View Full Version here: : Solar panel wattage vs amp-hours of charge

23-06-2012, 02:54 PM
Hi guys,

It looks like my 100 amp-hour battery doesn't cut it for my power needs. I was thinking about getting a second 100 amp-hour battery, but I'd also like to be able to head out on multiple night trips.

I've been considering a portable solar panel kit like this one (http://aussiebatteriessolar.com.au/solar/portable-120w-solar-panel/), but I'm not sure what wattage I'd need.

Does anyone know how many amp-hours of charge I can expect per watt of solar panel capacity in SE QLD during a sunny winter's day?




23-06-2012, 03:30 PM
The one you have shown will give you 10amps charging current under ideal conditions. Safest thing to do is put a solar charge controller between the solar panels and your battery to protect from overcharging.
I guess in real terms on a sunny day, you may get 7-8 amps out of it.
Just looked at the ad again, and it comes with a charge rergulator. Should do what you want nicely.

23-06-2012, 05:41 PM
Been thinking about these solar panels too David.Laurie's post is about right,for that size unit.

Have a look on ebay,I did see a 120W one cheaper than the one in your link,but your one might be a better build.

An important part is the regulator,many say the ones they come with are ok,but down they track get a better one and attached for better input.

Anyhow here is an ebay one

http://www.ebay.com.au/itm/150765161480?ssPageName=STRK:MEWAX: IT&_trksid=p3984.m1438.l2649#ht_554wt_ 952

I think getting one of these,or the one in your link is a good investment,I'd be interested to see how you go.Cheers C

23-06-2012, 06:00 PM
Based on what we get from our solar panels at home, you'll probably only get 6hrs of full charging during the winter months. So if the panels will produce 7-10amps (might be at the lower end of the scale during winter), you'll get a maximum return of 42-70amp.hrs returned to your battery.

Unfortunately this is where a generator coupled with a 15-25amp charger tends to make more sense, albeit noisier and smellier...

At Leyburn, I'm OK for one night, but use the generator during day if I'm there for two nights. The small panel on my shed recharges the battery array over a week or so.


23-06-2012, 06:05 PM
Here is another 120 watt unit Dave,might be worth a look.


You might need another 100 amp hour battery,if your out at Leyburn for a couple of nights in the Winter.Your still welcome up here if your after a DSO site-no problems with power supply.

Be very keen to see what portable panels you end up getting.

Cheers Chris

23-06-2012, 06:12 PM
For 12V solar, general rule of the thumb is 1 amp for every 20 watts of solar panels.

Many ebay sellers are okay, but not LHP. Their panels are dodgy and under spec.

23-06-2012, 06:39 PM
Thanks for the tips so far everyone!

I like to do a bit of observing through my 10" Dob while imaging, so between running dew heaters, laptop, cameras, mount, etc my whole kit is a bit of a power guzzler... especially when the ambient temps drops below 0.

Chris, those eBay prices do look much more affordable. I did notice that the company I originally linked to offers affordable padded aluminium cases for the panels ($89 for the folding 120W panels).


David, yep I can definitely see why people run gennies. I think I'll be happier dealing with solar for the time being, but once I start class again I'll probably be busy enough for multi-night trips to be moot!

It'd be great to hear from anyone who has bought these types of travel kits - any traps to be wary of, etc.

23-06-2012, 06:42 PM
Sadly you will never see 10 Amps.
Solar panel ratings are tricky to relate to real world conditions as they are determined in a lab, also, this retailer has provided no ratings which does not help.
It's not as easy as dividing the power by the battery voltage unfortunately.
Typically, the panels do not produce their rated output current until they hit about 16 to 18 volts and at about 20 - 25degrees(or less). This varies somewhat on the manufacturer and cell technology.
In full Sun, at midday in December -January, you could expect maybe 6 to 7 amps if the panels are aimed DIRECTLY at the sun.
Changing the angle of incidence does have a drastic effect on output.

I use a 70W and a 80W panel.
Here are the ratings for the 70W:
PMax: 70W (Power Max)
Vmp: 16.8v (Volts @ Max Power)
Imp: 4.16A (Amps @ Max Power)
Voc: 21v (Volts open circuit)
Isc: 4.63A (Amps short circuit)
TEST: AM1.5 1000W/m2 @ 25c
In summer you may get close get 1000W/m2 of Sun, at midday, but the panel will most certainly get hotter than 25deg C (even on a sunny winters day) so the output will be derated further.

If we say these 120W panels have similar characteristics and test conditions, this BOM map (http://www.bom.gov.au/jsp/ncc/climate_averages/solar-exposure/index.jsp?period=jul#maps) shows an average of approx. 15MJ of Sun per m2 for a whole winters day in July in Brizzy.
This is about 4000 watt/hours per m2.
Day length in Brisbane is about 10.5 hours in July, so 4000/10.5 = ~380W/m2
0.38 X 120W = ~45Watts at 17volts, even less at ~14v!!

There are special charge controllers that will keep the panel at its optimum output, these are called MPPT or Maximum Power Point Trackers.
Basically they are step-up/step-down (Buck/Boost) DC-DC converters that maintain the panel at (in my case) 16.8v and charge the battery at the correct voltage(11-14.4v depending on state.) They're reasonably efficient too at around 80-90%, the rest disappearing as heat.

The most current I have ever seen from my 70W panel is about 3.5A
The most current it can produce is 4.63 amps but then the voltage is too low to be useable.
A far cry from the 5.8Amps you would expect from 70/12 ;)
Last weekend it produced 2.9 amps in full midday winter sun, aimed directly.

23-06-2012, 06:52 PM
A-ha... that's what I'd suspected! Ideally, it'd be great if I could get close to fully recharging my overnight power consumption (I'm guessing somewhere between 20 - 40 amp-hours).

It sounds like the 120 W kits would be the minimum for what I'm after, and 180 or 240 W would be better.

I suppose I should probably also look into keeping the battery insulated in the car, rather than exposed to the cold.

23-06-2012, 07:19 PM
Hi Dave
Simon is pretty right about solar panel output. I have 3x20watt panels on my shed roof, and they feed through a charge controller to 4x40amp hour deep cycle batteries and then to a 500watt pure sine wave inverter. This provides emergency power to run my instant gas hot water, and also 12volts for my astro equipment.
The most I have seen in amp hour output is 3.5, but this should suffice for what you want.
Solar has the advantage of reliability-virtually nothing to go wrong with it,and is much cheaper than a GOOD generator and more portable.

23-06-2012, 07:27 PM
Sounds like your set up is rather drawing a lot Dave-a cheap generator might be the way to go for astro use at Leyburn.

I still like the idea of solar panels over generators,my engal fridge and basic power use will be fine with them,but for hardcore astro use at Leyburn in mid winter will require much more re charging power.

A 180 or 240 watt portable solar panel might be worth a look at,most of those ebay sellers are quite good,there was one there that had solar panels-but the answers to my questions were very vague.So worth the time to ask around.

24-06-2012, 06:39 PM
+1 Simon is on the money with the correct figures, it's nothing to do with manufacturers being devious, Solar panel power has always been calculated at the point of maximum output, not at 12 volts. My 85 watt panels average around 4.5 amps each in to my batteries. A rough guide is to simply divide the rating by 18 to get the current available for charging from the panel.
Another thing you need to consider is the amount of sunlight hours available over the day, not sure what you could expect in Qld but in Summer in Tassie I get around 60 amp hours per day from 170 watts of panels, that equates to 6-7 peak hours of sun per day.

25-06-2012, 07:38 AM
yep i agree with the rest 5 to 6 hours at optimum photon gathering angle. watts divided by volts gives you amps, as I remember. Also in my experience these numbers always fall short of reality in the field.

26-06-2012, 08:43 AM
Except LHP really are devious. Buy one from them and find out. :P

General rule of the thumb of 20 watts per amp is not unreasonable in real world conditions. Some days you get a little more, some less but it's a good average figure.