Solar powered lights

Capturing the free energy from the sun to power external domestic lighting seems an obvious thing to do and there are plenty of stand-alone solar powered floodlights and courtesy lights that supposedly do the job.   I use the word ‘supposedly’ quite deliberately so let me tell you why.

This website encourages sustainable, low carbon footprint solutions for preventing crime, but I have to be confident that the products that this site recommends will indeed return these carbon savings.  As I have already suggested we are wasting a lot of energy by lighting the wrong places around our homes, often using wholly inappropriate lights.  By understanding how light can help us prevent crime and by using existing tried and tested low carbon technology we can already reduce our carbon footprints and our electricity bills. 

The standalone solar powered lamp is a developing technology and judging by the many reviews on the internet it is quite clear that there are lots of users who have been disappointed with their purchases.  I am also rather sceptical about the bold claims made by the manufacturers and retailers, particularly those made about so called ‘security’ floodlights.  I am equally unconvinced by the large number of 5 star reviews for certain products, many of which are suspiciously ‘over the top’ in their praise for the unit.

In order to satisfy my own curiosity about the effectiveness of solar lamps I purchased a leading brand solar powered floodlight for my back garden and a leading brand courtesy lamp for the front door.  In both cases I will be able to point the solar panel towards the southern sky (See How solar powered lights work below).  I will report the findings in this section later in the year (2012).    

At the end of this section are some of the reviews I’ve picked off some well known on line shopping sites.  They are not all bad, but they do highlight some of the problems with solar powered lamps, which I hope will encourage the manufacturers to improve their products. 

How solar lights work

Solar lights work by capturing energy from the sun on an integral or wire connected solar panel which is converted into electricity and stored in batteries.  The stored electricity is then used to power an LED lamp at night.  In effect then, solar powered lamps are really battery powered lamps.

In respect to motion activated solar powered lamps there are several things that will affect the performance:  The available energy from the sun on any one day, the charging capacity of the solar panel, the capacity of the rechargeable batteries, the power requirement of the lamp itself, the sensitivity of the passive infrared detector and, where fitted, the photoelectric cell (light sensor).

The Sun

The amount of energy from the sun that reaches the solar panel is affected by the length of the day, the angle of the solar panel in respect to the sun and the cloud cover.  In the UK, the mean daily sunshine figures reach a maximum in May or June, and are at their lowest in December. So immediately you can see that available energy from the sun will vary enormously throughout the year. 

Most and least sunny places in the British Isles

COUNTRY	MOST SUNSHINE PER YEAR	LEAST SUNSHINE PER YEAR	PLACES THAT RECORDED THE HIGHEST SUNSHINE PER MONTH
ENGLAND	South coast 1750 hours	Mountainous areas 1000 hrs	Eastbourne 389.9 hrs July 1911
NORTHERN IRELAND	Coastal area of County Down 1400 hours	Mountainous areas Less than 1100 hrs	Mount Stewart 298 hrs June 1940
SCOTLAND	Angus, Ayrshire, Fife, Dumfries and Galloway The Lothians 1400 hours	Mountains of the Highlands region Less than 1100 hrs	Tiree 329 hrs May 1975
WALES	South West coastal area of Dyfed 1700 hours	Mountainous areas Less than 1100 hrs	Dale Fort 354.3 July 1955
BRITISH ISLES	St Helier, Jersey 1915 hours	Source: Various including Met Office 2007 data

 

The Solar Panel (Photovoltaic cell)

For the solar panel to work at its best in northern latitudes it needs to be pointing south and angled towards the sun.  During the summer the optimum angle is your latitude minus 15 degrees.  During the winter the best angle is your latitude plus 15 degrees.  In theory this means that you will have to adjust the panel throughout the year to get the best charge from it, but in practice most of us will fix the panel to a convenient wall, aim it in roughly in the right direction and leave it alone. 

The solar panel will provide some charge to the batteries when it is overcast or not pointing directly towards the sun, but quite obviously a bright sky is best.  Based on the figures in the table above the same properly directed panel on the coast of County Down will potentially generate 20% less power per year than if it were on the south coast of Sussex.  

For a ‘security’ floodlight it seems sensible to obtain a solar lighting unit that has a separate solar panel that is connected by wire to the lighting unit.  This will enable you to optimise the collection of energy, but have the lights pointing into a shady part of the garden.  Solar lighting systems with fixed solar panels will have to be positioned in the most sunlit places to work effectively.  Solar panels must be regularly cleaned for them to work efficiently. Some solar powered floodlights can be supplied with an additional solar panel, which sort of suggests that one might not be enough in some locations and at certain times of the year. 

The Batteries

The most commonly used battery packs use 1.2V Nickel Metal Hydride C, AA and AAA batteries of various capacities, although 3V Lithium Ion batteries are also used.  The rechargeable batteries will need replacing after three years or maybe more and replaceable ones are fairly easy to find.  Don’t buy cheap batteries as they simply will not perform as well.  The cost of replacing a battery pack in 2012 is between £6 and £8.     

The batteries will only fully charge if they are in good condition and the solar panel is working at its optimum performance.  Higher lamp output will require higher battery capacity, because obviously the batteries will discharge at the rate at which power is being drawn from them.  In the experience of some users of motion activated solar powered ‘floodlights’ they operate reasonable well in the summer, but less so in the winter.  In order to ‘improve’ the light output they have turned down the ‘on’ period to less than 30 seconds.  My low power ‘effect’ lighting in the garden, which simply creates a glow around the shrubs, draws very little current and will stay on for around 5 hours after a long sunny day.  They hardly work at all in the winter and so I take them in.

Lamps

The majority use clusters of Light Emitting Diodes (LED) of various powers encased within luminaires with varying telemetry.   The LEDs are generally not replaceable given that the lifetime of the LEDs is likely to outlast the unit as a whole. 

Motion detector (Passive infrared detector PIR)

When buying a PIR sensor operated solar lamp look for one that allows you to alter the direction and sensitivity of the detector to reduce the number of false activations.  Be careful not to place the sensor close to sources of warm air emanating from the building, such as the vent from a central heating boiler.   Most PIR sensor lamps will allow you to adjust the amount of time the lamp stays lit when it has been activated.  Depending on the product this means that you can time the light to stay on from around 10 seconds up to about 2 or 3 minutes after each activation.  Clearly, the longer the light stays on the more it will drain the battery, so if the battery isn’t getting sufficiently charged you may have to turn down the light on period.  (See   Passive infrared detector (PIR sensor) in this section)

Photoelectric cell (Solar switch, dusk to dawn switch)

A photoelectric cell in a light fitting is used to switch the lamp or PIR detector on when it gets dark and to switch the lamp or PIR detector off when it gets light.   Solar powered lamps fitted with a solar switch will prevent the lamp operating during the day when illumination is not required thus enabling the battery to be charged by the solar panel. (See Photoelectric cell switch (Dusk to dawn) in this section)

User reviews of solar powered ‘security lights’ (floodlights using LED lamp clusters in one or two luminaires)

“Rated this as OK, since for the money that's what it is.  It didn't look very weather-proof so I felt I had to do a bit of DIY on it, like sealing the light unit and the solar panel with silicon.  On dull days the battery doesn't charge very much, so if it's in a place where it's triggered quite often you will have to turn down the sensitivity and duration.  With the recent cold weather, the unit didn't work at all, so probably the electronic components aren't rated for low temperatures. Overall....could be better.“

“I have owned this product for a couple of months now and I haven't had to recharge the batteries once.  I use it to light up the patio and although it isn't as bright as next door’s it's still good enough to scare the neighbour’s cat away and is ideal when it comes to finding your keys.  The motion sensor is OK, although one night when we had a lot of rain and wind it didn't seem to work very well, but it has fixed itself and I haven’t had a problem since...”

“I bought one of these, as I didn't want to have to employ an electrician to install one with the new P regs [Electrical safety regulations].   I read the reviews and they all seem to say works great even with lack of sunshine.  I have put mine up and originally I had the solar panel facing east.  I found that if I activated the light at night, I'd get 4 activations then it'd run out for the night.  I moved the solar panel to the South facing wall as specified, even though it does say [it] works well even in low sunlight.   Again it's still the same, works around 5/6 times then runs out for the night.  Pretty rubbish if you ask me.  Recommend an electrician to hard wire one.”

“Despite being placed on a sunny wall, this unit has never worked reliably and has generally not worked at all. OK, it was cheap but, even at that price, it is disappointing.  I am now arranging to have a decent security light installed and will have the old one taken down and disposed of.”

User reviews for solar powered courtesy lights for use by the front door

“This solar-powered outside light gives you a cheap and easy way of throwing light on your situation. But don't think you are getting a security light; it is just not powerful enough for that.  But if you have an unlit exterior entrance this is just the job to help you find your keyhole, or your dog, in the dark.  Of course, the great advantage with this product is that you don't need an electricity supply and you don't need an electrician to fit it.”

“I bought this as a security light at the front porch and was attracted by its portability and its equivalent to a 60 watt light bulb output.  You must remember once a week to turn it to the "off" position so as to give it a charge for a day in the daylight, no real problem there.   However, initial charging seemed to take forever.  [Mine] only having a partial charge after 3 days in the sun as stated in the instruction book.  I ended up opening the product and charging the rechargeable batteries with a mains charger for a boost and this did the trick.” 

“Its size is also worth a mention as it’s tiny and has a lockable key for fixing it to the wall.  So it was outside working fine for a day or so and I noticed water had collected in the base, so I placed an additional seal of silicone around the panel and sensor and this did the trick.  [The] range is pretty good, as is the light level and therefore it would gain top marks except for 2 main points which are that you have to weatherproof it yourself if it's not located somewhere sheltered from the elements and secondly it is not charged fully on opening the product as described in the instruction book. I may sound negative but so far the benefits have outweighed the initial disadvantages by a long margin.  I would therefore give it 7 out of 10.”

Typical retailer or manufacturer description picked off the internet

“The spot light is by far the most powerful spot light we sell, easily illuminating up to 50 foot distance through its 8 powerful LED's. Because of its range the [lamp] is ideal to light trees, bushes or features. Testing over months has demonstrated that the [lamp] performs powerfully on build quality, brightness and weather resistance. Testing has demonstrated the consistency of performance as shown by the fact that, even following a number [of] overcast days, the large solar panel collected enough charge to perform at night.”