PV to run a laptop - recommendations

MrG · Post by **MrG** » 05 Jul 2011, 10:46

I'm looking to get some advice on the feasibility of running a laptop off solar power. Also recommendations of panels / system setup as I know a few of you people have small 12V PV setups. I'm talking about summer time only here (for now at least) and I'm talking about continuous use of the laptop (8 hours a day) because I work online and I'm wondering if it's going to be feasible for me to go away fro a while and work off my laptop off grid.

This isn't a peak oil prep for me but I'm posting it on the preparations forum because I'm sure the likes of adam and mike are going to come back with a detailed response

and it'll be useful to others. For me its just about going to stay with a friend who happens to live off grid for a couple of months because everythings going wrong and I just want to spend some time with my mate and some peace and quiet to get my head together - so summer only is fine but like I say it could be useful to others.

Laptop is an IBM thinkpad, I know the way to run it is not via inverter and the mains power adaptor as that's inefficient but rather to by the 12V car charger and run that off a battery.

That

http://www.DODGY TAX AVOIDERS.co.uk/Laptop-Charger- ... B002HG9KJ2

is rated at 75W.

Battery life is 3 hours and spare batteries are 20 odd quid each on ebay so I'm going to by 6 spare batteries at a cost of roughly £120 so that even if it all goes tits up or theres no sun for a bit, I can just run off laptop batteries for a couple of days.

I guess this

http://www.hrelp.org/archive/advice/solar.html

roughly lays out the system you need, though speaking to a friend that sells solar panels they are a lot cheaper than that now.

Post by **adam2** » 05 Jul 2011, 12:45

It is certainly possible to run a laptop from a PV and battery system, but for continous use may need a larger system than expected.

The first step is to find out how much power the laptop uses, the 75 watt rating given is hopefully the maximum power of which the adaptor is capable of supplying, not the average actually used.

Assuming an actual consumption of 60 watts* at 12 volts, proceed as follows.

60 watts at 12 volts is 5 amps.
It is stated that 8 hours use is required every day, therefore 40 ampere hours per day are required.
Allowing for 10% battery losses, you need a PV array that can produce 44 A/H a day.

Now decide how many hours bright, direct sun is likely in the summer, in the absence of detailed data, 4 hours might be a fair estimate.

Therefore to produce 44A/H in 4 hours, a PV array that produces 11 amps is required.

Experience shows that in real world conditions, PV modules put about an amp into a 12 volt battery for every 20 watts of rated output.
Therefore to produce 11 amps would need an array of about 220 watts.

Now size the battery. This depends on how many days reserve you wish for bad weather. It is largely a subjective decision.
I would be inclined to go for about 200 A/H as that would give about 5 days reserve (in prolonged cloudy weather the array will still produce a little current, so the battery will run down, but not as quickly as one might expect)

In practice a lot depends on how reliable the installation is required to be, and on whether or not any source of electricity is available.

If lack of electricity would endanger ones company or employment, I would DOUBLE both the battery and PV array, to be certain.

If on the other hand, some other source of power is available, such as a vehicle engine, then a smaller PV array could be used.
Running a vehicle engine for battery charging is not normally a wise use of fuel, but justified in some circumstances.
10L of petrol, at a cost of say £14 might save hundreds of pounds on sizing the RE system for the worst case, rather than for typical circumstances.

Like wise consider is 8 hours use ESSIENTIAL or merely desireable ? If 5 hours would do, but 8 hours would be better, then a smaller system might suffice.
Is the 8 hours every day, or only every weekday ?

I would not buy more than at most 2 spare laptop batteries.
An additional 12 volt battery or PV module might be a wiser use of the money.

Presumably internet access is required ? remember that mobile internet via a plug in dongle uses a few watts in addition the consumption of the laptop.

I would agree that the laptop should be powered directly from 12 volts via a car power lead.
These do not allways prove reliable in continual use and I strongly advise a spare, perhaps 2 spares in a remote place.

Finally, you may find a larger system worthwhile for basic lighting, a 12 volt fan, mobile phone charging, a radio etc etc.
If useing 12 volt lighting, keep an oil lamp as standby for cloudy weather thereby preserving more of the limited power for the PC.

* if the loading is more or less than 60 watts, then adjust the suggested PV and battery sizes in proportion.

DominicJ · Post by **DominicJ** » 05 Jul 2011, 12:59

Now decide how many hours bright, direct sun is likely in the summer, in the absence of detailed data, 4 hours might be a fair estimate.

I was under the impression that summer would be much more than 4 hours.
You should be expecting 1 hour of output even in deepest winter.

But, I'm not Adam2....

My first thought was a 100-150w panel and a battery of 50-100A/H

But it really depends on how much you want/have to spend.

Bandidoz · Post by **Bandidoz** » 05 Jul 2011, 13:04

Do bear in mind that you wouldn't want to consume more than a fraction of a battery's AH capacity....

Post by **adam2** » 05 Jul 2011, 13:25

DominicJ wrote:
Now decide how many hours bright, direct sun is likely in the summer, in the absence of detailed data, 4 hours might be a fair estimate.
I was under the impression that summer would be much more than 4 hours.
You should be expecting 1 hour of output even in deepest winter.

But, I'm not Adam2....

My first thought was a 100-150w panel and a battery of 50-100A/H

But it really depends on how much you want/have to spend.

One would certainly expect more than 4 hours sunlight on most summer days, but 4 hours "peak sun equivalent" is probably a fair estimate, maybe 5 hours in midsummer in the south west.
Remember that for much of the day, the sunlight will strike the modules at a non optimum angle. Power will still be produced but much less than the rated output. Early morning and late evening sun contains much less energy than noon sun. So 4 hours equivalent output is a fair estimate/informed guess, power would be produced for 12 hours a day or more, but the total output would be very roughly equivalent to 4 hours of optimum sun.

Post by **adam2** » 05 Jul 2011, 13:29

Bandidoz wrote:Do bear in mind that you wouldn't want to consume more than a fraction of a battery's AH capacity....

Absolutely for regular use.
However in the system suggested the battery would normally be at least 80% charged. Deep discharge should a rare event caused by prolonged cloudy weather.
A deep discharge once a month or so wont much reduce the life of the battery as compared to the standby life.
DAILY deep discharge would drasticly reduce the life of the battery.

MrG · Post by **MrG** » 05 Jul 2011, 13:44

Nice one adam I knew you'd come through on this one!

Likewise consider is 8 hours use ESSIENTIAL or merely desireable ? If 5 hours would do, but 8 hours would be better, then a smaller system might suffice.
Is the 8 hours every day, or only every weekday ?

I would be needing 8 hours a day. I need to be available all day every working day (I support a database system). How much I'm actually doing is dependent on how well everything is working! (as is how much time I spend on PS!) but I do need to be AVAILABLE. Actually that's a good point I need to power my phone as well.

In actual fact I only work 4 days a week so it's 30 hours a week or thereabouts of battery power.

If lack of electricity would endanger ones company or employment, I would DOUBLE both the battery and PV array, to be certain.

Loss of electricity would indeed mean I would lose my job so I'm with you on doubling the battery and PV module idea. There is a vehicle engine available for emergencies as well but it's my mates car so I couldn't rely on it if he was off somewhere.

I would not buy more than at most 2 spare laptop batteries.
An additional 12 volt battery or PV module might be a wiser use of the money.

I had in actual fact considered shelling out £200 which would give me a WEEK of battery power (well a working week for me 10 x 3 hour battery is 30 hours). But I take you point, that's about the same money as doubling the PV array which you already suggested.

That's based on my friend saying PV panels now about a pound a watt?? and your saying I need a 220 watt panel.

My basis for buying loads of batteries idea was that altough the place is remote it is cyclable from the train station and if push came to shove I could just go round a mates house at the weekend and charge up a weeks worth of battery power!

Presumably internet access is required ? remember that mobile internet via a plug in dongle uses a few watts in addition the consumption of the laptop

Indeed it is. The 3 hour battery life is including that. I know this because I use the dongle all day every day normally. I'm using it now.

Finally, you may find a larger system worthwhile for basic lighting, a 12 volt fan, mobile phone charging, a radio etc etc.
If useing 12 volt lighting, keep an oil lamp as standby for cloudy weather thereby preserving more of the limited power for the PC.

Yeah my mate already has a basic system for a bit of lighting and phone charging.

MrG · Post by **MrG** » 05 Jul 2011, 13:54

So basically I should have a 220 W panel (roughly 200 quid?) and a 200A/H battery - how much would that cost roughly?

And to be on the safe side I should double it.

Like I say there I'm not intending to actually move there just get away from my house for a couple of months so I could proabbly just get away with the one panel and one battery and see how things go.

**Edited to say.. solar panels much more expensive than that on the web my mate must be offering a good deal or I didn't hear him right**

Bandidoz · Post by **Bandidoz** » 05 Jul 2011, 14:15

...and make sure you get a deep-cycle (leisure) battery.....

DominicJ · Post by **DominicJ** » 05 Jul 2011, 14:26

They were heading towards £1w a while back, before sterling crashed 25%...

http://www.windandsun.co.uk/Prices/prices_modules.htm

Now they're over £2 a watt

http://www.windandsun.co.uk/Prices/prices_batteries.htm

Any chance you can get work to pay for it?
Or reclaim VAT somehow?

http://www.navitron.org.uk/product_deta ... &catID=126
180w for £360
Another £317 for a 12v 180ah battery from wind and sun, both inc VAT.

MrG · Post by **MrG** » 05 Jul 2011, 14:34

Ha! work pay...

No rolling round on the floor laughing emoticon.. they'd just suggest that I should really live in a house I think

Hmm I overheard him quote £1Pw to a different friend that deals in solar panels and the other guys reaction suggested to me that was expensive

Post by **adam2** » 05 Jul 2011, 14:59

MrG wrote:So basically I should have a 220 W panel (roughly 200 quid?) and a 200A/H battery - how much would that cost roughly?

And to be on the safe side I should double it.

Like I say there I'm not intending to actually move there just get away from my house for a couple of months so I could proabbly just get away with the one panel and one battery and see how things go.

**Edited to say.. solar panels much more expensive than that on the web my mate must be offering a good deal or I didn't hear him right**

For reasonable reliability and PRESUMING THAT THE PC USES 60 watts, then yes for reasonable reliability I would go for 220 watts of PV and about 200 A/H of deep cycle battery.
If reliability is vital, then twice that would be advisable, it might never be needed, but that is the penalty for requireing high reliability.
PV varies a lot in price but £500 might be a guide.
100 A/H deep cycle batteries are readily available at about £100 each.
Charge controller about £50 ?
A basic deep cycle battery only reliably lasts 4 years, but PV modules should last decades and will allmost certainly be useful in the future.
Confirm by measurement how much the laptop uses, 60 watts is only my estimate, they vary a lot.

Does your friend drive the vehicle regularly ?
If so it might be worth charging a battery from the vehicle whilst it is in use anyway. The EXTRA petrol cost to charge a second battery is small and could be well worth it.
Care must be taken that the second battery is correctly secured, and only connected to the main vehicle battery when the engine is running.

MrG · Post by **MrG** » 05 Jul 2011, 15:48

PRESUMING THAT THE PC USES 60 watts

Its very probably better than that.. this page suggests

http://www.upenn.edu/computing/provider ... usage.html

Its a thinkpad T400 which says "not testable" for moderate use.. great! wonder what they mean by that. The other thinkpads are rated at 20 - 40W though so they are fairly efficeient machines.

Does your friend drive the vehicle regularly ?

Not really. He lives in the woods and doesn't really go anywhere much. Didn't even have a car for a while. I think of late though he's been going a bit stir crazy out there on his own and has started driving a bit more on weekends and things - hence he's keen for the company. He suggested I move there but I don't think I'd be able to power the laptop off solar in winter. Happy to keep him company for the rest of the summer though I could do with being a hermit for a while having recently split up with girlfriend and generally very depressed.

He does have a generator but Its old and a bit f***ed and I wouldn't want to rely on it.

phobos · Post by **phobos** » 05 Jul 2011, 16:21

As with all things electric, can you reduce your usage rather than up your supply, i.e. could you work from a ipad or another tablet which has a much longer battery life and less demanding on recharge? It may be cheaper and more portable to do this than a massive Solar Panel and batteries etc?

Post by **adam2** » 05 Jul 2011, 17:03

Even an unreliable generator could be very usefull.
Use it briefly perhaps every other day to keep the large 12 volt battery full. If the generator fails and cant be readily repaired, you still have battery power for several days "thinking time" or perhaps over a week with some solar charging.
In that time repairs could be organised, a very cheap generator purchased, the battery charged from a vehicle, or some other arrangements made.

In the longer term, solar powering in the winter is possible, but rather more costly.
A small wind turbine might be a better bet than ever more PV that does not produce much in winter.

I use a Rutland 913 in winter and easily power a large power gobbling laptop for several hours a day. A more efficient one for 8 hours should be fine, and of course you would still get a little from the PV.
Dont forget some 12 volt lights to use all that electricity when you have plenty, and some oil lamps so as not to be in dark when power is short.