Summer is here and the days are getting longer, and hotter. But is there enough sun here in the UK to power a laptop? We decided to test out some solar-charging kit to see if it's possible to run a MacBook entirely on the sun's rays. Read next: Best solar chargers

The solar-charging kit you'll need

Here's the first reality shock when solar-charging gadgets and laptops: you shouldn't just plug them directly into the solar panels. This would result in too spiky a voltage due to clouds rolling across the sun and so on, and could damage the charging circuitry or batteries. Instead, the solar cells typically charge a battery pack, which is then used to charge a computer or devices.

Pretty much the only serious contender in the dedicated computer/gadget solar-charging game is Voltaic. Although the company has a wide range of products, I chose the Voltaic Arc 20W Solar Charger Kit.

How to solar-charge a MacBook

This costs around £295 here in the UK (at time of writing you can get it for £289.99 from FunkyLeisure or £295 from CyberEnergy) and features the highest-output solar panel the firm sells, plus the V72 battery. At 72 watt-hours this is also the largest-capacity battery Voltaic sells.

The solar panel is a monocrystalline 20W unit spread across four interconnected sheets, and when not in use these fold to the approximate size and weight of an older-model iPad or A4 book. Also included in the kit is a wall-wart mains charger for the battery, which effectively turns the battery into a standard external battery pack, and you get a car cigarette-socket cable to charge the battery on the move, plus a cigarette-lighter-sized socket for attaching devices that might use cigarette-charger plugs.

Additional bits and pieces in the boxes include a host of plugs so you can power most laptops and mobile phones... but you don't get MagSafe adapters for older-model MacBook Pros or Airs. That'll cost a further £14.99, and if you have a modern MacBook Pro/Air with MagSafe 2, you'll need to buy the MagSafe-to-MagSafe 2 adapter from Apple for a further £10. (Voltaic sells a specific MagSafe 2 adapter but it doesn't appear to be available in the UK.)

Those with 2016 or later MacBook Pro models, or a MacBook, should be able to simply attach their existing USB-C cable to the single standard (type-A) USB port provided on the V72 battery and fill their batteries that way. Charging will be relatively slow because the USB port is limited to 10 watts. Current-generation 13in MacBook Pro models come with a 61 watt charger, by way of comparison.

However, iPhones or iPads will charge at their usual (or better) speeds via the USB port - simply attach your existing Lightning cable. Read next: Apple 'trying to make solar powered MacBook'

How to solar-charge a MacBook: MacBook Pro 2016

Where to place your solar panels

To give the Voltaic kit a really hard time I did my testing in Manchester during May/June, and my test MacBook Pro was a 15in 2015 model. This contains a 99.5 watt-hour battery, the largest of all possible capacities, and certainly larger than the 72 watt-hours quoted for the Voltaic battery. Most laptop batteries are between half and two thirds of this capacity.

Trying to find a good place to leave the solar panels and battery proved genuinely challenging. Charging takes the best part of a day - Voltaic quotes at least 6.5 hours of unadulterated sunshine to get the full charge required to power a laptop (although as little as one hour might be enough to fully charge a phone, for example).

In other words, you need to ensure the spot will not be covered by shadows as the sun moves across the sky. I ended up leaving the panels on my car bonnet.

How to solar-charge a MacBook: Car

A note on rain

When using the kit you always have one eye on the weather, because none of the kit is waterproof. It's not unlike drying clothes outside - as soon as you hear the pitter-patter of rain drops against the window you instinctively sprint outside.

Before you ask, no, putting the solar panels under glass or Perspex isn't feasible because that filters out most of the vital UV light that's converted to electricity - although this perhaps isn't the entire truth, as I'll discuss later when talking about test results. Read next: Best festival tech kit

Setup

I have to dock the Voltaic kit a mark when it comes to the supplied instructions, which comprise a brief leaflet and a small diagram on the solar panels themselves. Connecting everything together was initially quite puzzling.

The charge and discharge ports on the battery are the same type and size, which aids confusion, and the inclusion of so many laptop adapters muddied the water even further. But in the end it was actually very simple: the solar panel unit has a lead that plugs straight into the INPUT port of the battery, and when it came to discharge I simply plugged in the MagSafe adapter into the port marked OUTPUT. All other cables and adapters can be ignored.

Testing results

My testing was not scientific but to get a baseline reading I initially charged the Voltaic battery from the mains until it reported it was full, and then used it to charge the MacBook Pro when it was on virtually zero battery charge. (At this point the MacBook Pro had shut down and showed the battery symbol whenever I attempted to turn it on.)

Here came the second surprise during my testing: even charged fully via mains power, the Voltaic battery could only charge this particular MacBook Pro to around 50% before becoming entirely depleted - and if the MacBook Pro was in use while charging happened, the maximum MacBook Pro charge dropped to around 35 percent. The Voltaic battery also got almost too hot to touch while discharging, although I was assured by Voltaic's tech support that this was safe and normal.

In other words, it isn't simply the case that the full 72 watt-hours of the Voltaic battery is transferred to the laptop. The discharge process isn't 100 percent efficient and Voltaic's tech support quotes a 25-30 percent loss.

The real-world reality is that getting a 100 percent charge for my particular MacBook Pro via solar power would take around two lots of circa 6.5 hours worth of charges from sunlight - just about possible during the long days of summer if you get up early enough!

For comparison, here's how the current MacBook models line up in terms of battery watt-hour ratings, as of mid-2016:

How well the Voltaic battery would charge each is hard to tell, and your guess here is as good as mine because I wasn't able to test every model. However, I suspect with the exception of 15in models, a single blast from the Voltaic battery would come close to charging all the existing MacBook (Pro/Air) range.

Before attaching the Voltaic battery to a laptop you have to set the voltage to match. A physical switch selects between 12, 16 and 19 volt output.

You can find out the voltage setting for your MacBook by looking at its power brick - my MacBook Pro required 19V - but this was perhaps the hairiest part of the whole adventure and I feared blowing up the computer. Although an LED indicator shows the voltage setting as soon as anything is plugged into the OUTPUT socket, I still worried that the switch might get accidentally adjusted if the Voltaic battery is thrown in a backpack, for example. It's something you'll need to check every time you use the Voltaic battery.

So, here are the results of our testing - and do remember that none of this was in any way scientific:

Sunny weather

In the middle of May I put the panel out to charge the Voltaic battery at 9am, and took it back in at 3.30pm. This provided the recommended 6.5 hours of charging time, and there wasn't a cloud in the sky. The Voltaic battery's charge-level indicator said it was fully charged and sure enough the MacBook Pro was again boosted from zero to circa 50 percent charge - the maximum possible charge it can provide for this model of MacBook Pro.

Cloudy/overcast weather

On a day that the Dark Sky iPhone app reported as "Partly Cloudy", with occasional outbreaks of sunshine, 6.5 hours outside showed via the Voltaic's charge indicator lights that the battery was around 75 percent charged. However, the MacBook Pro was boosted merely from zero to 13 percent.

How to solar-charge a MacBook: Clouds

Voltaic reports that clouds are a significant problem for solar charging and can reduce the charging capacity by anywhere from 50 to 100 percent. (That is, no charge at all!) However, a repeat of this test on a similar day, leaving the panel outside from 8am to 8pm, resulted in the Voltaic battery showing a full charge; and again the MacBook Pro was boosted from zero to around 50 percent.

The moral of the story is that leaving the panels out all day long, if you can, produces best results given typical UK weather conditions.

Indoors

With the panels arranged against a sunny window indoors, a full day's charging from 9am to 7pm provided a 15 percent charge for the MacBook Pro, which is actually better than we had anticipated.

Bear in mind that the Voltaic battery is designed to be trickle-charged, in addition to being charged relatively quickly on a sunny day. Leaving the solar panels against a sunny window indoors for the best part of a week during unbroken sunny weather will very likely result in the Voltaic battery getting a 100 percent charge.

This is arguably one of the most effective ways of using the Voltaic in the UK: when not in use just leave the Voltaic battery connected to the cells and trickle-charging.

One issue to watch out for, and admitted by Voltaic's tech support team, is that the battery charge indicator lights on the Voltaic battery can be optimistic. We came to learn that there are only two definitive states you can trust: empty, in which case the charging lights either won't come on, or will blink a single light, or fully charged, in which case all five charging indicator LEDs remain lit and non-blinking when the solar panel is attached.

Conclusion: Is it feasible to solar-charge a MacBook?

It's likely you'll be charging your MacBook (or your iPhone/iPad) from solar for pure-minded environmental reasons, or maybe just for fun. If you're doing it to save money then, well, you're in for a wait - it will take around 60 years by our calculations for the kit to pay for itself, and that's using it 365 days a year, which is simply impossible in the UK.

And this leads us to the saddest truth about solar charging in the UK. This has little to do with the Voltaic kit, which we found to be admirably effective once the initial connecting-everything-up teething troubles were overcome. The problem is simply the UK itself.

Not every day is a sunny summer's day - not in Manchester, at least - and it often rains when it's not expected to.

And then there's the damning physical reality of living on an island located relatively far up in the northern hemisphere. For six months of the year, roughly October to March, the UK gets little UV light thanks to the low position of the sun on the horizon. The days are also short, of course, so there's simply fewer hours for solar charging in any event.

Getting a full charge from the solar panels during this period will likely take weeks of day-long exposure outside but, of course, it's also a period of the year when it rains persistently, so the panels can't be left outside.

How to solar-charge a MacBook: MacBook Air

However, putting the solar panels behind glass with a view to trickle-charging the battery over several days is a realistic option during the spring/summer period, and means changeable weather is less of an issue. You might stick the panels in your conservatory, on your vehicle dashboard, or even in your greenhouse, but watch out for the ambient air temperatures getting too hot. (We couldn't find an operating temperature listed for the Voltaic battery but guides online say Lithium-Polymer batteries shouldn't get above 60 degrees centigrade.)

The UK's miserable weather and position on the globe isn't the only concern. There's the risk of theft. For best results you have to leave the panels and battery outside, in the open, all day long. Your own back garden or yard is probably secure but that's about the only place I can think that is. We also found during our tests that the panels could get dirty when left outside for long periods, so you'll have to clean them occasionally.

Where does solar charging like the Voltaic kit make sense? The customer reviews on Voltaic website show that field workers and travellers are getting some good use out of the kits.

If the weather meets the specifications and the day is long enough, a laptop can be run essentially indefinitely away from the mains power. That's pretty damn cool, and don't forget that the USB port can also charge phones or iPads too. Because the panel folds into a compact form no larger than a large book, transport isn't much of a concern either.