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Subject Topic: What size solar panel/battery Post Reply Post New Topic
Personf13dev
12/7/2017 at 11:06am
 Location: Margate
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Hi all,

We're toying with the idea of getting a solar setup to open up the sites we can go to/get cheaper site fees for the other sites by being able to go without an electric hookup.

Ok, might sound a bit ott for camping but I'm an OU student and need to power a laptop all year round for uni work (very few actual holidays).

What I would be looking at powering is:
* Trailer tent lighting (say 5w 3 hours a day) - 15w
* Charge 2 mobiles once per day - 15w
* Charge 1 laptop, once per day (60w charger, approx 2 hours for a full charge) - 120w
* Charge vape batteries - 15w

Total = 165w

Although I'm fairly confident with electronics I'm struggling to calculate the efficiency of a solar setup and just wanted to check my calculations with others.

Everything would be run from 12v car chargers or through 5v buck converters to minimise loss.

A solar panel of 75w would provide around 150w of battery charge per day on an overcast rainy day or 300-450w on a bright sunny day? If so, 75w would be suitable as the laptop wouldn't need a full charge daily, phones in reality last 2 days on a charge and can be topped up while on the road, vape batteries usually last a couple of days.

Battery wise, 165w at 12v is 13.75a and I wouldn't want to drain the battery to empty and allow for deterioration as it ages, so a 20a or above battery would be suitable?

Would these calculations be acceptable or am I under estimating the requirements?

Jim


woodman2
12/7/2017 at 2:40pm
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Hello,

I am no expert here, but are you proposing to have the solar panel top up the car battery whilst you are on site and then charge everything from the car?

Or do you have a separate battery system in the trailer tent?


Personf13dev
12/7/2017 at 4:12pm
 Location: Margate
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Hi,

I would be looking at using a separate battery for the solar setup to avoid running the car battery flat or forgetting to unhook it before driving off on a day out.


Personboff
12/7/2017 at 4:43pm
 Location: N Wales
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This is my setup. 150W roof mounted solar panel feeding 2x100 ah batteries. From this I can power all the vans requirements lights, water pump, central heating pump, radio, TV, chargers for iPads Kindles and phones. This set up can struggle end of October to end Feb ish when I say struggle the charge doesn't keep up with demand so batteries get flattened over several days. This time of year no issue batteries fully charged often before I get up. I don't even bother looking.

Making this practical for you. The battery get a decent sized battery 75ah or more. Think of it as the bucket where you keep your electricity. Secondly I said I have a 150w panel roof mounted. What I didn't say is roof mount a panel is very in efficient, so I would think that a 75W panel will be enough as long as it's pointed at the sun.


Personinsiorc
12/7/2017 at 8:14pm
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Deleted - Sitting in a campsite, not reading posts right, writing rubbish after too many beers...


Post last edited on 12/07/2017 21:34:09


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Personnavver
12/7/2017 at 9:00pm
 Location: West country
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Not sure when you want to be away but output varies considerably from month to month.

This is the typical output in watt hours from a 1w peak panel for each month. You need to divide by the number of days.

You have calculated you want 165 watt hours per day. Worst month is Dec with only 20 watt hours per 1 watt panel so you need over 240 watts. You probably need a fair bit extra for system losses etc say 50% so a 360watt panel.

In April you have 103 watt hours so 3.3 per day. You only need a 50 watt panel +50% = 75 watts.

Jan 26
Feb 42
Mar 78
Apr 103
May 124
Jun 125
Jul 122
Aug 114
Sept 92
Oct 61
Nov 36
Dec 20


Personf13dev
12/7/2017 at 9:36pm
 Location: Margate
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The wattage draw would be the same at either voltage (although a small amount of power is lost in converting voltages). Although the laptop charger and lighting would draw at 12v with the phones and batteries drawing via a 5v buck converter. In theory all devices are unlikely to be run at the same time, but I'd want to account for it to be on the safe side.

By keeping the battery at a minimum level of 60% I would require a 35a battery, assuming it can handle the current draw.

I plan to keep the panels portable so they could in theory be moved during the day to accommodate tracking the sun, although in reality there's a good chance I won't remember to move them throughout the day :D

Navver, those solar efficiency calculations are very useful, going by that a panel of 100w should provide enough power between March and September (with March being the lowest month).

78w / 31 days = 2.5w per day, per watt
165 / 2.5 = 66w
66 + 50% = 99w


Personnavver
12/7/2017 at 10:15pm
 Location: West country
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Don't forget, if you charge the battery at home you start with a full one and can return with an empty one. That gives you a bit extra, certainly enough for 1 day.

Also presumably the laptop and mobiles etc are charged at home first adding a little more bunce.

A 35A battery is 35 ampere hours at 12 volt. That is 35 x 12 = 420 watt hours. That should cover your 165 watt hours daily draw easily.

Those figures are for a commercial solar farm near Bristol with the panels angled but fixed. Of course, if you holiday in Margate, you will get many times more power as the sun will always be shining even when the rest of us have snow.


Personinsiorc
13/7/2017 at 8:30am
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f13dev, when re-reading my post I noticed my error on watts staying the same, and realising I was another beer on I reckoned it safer to delete the post to save confusion.

For batteries, the Ah rating is achieved using a standard often calculated over 20hrs (you need to read your battery spec sheet). So would be 1.75 Amp drawn over 20 hrs to get the 35Ah rating. Any draw higher will decrease the 35Ah capacity. 13.75A draw is still a big draw from a small battery.
But as I'd wrote, if charging during solar times then the battery wouldn't be providing all the power.

But your calculations are not correct as for example your laptop is not 120 watts, it's 60 watts for 2 hours which is still 60 watts draw. The light is 5 watt draw, for 3 hours which is not 15W draw it is still 5W. Phones, 15W each or both together? I'm on a phone so I'm not going to scroll up and down to check more but you need to re calculate your power draw as it is. This helps both your solar panel and battery size.
Then you can calculate the Ah draw of each device which will better enable you to work out your battery size to stay above the recommended 60% minimum discharge.


Personf13dev
13/7/2017 at 9:00am
 Location: Margate
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Insiorc, my calculations above were for the capacity, I've added in figures for the Wh and current draw at 12v.

Max draw in Wh
• Lighting - 5Wh (0.42Ah at 12v)
* Phone charge - 7.5Wh (0.63Ah at 12v)
* Laptop - 60Wh (5Ah at 12v)
* Battery charger - 5Wh (0.42Ah at 12v)

Total draw = 77.5Wh or 6.5Ah at 12v

So I would need a minimum capacity of 13.75A per day that can handle a draw of 6.5Ah at 12v. Using the draw over 20 hours I would need a battery with a 130A capacity which could be easily sourced.

Navver. As for camping in Margate I could just run an extension lead from the house :D If Margate truly got that much sun I'd have solar panels on the house and be making a mint selling electricity back to the board.


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Personinsiorc
13/7/2017 at 9:37am
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I wasn't clear - the watts drawn makes a difference for Amp draw for the battery.

So you now have a draw of 85 Watt if everything is plugged in, or 7 Amp at 12 Volt - much healthier for a small battery than 13.75 Amp.

No not 6.5 Ah over 20 hours for a 130 Ah battery, that was an example of how battery Ah ratings are calculated, to show that 13.65A will hugely decrease a 35Ah battery.
You'll get away with much smaller than that, especially as it's only 7 Amp draw, albeit for a couple of hours.



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