Solar What solar do I need?

Chriso100

Member
Apr 26, 2020
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Rosebud Victoria
Hey there all. I’m sure it’s been done to death...but. What size portable solar panel do I need? Is there a specific thread I can go to? Just recharging the aux battery in our van etc.
Chriso
 

Crusty181

Well-Known Member
Feb 7, 2010
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You first need to determine how much power you consume from the battery. Its often suggested that wet deep cycle batteries are only good for 50% of their capacity ie a 120ah battery is good for 60ah, and that is a good place to start (50% is just a arbitrary point where someone has decided its a good balance between practical usage and battery health of life expectancy. You can use more or less, it will just affect the batteries overall life expectancy)

Make up a list of what appliances (watt, get it) you are using, and for how many hours per day you use them. Combined the consumption of those items over a 24hr period. Consumption is measured in either watts or amps, but you'll need the amps for this exercise and the amps you will mean amps/hr.
eg

Screenshot 2020-09-22 074820.jpg


You get the idea

Many appliances are measured in watts, so just convert it watts / volts = amps (per hour) My TV is 48watts, so 48w / 12v = 4amps/hr. Hopefully the totals are less than the magical 50% of your battery capacity, or you'll need more batteries before you look at solar panels.

The size and type of the solar panel or panels will then be determined by how much total current you have calculated that you will be consuming from the batteries each day, and need the solar panel/s to replace. Solar panel output amps are calculated as the panels wattage / panel voltage ie a 200watt panel / 20volts = 10 amps per hour. ("12v" solar panels actual voltage is around 20v ish) I would be counting on a maximum average of 4 hours per day that youll will be harvesting solar power, so that 200w panel will produce 40amps per day in the perfect world as determinced on the mountainside of a far away Scandinavian village surrounded by half naked nymphs. Its a bit like the fuel specs you thought you be getting in your new car, that the dealer showed you in the brochure. Down here in the real world, so you could easily loose 25% of that so now we have 30amps per day from our 200w panel if we have lots of clear sky and beautiful sun.

If youve worked out youll consume 30amps per day, one 200w will go ok. If you worked out youll consume 60amps, youll need 2 x 200 panels. If you want or only have room or funds for one 200w panel you have no choice but to curb your power consumption to 30amps per day. When its overcast, youll have to curb your consumption anyway because youll be harvesting less

You can do what I did and throw calculations and techno mumbo jumbo out the window and super pimp it up by dropping in 260amps of batteries, and slapping 600w of panels on the roof, and throwing another 600w of blanket panels on the ground and without a clue how much solar your sucking from the sky you only know its too much because the grass is charred around the van, the neighbours are vaporising, the tv brightness is like a welding flash and a black hole forms over your campsite
 
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mikerezny

Well-Known Member
Sep 11, 2016
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Mount Waverley, VIC
All of the above!
Here is some further information to add to the conversation.

Basically you need to determine normal daily usage in Ah. Then you need at least enough solar to generate that power each day, in reasonable sunlight in about 5 hours. Add another 20% if on a roof where it will not be positioned optimally to face the sun.
Then you need, at the very least, enough battery capacity to at least provide power for the other 19 hours when there is little or no sun.

As stated above, 50% is a good trade off. But there is no magic about 50%. A deep cycle battery is designed to be run down to 0%. The more deeply they are discharged, the less cycles before they reach end of life. Contrary to what is written by many on various camping forums, discharging a battery below 50% will NOT kill it.
Now, if you are a weekend warrior, and venture out around say 50 days a year. Then the battery will be discharged 50 times a year. The typical life of battery is rarely over 6 years when used in a van. So that would be 300 cycles over this 6 years. The specs for a typical GEL battery show that you can expect it to provide about 500 cycles if discharged down to around 10V. From that you can see that the battery will typically die of old age before it dies from being regularly fully discharged 300 times.

The caveat is that the battery must be maintained correctly and fully charged as soon as practicable. Batteries deteriorate much quicker if left flat.

The economics, in the example above, are that using two batteries will double the weight, double the cost, but you will now discharge the battery down to only 50%. That means you can expect the battery to provide about 1,100 cycles. That is 800 more than the 300 you actually needed. They will still typically die of old age after 6 years and need to be replaced at twice the cost. Weight is an important consideration and an extra 30kg needlessly greatly impacts on the number of slabs that can be taken away on holiday.
You can do the same sums for varies days of use. You need to use the van for more than 80 days a year before the battery will typically die of cycling at 100% before it dies of old age.

The most common causes of battery failure for new owners:
Coming home and storing the van for months and letting the battery become completely flat.
Leaving phantom devices running off the battery or not charging it at all.
Never bringing the battery back up to full charge and maintaining it at full charge.
Not charging the battery correctly: Incorrect float voltage.
Overcharging the battery and boiling it, at the worst causing an explosion and a fire.

A simple vague analogy is personal finances.
Income is solar power
Living expenses is power consumption
Battery is the bank account. (But anything over 100% in the battery goes in the bin)
Bank balance needs to be enough to survive a rainy day or two when the Income runs out.
Same as with finances, when the income stops, you need to know were the money is going and trim outgoings to live within your means to avoid draining the account.

Implicit in this is the means to measure solar generation, power usage, and battery State of Charge (SOC).

I have attached the spec sheet for a battery that was / is supplied by Jayco.

take care
Mike
 

Drover

Well-Known Member
Nov 7, 2013
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Well thats got your head spinnning now and all good wisdom......................when you work out how much power you need for 24hrs, add 10%............... then work out how much solar you will need to cover that amount going by the rated output of the panel/s, to save heartache deduct 30% from the rated output and you may go close to what actually comes down the cable to your battery.
I'm with @Crusty181 in that the theory is all well and good but adding more than the eggheads on paper reckon will ensure it works and you enjoy your holiday..........

Mine has only 205ah of GEL batteries (prefer AGM) 200w on the roof and 160w on the ground, the portable actually shunts more juice during the day than the roof due to ability to keep facing the sun god................ Van chews a roughy of 35 -45a/day and while in theory I should never get low I'm adding at least 180w on the roof to keep things happy, would like more storage but the panel is cheaper and another 25-30kg is not wanted.......

In a nutshell the bottom line would be 250w of panel on roof, 200ah of AGM in boot and a MPPT regulator......................of course if mostly using parks then dont bother and save money.

Some reading to make your eyes roll.......

...........https://caravanchronicles.com/guides/how-to-connect-two-batteries-in-parallel/

..............https://www.gorv.com.au/10-rv-solar-myths/

...................................https://12voltblog.com.au/a-guide-to-using-solar-panels-and-regulators/
 

mikerezny

Well-Known Member
Sep 11, 2016
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Sort of like buying the biggest heaviest toughest vehicle you can find before working out what size van you want to tow, where you actually want to go, and before you have gained any experience or knowledge on how to maintain a vehicle or tow a van.

It never hurts to spend time to understand how things work. The more difficult it is, the more important it is to attempt to understand it.

There are heaps of posts on other social media by people optioned up to the hilt, no understanding of how any of it works. And then replacing batteries prematurely and all other bits and pieces without a clue why they have failed, why they dumped a freezer full of food and had to come home early, and then blaming everyone else for the problem. So refreshing that this doesn't happen on this forum.
:cheer2::cheer2::cheer2:

take care
Mike