Earth Notes: A Note On Going Solar in the North West UK

Solar PV and LED lighting @Home?

Jason Garner sent me an interesting email on going solar at home headed "Powering a 5W Deltech 12V LED Bulb Using Solar Power: Help!" Here is some of our exchange:

I am interested in making our home more energy efficient and have made lots of changes in trying to reduce consumption.

I haven't had any experience with solar power.

I am now trying to gain experience by installing an outdoor porch light to highlight our front door and step- driven by solar power.

I have read a number of different pages about small solar power installations and am now feeling confused.

I bought a 5W 12V deltech cool white led bulb and a 12V 1A max. led driver.

I've also got an MK Guardsman PIR (but it is designed for 240v).

I'd like the light to come on when someone walks up our path for 5-10 minutes before switching off.

What PIR device would be appropriate to use on to a 12V system?

If I dont use a PIR type device for occasional lighting I may want the light to come on during darkness, until sunrise with some sort of 12V friendly photocell.

I have been looking at the following webpages to try to figure out what battery and solar panels I may need.

The LED bulb draws 0.416666* amps.

The battery would have to accommodate minimal sunshine in winter in North West UK, with the solar panel able to keep the battery sufficiently charged.

If you wanted to power an MR16 5W LED Bulb on 12V, what size battery and solar panel would you use?

How long could the bulb be on per night?

How large would the solar battery be?

How large would the solar panel be facing due south in a sunny position?.

Would you use a PIR or photocell or alternative setup?

I'd appreciate your help and advice.

Life isnt a one way street, so If you need any ideas off me, feel free to ask me a question.

Have just installed SMD warm white LED's in our triangulated kitchen lights and now our 8 lights use 21W.

Each SMD LED is equivalent to 17-22W Halogen Capsules.

I've done alot of experimenting with lighting - Halogens, CFL's and now am learning more about LED lighting as the technology evolves.

We've got an extremely energy efficient cooker too which we love- which was a revelation to us.

It cooks better whilst being very energy efficient.

I'd appreciate it if you could help me with my first solar project.


I said that I hadn't yet found a 12V PIR that seemed to be energy-efficient (at least for RE purposes) and that 5W was quite a lot of light at night; maybe have instead a 1W LED on all the time or controlled by a photocell. That also makes the arithmetic a little easier...

To supply 1W for 24h is 24Wh, which is 2Ah at 12V minimum (there will be various inefficiencies), so call it at least 36Ah and 3Ah@12V, if you don't use a photocell or at least don't much reduce consumption with it at the worst time of year.

To really run it all year round purely off solar PV (eg including at the worst time of year, about now) you should cover 3-to-7 days' consumption with the battery for a run of dark days, so ~20Ah+.

To generate enough power from PV from an optimally tilted, south-facing, unshaded panel at the worst time of year you will need roughly 1Wp of panel for each Wh of consumption per day, eg 36Wp of solar at least.

If you want to do 5W on that scheme, multiply up the battery and panel size by 5.

I thought that his other energy-efficiency stuff sounded good too, and he gave me more of his thoughts and experiences:

In the quest to create energy lots of work needs to be done.

In the quest to use energy efficiently I am doing pretty good.

What I've done well is to systematically look at what I have and how I use it over time.

Energy saving is a factor in my decision making, not the reason d'etre.

"A washing machine needs to wash clothes well, be reliable and then be energy efficient".

I'm not suggesting that in buying a washing machine energy saving wasn't important to me, it was.

Merely trying to say - "no sense in buying an ultra efficient washing machine that washes clothes poorly".

A good example to illustrate this point is tumble dryers.

Why buy an ultra efficient tumble dryer that takes for ever to dry your clothes? You won't use it productively.

At times I've been prepared to invest in good quality appliances that are fuel efficient and don't break down.

I've learnt many things- Do your homework, Don't be afraid to pay for quality.

Some of the most useful below:

  1. Buy a decent energy efficient cooker. We have an Electrolux EKT6045X which is fantastic.
    • It has a flat top - easy to clean.
    • It cooks food, incredibly well throughout, quicker than a conventional cooker.
    • It is energy efficient.
    • It has rings that turn on and off- the user sets duration of time between on and off, not heat of rings.
    • User can also set no of concentric rings per ring.
    • It is very fuel efficient when you get used to getting the best out of it.
    • I paid £520 for it. It was a show room model with 1 minor scratch on its edge.
    • Sold our old cooker for £100.
    • Net outlay: £420 for a £795-£895 cooker.
  2. Buy a good quality washing machine i.e a Miele.
    • It has taken me many years to realise that there are no short cuts with washing machines.
    • I now pay for a Miele to save my heart having the stress of replacing parts on a poorly designed electrolux machine that just wont last.
    • Wait for Mieles to drop to a low historic price then buy.
  3. Buy a good quality combination boiler- Worcester Bosch boilers are a good example.
    • Ensure the boiler is sited upstairs.
    • When your boiler is fitted ensure the pipe work has an entry point to pour new fluids into the closed loop, should you want to flush your system or add new inhibitor.
    • Control your boiler with a good radio wave programmable thermostat i.e Honeywell.
  4. Lighting: Megaman CFL light bulbs are the best. They are expensive but cast the most authentic light.
    • They are expensive so try to find megaman myplanet bulbs subsidised by Eon.
    • LED light bulbs aren't good enough yet, they are too dim and the light too harsh- except for highlight lighting.
    • Best Halogen MR16 IMHO Phillips Masterline ES 20W. They give out a good light to power ratio.
    • LED bulbs will be ready to replace these bulbs soon, but not for a while.
    • Kitchen lighting 10 SMD LED warm white 2.6W G4 replacement bulbs are there and an excellent replacement for 20W Halogen capsule bulbs.
  5. Buy a good freezer or 2- that freezes food efficiently, whilst being energy efficient.
    • Some of Whirpools A+ rated freezers are really good, but do your research and choose carefully.
    • We use Whirlpool upright freezers to good effect to keep the economics of living down.
    • Yes they do use a energy, but you save money in living buy being able to store food at cheaper prices and store it.
  6. Buy some EcoTEK Standby Saver's for your PC and your tv setup.
    • They turn off the power to appliances when your tv and pc switch off.

This is just a fraction of the stuff we've done.

I'm not into the save the planet, to extract money from your pocket nonsense.

I'm into saving energy to live efficiently because it makes sense- why waste resources?

Do your research, buy good quality items that will last and dont be afraid to pay for it.

Use your common sense, make sensible decisions, plan for the future.

There is little evidence of the latter in this over busy, manic world we now live in.

Take time to do nothing. Its essential.

(We're going to have to disagree on the "extract money from your pocket nonsense" part!)

After some further research, Jason then came across an efficient 12V PIR...

12V PIR sensor: £23.86 plus VAT and delivery.

12V PIR Movement Sensor (IP44) External - Inbuilt 10A Switching Relay...


  • 'On' timer adjustable between 5 seconds & 15 minutes.
  • Adjustable light level sensor - 5 lux to 1000 lux.
  • Ultra low self-consumption (3.5mA, 12Vdc @ standby).
  • 10A Switching load capacity @ 12V to 30V DC.
  • IP44 Rated - can be used outside.

I am going to use this sensor with the Deltech MK7 LED bulb which has a power draw 3-4W.

It has a light output [equivalent to incandescent] 20-25W.

I might connect it to mains via an LED driver or I may buy a deep cycle battery and solar panel...

The sensor chosen isnt cheap, but its the cheapest way of doing it that I could find.

I looked into making the electronic circuits to connect to a 12V security PIR, but when all the components are added together the cost exceed buying the above PIR.

Hope the above is some use to you and others.

(Jason found that the PIR was also available from Conrad Electronic, cheaper.)

Following this sterling find, Jason sent another email:

If I were to run this porchlight off a deep cycle battery with solar panel, what battery size and solar panel size would you use?

Assuming the bulb consumes 4W and the PIR 3.5mA in low use mode and 40mA when the relay energises.

The PIR has a photocell so the light will only switch on during the hours of darkness.

The LED bulb would come on sporadically in the night and turn off after 5 minutes- not very often but not sure how much it would trigger.

What battery would you use combined with which solar panel and why?

Unfortunately we would need some more numbers that only Jason can provide, so I said:

Compute/estimate the total Watt-hours that the light + everything else should consume each day.

Your battery, if the thing it to be entirely off-grid, should be several times that since you don't want to discharge lead-acid more than about 50% ever and you'll need to cope with runs of dark days.

You'll need about 1Wp of south facing panels for every 1Wh per day of energy that you will consume.

You'll have to estimate on-time or else you'll have to treat it as if on all the time in the dark as I did.

Some Important Units

Jason was feeling a little overwhelmed by some of the terminology, and he's certainly not alone. I suggest borrowing or buying a very basic book on electronics or renewable energy, or looking at an online glossary.

In particular Jason wanted clarification on:


The Wp (Watt-peak) rating, usually on the rating plate on the back, is the most power you can draw from a given solar panel in optimal conditions, eg a sunny cloudless summer's day with the panel directly facing the sun ('normal' to the incident radiation), and (and this is often overlooked) into a perfectly-matched 'ideal' load.

(In the days when I used to edit a supercomputing trade rag, this would have been the "will not exceed" rating in the headlines!)

About 1kW of sunlight is striking every square metre (m^2) in those conditions (the same power as one bar of a typical electric fire) and the very best solar panels as of 2009 will, including all loses, return you about 180W per m^2, ie are about 18% efficient.

The rule of thumb as I mentioned above for the UK is that a well-sited and unshaded 1Wp panel will generate on average 1Wh (1 Watt-hour) per day in winter (and ~5Wh per day in summer).

That's the same as saying that the UK averages the equivalent of one full hour of bright sun per day in winter and five in summer.

To make sure that the light never fails when you need it most then you must select panels and batteries for the worst (winter) case.


1Wh (Watt hour) is enough energy to run a 1W LED/appliance/etc for one hour, or a 2W LED/appliance/etc for half an hour, so if you wanted to run a 1W LED 24 hours per day even in the depths of winter, ignoring battery and other loses, a perfectly-set-up 24Wp solar panel could provide enough energy.

1000Wh is also written as 1kWh (one kiloWatt-hour) and is one 'unit' of electricity as measured by your supply meter, and is the amount consumed by a one-bar (1kW, one-kiloWatt) electric fire each hour, and is about the solar energy hitting each m^2 of ground each hour at summer noon.


Battery capacity is commonly quoted in Amp-hours rather than Watt-hours. If you multiply the nominal battery voltage by its Ah capacity you get an idea of its maximum Wh capacity. For example, a 12V 2Ah battery holds about 24Wh of energy. (Be aware that the voltage is not exactly 12V, nor completely steady.)

However, when specifying a battery for a particular application a few wrinkles have to be kept in mind.

Firstly, the process of putting energy into a battery and getting it out again is not 100% efficient. For example, for good lead-acid batteries the "cycle efficiency" is about 80% meaning that about 20% of the energy is lost. Some battery types have better cycle efficiency (such as Li-ion I believe) and some much less good (such as NiMH, for all its other good qualities).

Secondly, some battery types do not like to be fully discharged. In particular, even with a "deep cycle" lead-acid battery you will severely shorten its life if you discharge it below 50%.

Thirdly, if you are running a completely "off-grid" application you have to allow for several days of dull and cloudy weather when the sunshine will be a tiny fraction, maybe less than 1%, of average. (This behaviour, known as 'intermittency' is one of the chief challenges in using renewable energy such as solar and wind in the grid.)

So, if we wanted to keep that 1W LED running 24x7 through the winter it would consume 24Wh of energy per day, which implies 24Wp or more of solar PV.

To allow for battery losses of ~20% we need to bump that up to at least 29Wp (24Wp * 1.2).

And to accept that energy we need 2.4Ah of lead-acid 12V battery.

To allow for runs of (say) 5 days of cloud, rain and general misery in winter we are now up to 12Ah of battery (2.4Ah * 5).

To avoid ever going beyond 50% DoD (Depth-of-Discharge) we need to double up again to 24Ah of battery.

My 4W SheevaPlug computer should by extension have about 96Ah of battery (and ~115Wp of solar PV), and as of the end of 2009 my 40Ah battery is not enough to carry me through the dark times; 200Ah would give me a decent safety margin.

As a final point, you should generally be using a solar controller to protect your expensive new battery from overcharging (or over discharging), but unless it is an MPPT (maximum power-point tracking) type then you will never be able to extract all the energy from your PV panel(s), and so you should round up your panel capacity a little more, maybe 20% again.