??? Portable solar panels for keeping tablet charged w/ heavy use - UK winter ?

Hi - I wonder if you can help me.

I've been trying to figure out the optimal system for charging basically just a samsung tablet while bike touring through winter in UK. The system I calculated [see below] based on this guide (for 12v system) would involve maybe 2x 100w panels (or equivalent to produce that output, e.g. 2x 150w?) and a 120,000+ mAh power pack / battery - which seems needlessly expensive and overpowered given that many portable solar power kits on the market are much smaller.

Basically I want to ensure uninterrupted use of a tablet if necessary as I'll be using it for work. I calculated based on it taking 7 hours to charge and that being enough for a day's use - but ideally I'd want to be able to charge at least once daily. And I want to be able to rely on it through the winter specifically - so assuming the worst conditions - again, in the UK. That guide I linked to above helpfully (if accurate) estimates that one full day in a UK winter will provide the equivalent of 1 hour continuous sunshine.

What do you think? Would really appreciate some advice!

Here's what I worked out based on that guide, and having solar panels out for half of each day (or the equivalent, e.g. a full day in bad conditions):

PANEL

• Tablet wattage = 15w (probably 10w actually but allowing for minimal other charging requirements, e.g. e-reader every couple of weeks and similar)
  
• Daily requirement = 15w * 7 hours [including full charge and drain each day - deliberately overestimating I think] = 105wh
  
• Panel size [winter] = 105wh / 0.5h [half a winter's day] = 210w
  

BATTERY

• Weekly requirement = 105wh * 7 = 735wh - in amp hours = 735/12 = 61.25
  
• Battery size = 61.25 * 2 = 122.5ah
  

CHARGE CONTROLLER [necessary or usually included?]

• Amps produced by panels = 105/16.5 [as per guide for 12v system] = 6.35 amp
  

https://uk.bestreviews.guide/solar-phone-chargers?origin=google&google_params[matchtype]=b&google_params[network]=g&google_params[device]=c&google_params[creative]=404934647038&google_params[keyword]=&google_params[adposition]=&google_params[adgroupid]=91896025371&google_params[campaignid]=8489374211&bs=&google_params[feeditemid]=&google_params[targetid]=dsa-400926840172&google_params[loc_interest_ms]=&google_params[loc_physical_ms]=9045839&google_params[devicemodel]=&google_params[target]=&new_api=true&dest=0&sys_id=0|357&gclid=EAIaIQobChMI4PGG4t786gIVW-ztCh3HjAG_EAAYASAAEgJLkvD_BwE

Please realise that the quoted outputs of solar panels are usually, if not always, in optimum full sunshine. I'm not sure what assumptions are made about lattitude.

Winter sunshine is already weaker and heavy cloud attenuates it more. On a really dull (typical) UK winters day I would guess that you will be lucky to get more than 10% of rated output. 

I'm not trying to put you off, but look closely at the claims made for these devices.

Thanks Geoff - yeah I'd expect that although hadn't thought it would be that small a % of rated output

Based on 1 winters experience I was surprised how much our panels produce on a winters day ............ sure not up near the 3.6Kw but I would say 20% or more is very usual, and when the sun comes out, it goes right up 

Our winter quarter lecky consumption was 25% down on the previous year

gscott69 said:

Thanks Geoff - yeah I'd expect that although hadn't thought it would be that small a % of rated output

Click to expand...

Well, as I say It is only a guess. Others on the forum may have different views or maybe a more scientific way of estimating.

Geoff1946 said:

Well, as I say It is only a guess. Others on the forum may have different views or maybe a more scientific way of estimating.

Click to expand...

That said as portable devices, I'm thinking that the user can move them into optimum positions - as such, which is tricky on a house roof

There is that yeah but I would assume (based on little experience or knowledge) that portability also compromises on efficiency/quality of panels. That's interesting you got 20% as a kind of minimum though

The best I can get on a perfect summers day is 13kWh but more like 10kWH, so on a cloudy winters day 1 kWh, which means my winter production is about 10%.

Thanks - that's helpful

Murdoch said:

That said as portable devices, I'm thinking that the user can move them into optimum positions - as such, which is tricky on a house roof

Click to expand...

Winter output is improved by angling at a steeper pitch, ideally your panel should be 90deg to the sun, which with  a portable panel just means propping it up at the appropate angle.

gscott69 said:

There is that yeah but I would assume (based on little experience or knowledge) that portability also compromises on efficiency/quality of panels. That's interesting you got 20% as a kind of minimum though

Click to expand...

 If I'm reading your calculation correctly, then yiu aren't allowing for the fact that the panels can be generating whilst you are charging, so if your load is say 20W total during chaging then your panel only needs to be outputting 20-30W for the 7 hours of charge. Most motorhomes work on a single panel, 200W or less. OK they can also charge up from engine and other sources, and have the benefit of a large battery to last several days, but 200W of panels does seem a little large, and as you can point panel directly at the sun, your efficiency will be far higher than a fixed system on a house roof or van roof.  

As you are cycle touring, I think yuo will truggle to find anything small enough and light enough to meet your needs effectively. Perhaps you might be better off taking the approach that you need to charge from mains every couple of days and any soalr panel mearly increases battery life, rather than fully charge it? 

This may suite your needs https://www.selectsolar.co.uk/prod/1230/hubi-lighting-and-power-system

Well yeah, it's a good idea , but do not forget that your calculations if it's 100% sunshine, that is on a cloudy day , which is not enough , you will find it difficult to charge your device.

If you are cycling, have you considered the modern-day equivalent to the old bike Dynamo?

e.g. instead of just working a pair of lights of a dynamo..

there are now dynamos with USB charger outlets..

Which may be a useful alternative or addition to compliment any solar option you choose..

e.g.  Whilst on the move have a dynamo to charge a USB battery pack...

When finished cycling either use your battery pack or Solar panels to charge your tablet..

https://www.cyclingabout.com/best-dynamo-usb-chargers-bicycle-touring-bikepacking/#:~:text= All About The Best Dynamo USB Chargers,Power Pack II is an aftermarket... More

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