solar charge controllers:

PWM (Pulse Width Modulation)--The solar charger is a simple computer controlled on/off switch. They can work well, but you need to "match" the solar array to battery bank voltages. Ideally:

• ~17.5 volt Vmp (voltage maximum power) solar array
• 12 volt battery bank

The "match" of Vmp to Vbatt (17.5 volt Vmp to 12 volt Vbatt) allows for (pretty much) all of the solar energy power (Watts) to be sent to the battery bank for charging/running loads.

With your solar panels having Vmp~35.1 volts -- Using a PWM controller with a 12 volt battery bank--You will only harvest about 1/2 of the available power from your solar panels. It is like peddling a 10 speed bicycle in "high gear" while going up a hill. It does work, but it is not very efficient.

MPPT (Maximum Power Point Tracking)--This type of solar charge controller is a computer controlled buck mode switching power supply. This type of controller will efficiently match the high voltage/low current of the array and down convert (with ~95% efficiency) the lower voltage/higher current of the battery bank. (of course, this needs to be within the electrical specifications of the MPPT charge controller).

We can go into the electrical details of an MPPT controller if you wish--But the "simple" analogy is comparing to a car. A PWM controller is like a 1 speed transmission (low gear good match for going up hills, high gear good for freeway use). Vs an automatic transmission which matches engine RPM and torque to vehicle speed and conditions (up hill, freeway, etc.). The MPPT controller will "match" Solar array to your battery banks and loads.

You can use a 12/24 VDC capable PWM controller (i.e., a controller that can charge either 12 or 24 volt battey banks). The controller will (typically) set itself for charging your 12 volt battery bank (connect controller to battery bank first, before connecting solar panels).

Connecting a Vmp~35.1 volts solar array to the PWM + 12 volt battery bank--Simply the current of the solar array (i.e., 4 amps in, 4 amps out) to charging the battery bank:

• 4.55 amps Imp (full noontime sun) * 14.5 volts charging = 66 Watts from solar to battery bank per panel (parallel connect panels)

Vs a MPPT controller with the Vmp~35.1 volt array

• 160 Watt panel * 0.77 panel+controller deratings * 1/14.5 volts charging = 8.5 Amps charging
• 160 Watt panel * 0.77 panel+controller deratings = 123 Watts to battery bank charging per panel

MPPT controllers are usually much more expensive than PWM controllers (more expensive copper coil inside MPPT controller). Which controller you choose (and the wiring parallel/series) depends on which controller you get (PWM vs MPPT; for MPPT controllers, there are different maximum input voltage ratings).

Which do you need--Have to understand your loads first. Lets say you need 3x 10 Watt LED floods for 10 hours per night.

Just as an aside--There are LED Lamps that have 12/24 volt input... You could get a 24 volt LED flood and a second battery--And use your present solar panels (Vmp~35.1 volts), a 24 volt battery bank, and a PWM controller. The 12/24 volt capable LED lamps already have a switching power supply inside each lamp to convert 12/24VDC input to the voltage/current needed by the LED lamp itself. For example:

https://www.amazon.com/QUANS-Floodlight-Security-Outdoor-Waterproof/dp/B01J2TF9EQ

The above LED floods would work with either 12 or 24 VDC battery bank (use same Power or Watts but less current at 24 volts vs 12 volts). Remember Power=Voltage*Current (more voltage/less current = same power in Watts).

The "math"

• Battery Capability (use 25% of battery energy per night): 78AH * 12 volts * 0.25 per night = 288 WH per night
• 288 Watt*Hours per night loads / 30 Watt loads = 7.6 Hours per night

Looking at your location for hours of sun per day by season. Fixed array facing south:
http://www.solarelectricityhandbook.com/solar-irradiance.html

London
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 38° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
1.27	2.04	2.76	3.67	4.17	4.20
Jul	Aug	Sep	Oct	Nov	Dec
4.25	4.16	3.26	2.41	1.53	1.05

Lets use February as "break even month" or 2.04 hours of sun per month:

• 2 * 160 Watt panels * 0.61 DC off grid solar efficiency * 2.04 Hours of sun per day (Feb) = 398 Watt*Hours per day (Feb breakeven)
• 398 WH / 30 Watts = 13 hours of 30 Watt lighting per night average

Now--For a full time off grid system (say fixed security lighting)--I would only use 50% of predicted output for "base loads" (loads that must be used 24x7):

• 13 hours Feb * 0.50 fudge factor derating for always used loads = 6.5 hours per night

Which loops back around to PWM vs MPPT type charge controller... If Vmp~35.1 volt array, 12 volt battery bank, and PWM controller--You would only get 1/2 the energy from the array. So the 13/6.5 hours per night February would be 1/2 that with "simple" PWM controller.

Anyway--A "simple question" gets complicated very quickly. You have some parts already that do not "match well" with PWM controller...

Your questions and corrections to my guesses?

-Bill

barney

Hey Bill,

That is some excellent information!  I'm going to have to digest all that and make sure I understand everything, but that was pretty much everything I was trying (and failing) to work out!!!

I'm only going to need lighting for around ~3h per night - it's mainly for being able to tend to various animals, and usually between sundown and just after.

I am pretty sure I'll likely have more questions, but thanks again, this is fantastic!

BB.

You are very welcome Barney,

With lightning--Before you spend a lot of money on the rest of the hardware... I suggest getting the lights first (and just clip them to your battery--That you are keeping charged with a 12 volt AC Battery charger) and see how much light you will need.

With our eyes--They are amazing "sensors". We are used to measuring things with a tape measure... 1 meter vs 2 meters is a big difference in "something physical".

For light (and sound), we see light (more or less) logarithmically. The amount of light from noon time sun vs evening/morning in the back yard:

https://en.wikipedia.org/wiki/Daylight#Intensity_in_different_conditions

Intensity in different conditions

Illuminance	Example
120,000 lux	Brightest sunlight
111,000 lux	Bright sunlight
109,870 lux	AM 1.5 global solar spectrum sunlight (= 1,000.4 W/m2) [3]
20,000 lux	Shade illuminated by entire clear blue sky, midday
1,000–2,000 lux	Typical overcast day, midday
400 lux	Sunrise or sunset on a clear day (ambient illumination)
<200 lux	Extreme of thickest storm clouds, midday
40 lux	Fully overcast, sunset/sunrise
<1 lux	Extreme of thickest storm clouds, sunset/rise

For comparison, nighttime illuminance levels are:

Illuminance	Example
<1 lux	Moonlight,[4] clear night sky
0.25 lux	A full Moon, clear night sky[5][6]
0.01 lux	A quarter Moon, clear night sky
0.002 lux	Starlight, clear moonless night sky, including airglow[5]
0.0002 lux	Starlight, clear moonless night sky, excluding airglow[5]
0.00014 lux	Venus at brightest,[5] clear night sky
0.0001 lux	Starlight, overcast moonless night sky[5]

Noon vs Full Moon lighing:

• 120,000 Lux / 0.25 Lux = 480,000x more Watts per sq meter at noon vs a full moon.

Why talk about it here--Because solar power is expensive (hardware, maintenance, new battery every 3-5 years, etc.) and choosing the "right light source" can save or cost you a lot of energy...

More or less, with our eyes, you can barely see the difference between two lights (of the same type) where one is 2x more light than the other (i.e., a 10 watt vs 20 Watt flood). But from solar power point of view, the 20 Watt light requires a 2x larger solar power system. And you will probably not even see the A/B difference in useful light.

On the other hand a 10x difference in power is a "night vs day" difference between light sources. The smaller 10 Watt light would hardly be noticable vs a 100 Watt light.

Similar with "spot" vs "flood" lamps... A spot lamp may light up 1 square meter--Vs a flood light which could light 5x5 meter or 25 square meters. So the 10 Watt flood will be 1/25th the light per sq meter vs the 10 Watt flood.

When picking lights... You may only want to light up the "working area" vs an entire yard--To save energy with solar. Small flood/larger spot farther way pointing at walkway, etc....

Also, there are some pretty nifty motion detector lights... Only light when you need it:

https://www.amazon.com/s?k=12+vdc+motion+detector+led+light

Of course--If you have Motion Detector light pointing a a pen/stock area--Then the motion of the animals could end up with the light running the entire night (not something you want either).

Other things to think about--A simple mechancial timer--Twist on for lights, and an hour later off they go--No forgetting lights overnight/into the next day:

https://www.amazon.com/BN-LINK-60-Minute-Countdown-Mechanical-bathroom/dp/B01LVTGKBR

Lots of ways to slice and dice the lightning questions.

-Bill

Photowhit

Renogy makes some reasonably prices MPPT type charge controllers that should work for you. 
If you hunt you should be able to find instructions on line.
Charge controllers are sized for their amp output, amps x volts = watts

So in theory you would have a rough maximum output of the total wattage, 2x160= 320 watts, divided by your system voltage 12 or 320/12=26amps. In reality it will be less than this, between the way panels are rated (you normally only see about 75% of their rating) and that most of your charging will be done before solar noon and at higher than 12 volts you are likely to never see 20 amps charging current.

...but 30 amp controller isn't much more expensive and allows for some expansion. 40 even better.

Here's a quick link to UK eBay listing. Amazon might be cheaper!

https://www.ebay.co.uk/itm/124879630487

barney

Hi again!

Having read through all of this and tried to make sure I understand it, I have a couple of questions...

MPPT charge controllers seems to be the way forward.  i was hoping not to spend quite so much, but there seem to be a few reasonably priced options around, and the advantages are big enough to justify the cost.  I took the plunge and ordered the Renology device @Photowhit suggested 

Does the make any difference wiring series vs parallel? A lot of the MPPTs accept an input in to 100v range, so both panels in series would be 72v @ 4.55A, or in parallel would be 36v @ ~9.1A.  The only difference is an extra connector, but does it make any difference in terms of the choice of charge controller?

In terms of lights, thanks for the advice @BB. I'm going to order a 10w spot and a 10w flood to see what the difference is.  I tried to calculate how much light i would end up with based on height and spread etc, but its almost impossible to really appreciate the difference without actually seeing it.

Thanks for all the advice, I have the panels mounted at 38.4 degrees, and my multi-meter tells me they are pumping out quite a bit of power, even at this time of year on fairly dull days!  I'm looking forward to seeing how well they perform once they're all wired up

BB.

For an MPPT controller--Their "sweet spot" is roughly Battery Votlage * 2 = Vmp-array voltage... I.e., charging a battery bank at ~15 volts = ~30 Volt Vmp (very approximate numbers--I.e., 14.4 volts charging and 35 Vmp-array is fine).

If you run the array at 70 Volts Vmp--The controller may be 95% efficient vs 96% efficient as the 35 Vmp array (not much more wasted energy).

However, the major advantage of a higher voltage array is that the cable run from the array to the "battery shed" can be much smaller in diameter--And this can save you lots of copper costs if you have a longer wiring run (i.e., 10's of meter long wiring run from array to charge controller). If the wiring run is short (less than, say a couple of meters), then cable thickness may not be a big cost issue for you (short runs with relatively low current array--Still not expensive).

With solar, there are major things to watch out for:

1. Voc-cold: In very cold regions, Voc (voltage open circuit) of the solar panel goes up--Need to make sure it does not go over the 100 volts (your controller?) on cold winter mornings. Panel Voc is larger than Vmp (i.e., 42 Voc vs 35 Voc--2xVoc=84 volts--Already getting close to 100 Volts Vpanel-max on a cold winter morning).
2. Vmp-array-hot: On very hot days, Vmp (voltage maximum power) falls (i.e., panel in hot sun gets hot--surprise!)--Should not be an issue here with your panels and battery voltage.
3. Sizing of cables (array, to loads, etc.).: Two calculation. Maximum sustained current. You do not want to overheat wiring. Have a fuse/breaker for wires that "leave the battery bus" to protect wire runs against short circuits.
4. Second calculation is voltage drop: Low voltage wiring and long (skinny) wire runs have substantial voltage drop. We try for 1% to 3% maximum drop (i.e., 12 volt * 0.03 = 0.36 volt drop from battery to your remote loads). Not like a 230 VAC circuit where >6 volt drop and your loads will work fine.
5. Don't under charge/over discharge your battery bank. For Lead Acid batteries, suggest only using, at most, 1/4 of their stored energy overnight. And ensuring that there is enough sun/panels to recharge the next day. If you draw more power and don't replace the power in the next day or two... The batteries will not last very long (months or year--Not the 3-5 years we would try for with smaller batteries).

-Bill

barney

Hi All,

Well the exciting news is that as of a couple of days ago, I have my panels connected up to a Renogy Rover controller happily charging my 78Ah battery, and powering a single 12v 10W flood lamp!

I have to say I am pretty surprised at how powerful a 10W flood lamp is when you're outside in the dark

All in all i am really happy with how things are going, and have been pretty impressed with how much power i'm getting out of the two panels.

I'm now getting to the point where i need to think about getting a bunch of lights.  The main purpose of all of this is to provide some lighting around a field and along a dark lane.  I'm not planning on running my house from this (well, not yet at least )

What I have been wondering about though is DC vs AC.  @BB. had mentioned the improved transmission of higher voltages, and i'm probably going to need to run cabling to lights up to ~70m away.

While I don't plan to run anything huge from the power, it does also seem like it might be useful at some point to be able to run standard "stuff", in which case AC seems like it might be a good idea.

So that brings me to my question: Is it worth the investment to get an inverter?  I've seen a couple online for sub-£100, which is pretty reasonable - like i say i'm not expecting to run a house from this, but more efficient power transmission and the possibility to run standard equipment would be useful.  as an example i've seen https://www.ebay.co.uk/itm/353686679401?hash=item52595e6f69:g:6QcAAOSw-XhhSuwv (1kw DC->AC

The lighting options with AC seem a bit more useful, for example with PIR sensors, and being able to use really standard components (i.e. switches etc) and cost wise they're either basically identical prices to the 12v systems.  That said, am I just adding more loss to the system than I am likely to gain in convenience?

Its easy to get carried away with this stuff.  What I don't want to do though is waste a bunch of money on things that really aren't that beneficial.  My budget is reasonably limited, and this all started as what was supposed to be a small project for some helpful lights, but i have honestly realised how little i really understand electricity  

Thanks in advance as always, the incredible advice here is massively appreciated

BB.

For sending power, AC or DC is not a big difference... It is the voltage and current. The higher voltage line voltage, the lower the current, and the less the voltage drop, and more voltage drop you can allow--I.e., picking 3% max voltage drop:

• Power = Voltage * Current; Current = Power / Voltage
• 10 Watts / 12 volts = 0.83 amps
• 10 Watts / 24 volts = 0.42 amps
• 10 Watts / 240 Volts =  0.042 amps
• 12 volts * 0.03 drop (i.e., 3%) = 0.36 "allowed" drop
• 24 volts * 0.03 drop = 0.72 volts
• 240 volts * 0.03 drop = 7.2 volts

What does all that mean... 12 volt lighting/wiring has much more current, and much more voltage drop, needing much heavier wire.

For larger solar power systems... It almost always makes sense to use an AC inverter to convert the low voltage DC (12/24/48) to higher voltage AC (120/240 in USA, 230 VAC in much of the rest of the world). AC lighting/appliances/etc. are all readily available, and usually very energy efficient these days (thank the newer laws for that), and usually much cheaper to wire.

At the cost of an AC inverter (inverters are getting pretty cheap these days), and (roughly) 50% larger solar ar--HOWEVER, the cost of batteries--Both for the larger battery bank (again ~50% larger) and replacements of larger battery banks every 3-5-7 years adds cost too.

Also, AC inverters "waste power" too (called Tare Losses)... A small 300 Watt inverter may use >6 Watts "just turned on". A larger inverter may use 20-40 Watts in Tare losses... For somebody using a few 10 Watt lights--That can be a fair amount of wasted energy (and larger battery bank and solar array to cover your losses).

Also, depending on how you setup the system--Your lights may be on for 3 hours per night--But the AC inverter will be on for 12 (over night) to 24 hours per day--Eating up more energy:

• 10 watt lamp * 3 hours per night = 30 WH
• 20 watt Tare on a mid size inverter * 24 hours per day = 480 WH per day

Or the inverter could (worst case example) draw >10 more energy than the single 10 Watt lamp...

The usual answer is to do several (or many) paper designs.

• How many XX Watt lamps?
• How many hours per night?
• Longest wire run?
• how many lights on the long wire run (one lamp on  a 100 meter run is different than 10 lamps on a 100 meter run)?

Then do a paper design of a DC only and an AC system. And nothing stops you from having 12/24 VDC lighting "local" to your power system, and few lamps 100's of meters away with a dedicated smaller AC inverter...

Anyway--This all adds up. And can easily push a small DC system into a "larger" system with the new requirements.

There are other options too... Such as multiple small DC solar+lighting units scattered around the property. Either your own "system designs" or off the shelf:

https://www.amazon.com/s?k=solar+street+light

There are even PIR units... Will be "dim" (just enough light for you to see ahead) and they will go "full power" when somebody walks near the light...

Be careful of specifications... You will see a "500 Watt" lamp. No--Just marketing throwing meaningless numbers around.

I would guess if these "inexpensive" lamps last 5 years--That would be a "good life". Simple point of use lamp system (light needs to be in full sun to recharge) vs your wired/home designed system that can have lighting inside barns and dark walkways.

Don't spend money yet... Research, paper design, an perhaps just a couple of sample lights to see how they work for you....

Once you have "answered" the 4 questions above--We can show you how to do the detailed calcuations for wiring up an off grid power system... Or you may opt for the "solar street lamp" approach.

-Bill

barney

Interesting!  I hadn't considered the "always-on" nature of an inverter.  Feels like its not likely to be worth it, at least not at this stage.

I actually bought a single 10W 12V flood lamp to see what it was like, and have been pretty surprised how bright that is in a dark field!  I had considered going 20W or even 30W from some online lux calculators, but after trying out a single 10W i think higher power would be massive over-kill for my needs.

As for the four questions:

• 10W flood lamps seem plenty bright enough, maximum 15 lights
• 2 hours per day, maybe an occasional 3, but 2 seems to cover things pretty well from testing
• The longest cable run would potentially be 120m, but could also be 60m - I'll comment on this below
• On the long cable run it would be max 8 lights.

The longest cable run is along a fairly wide lane.  I could run down just one side, but for complete ground coverage zig-zagging side to side would allow the triangular beams of light to tessellate and form a continuous lighting.  The plan was to have these lights relatively low to the ground (lots of trees etc make height difficult) so the beam wont be hugely wide.  This is an ideal layout, if it causes problems from a power perspective, it could easily be a run down one side and just accept the small areas without direct light.

Other lights would likely be on a second cable run of no more than 40m but with a bit of branching (again, unless that is problematic).  These would also likely have either switches or PIRs, and would be mounted much higher to cover wider areas.

Thanks again for all the help, its been a fascinating project and I really appreciate all your advice.

mike95490

15 lights at 10W each, is going to be 12.5 amps, and you will still get voltage drop at the far end, even using 10ga wire ( rated for 30A )
How your lights respond to low voltage is a question.   Will they just dim ?  Will their driver circuits start to flicker and blink ?

BB.

15 x 10 Watt lamps = 150 Watt AC load
150 Watt * 3 hours per night = 450 WH per day

Those are not "small loads"... And could make sense with an AC inverter (possibly setup a timer of somesort?).

There are "always on" inverters, and those with remote on/off (12 volt control signal from timer/light sensor to inverter?), and those with "search mode" (low power standby, turn on when >~8 Watts of AC loads.

And you could have lights on in areas (local switch, PIR), or you could have a single switch that turns on "everything".

An example of a (pretty muc bullet proof) AC inverter with those options (this is a 110 VAC 60 Hz, but they do have 230 VAC 50 Hz model too):

https://www.solar-electric.com/morningstar-si-300-115v-ul-inverter.html

You can probably find something local and less expensive (import)--But you probably want some of the features (remote on/off, search mode) to help save power... These options were almost non-existent in 12 VDC input small inverters--But now are getting more common.

Since I don't know UK Wiring (mm or mm^2, UK seems to use mm^2 ??) or websites that do voltage drop calculations for Metric/UK wiring--I will do US and you can convert as needed.

https://www.diydoctor.org.uk/projects/cablesizes.htm

If you are using the 12/24 volt Lights--I would highly suggest 24 VDC battery bank... Example (call it 60 meters/100 Feet) Mistake here--30 meters = ~100 feet... (Corrected from 60 meters = 100 feet) at 8 lights and 3% max voltage drop:

• 10 Watts * 1/12 volts * 8 lights = 6.7 Amps
• 10 Watts * 1/24 volts * 8 lights = 3.3 Amps
• 10 Watts * 1/230 volts * 8 lights = 0.35 Amps

Looks like 1 mm (really 1 mm^2?) wiring is good for 8 amps (inside insulated wall) minimum (roughly equal to 17 AWG wire size?).

And a voltage drop calculator. Using 14 AWG (smallest this one goes--About 2mm^2 wire), 100 Ft, 6.7 amps 12 volts:

https://www.calculator.net/voltage-drop-calculator.html?necmaterial=copper&necwiresize=0&necconduit=pvc&necpf=1&material=copper&wiresize=0.4066&resistance=1.2&resistanceunit=okm&voltage=12&phase=dc&noofconductor=1&distance=100&distanceunit=feet&amperes=6.7&x=0&y=0&ctype=nec

Result

Voltage drop: 4.17
Voltage drop percentage: 34.72%
Voltage at the end: 7.83

Way too much drop... To get 3% or less drop:
https://www.calculator.net/voltage-drop-calculator.html?necmaterial=copper&necwiresize=7&necconduit=pvc&necpf=1&material=copper&wiresize=0.4066&resistance=1.2&resistanceunit=okm&voltage=12&phase=dc&noofconductor=1&distance=100&distanceunit=feet&amperes=6.7&x=45&y=19&ctype=nec

Or 2 AWG (in USA, generally only "even" AWG cable is available). Around 33 mm^2 (want to price 200 feet of that)?

Result (12 volt @ 2 AWG)

Voltage drop: 0.25
Voltage drop percentage: 2.11%
Voltage at the end: 11.75

For 24 volt and 3.3 amps @ 100 feet:
https://www.calculator.net/voltage-drop-calculator.html?necmaterial=copper&necwiresize=2&necconduit=pvc&necpf=1&material=copper&wiresize=0.4066&resistance=1.2&resistanceunit=okm&voltage=24&phase=dc&noofconductor=1&distance=100&distanceunit=feet&amperes=3.3&x=53&y=26&ctype=nec

Result (24 volt @ 10 AWG, roughly 5.3 mm^2)

Voltage drop: 0.78
Voltage drop percentage: 3.27%
Voltage at the end: 23.22

And 14 AWG @ 0.35 amps @ 100 feet (2 mm^2) wiring:
https://www.calculator.net/voltage-drop-calculator.html?necmaterial=copper&necwiresize=0&necconduit=pvc&necpf=1&material=copper&wiresize=0.4066&resistance=1.2&resistanceunit=okm&voltage=230&phase=ac&noofconductor=1&distance=100&distanceunit=feet&amperes=0.35&x=35&y=27&ctype=nec

Result (14 AWG @ 230 VAC)

Voltage drop: 0.22
Voltage drop percentage: 0.095%
Voltage at the end: 229.78

Almost no drop... (0.1%)... So higher voltage is better. 230 line voltage--You can use very small diameter copper wire (Ultra Violet/sun/weather resistant) for the long wire runs.

And, depending on the actual wiring (i.e., 8 lamps, starting at battery bank vs 8 lamps 100 feet out, then 1-8 10 Watt lamps), the first segment has 8 lamps worth of current--The last segment as 1 lamp of current... Above is "worst case"--Your actual calculations (drop per segment) is probably "less drop" than above "simple" calculation.

And sizing the battery bank... You decide... But starting is 25% discharge per day, for two days, and 50% max planned discharge (or 1 extra day in "emergency"). Starting with 24 VDC (12 volt does not "work" for long cable runs):

• 150 Watt load @ 12 volts
• 150 Watts * 3 hours per night * 1/24 VDC battery bank * 2 days storage * 1/0.50 max discharge = 75 AH @ 24 volts

Or 2x 75 AH @ 12 volt batteries in series.

You probably need the most light in winter... Let's start with 450 WH per day of lighting (3 hours * 15 lights * 10 Watts) @ 24 VDC.
http://www.solarelectricityhandbook.com/solar-irradiance.html

London
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 23° angle from vertical:
(Optimal winter settings)

Jan	Feb	Mar	Apr	May	Jun
1.30	2.03	2.62	3.34	3.66	3.69
Jul	Aug	Sep	Oct	Nov	Dec
3.76	3.73	3.06	2.37	1.56	1.08

December, 3 hours per night, 1.08 Hours of sun per day December average:

• 450 WH * 1/0.77 panel+controller deratings * 1/0.80 FLA battery eff = 676 Watt array for December "break even"
  

Will you charge during "dark weather", will you need less light? 

Because of poor winter sun in your region--You need a pretty large array to keep up with a load like this.

And if you need 3 hours per night in December, then a 2x larger array would not be a poor choice (for solar, you are always over designing the system to have reliable power when you need it).

-Bill

BB.

I made mistake in previous post

100 feet = 30 meters. Not 60 meters.

Bill