Solar Panels and Regulators
Solar Panels convert the sun’s rays into electrical power – I think we all know that by now – but what are the if’s and but’s of it all, and the conditions that affect its operation and efficiency. Most of us also want to know how much solar is needed, and this question is covered in the section How Much Solar?
So for now let’s take a closer look at the panels themselves, and then see how that matches up with conditions in Australia.
Voltage and Current of Solar Panels:
Their full name, photo-voltaic cells, tells us that light is converted to electricity, and the efficiency of commercially available panels is currently 15-20%. The sun gives us about 1000 Watts per square metre (W/m2) and this is also part of the Standard Test Conditions (STC) that are used to measure the output of solar panels. Every panel has a label on the back which specifies its output parameters at STC: Maximum Power (Pmax), Open-circuit Voltage (Voc), Voltage at peak-power (Vpk), Current at peak-power (Ipk), and Short-circuit current (Isc). For a 12 Volt panel the open-circuit voltage will be around 22 Volts – or a volt or so either way.
This 1000 W/m2 is of course only true around midday in summer but it gives us a typical figure to work with, and thankfully it’s used consistently throughout the solar industry. So using this typical figure for solar input, for every square metre of panel we should get about 150 to 200 Watts (15-20% efficiency x 1000Watts).
Practicalities and Panels:
If we check this out on a typical 60 Watt panel that measures 770 x 660 mm then we have 0.5082 m2 of area so we’re getting a typical input of about 508 Watts from the sun onto that area. So our efficiency in this case works out at 60Watts/508W = 11.8%. What happened? Why is our efficiency less than expected? Where did that power go? Well it’s more a case that we failed to capture the full amount of solar available. Have a careful look at a typical panel and you’ll see plenty of white space and wires, they are an essential part of the panel but it’s also space not covered by solar cells, so the efficiency we can actually achieve is less. So if we adjust for real-world factors then it’s close enough to give us some confidence in our solar panel, so that’s the good part.
Mono and Poly – over to you
And while we’re on theoretical versus practical, I am not going to get into the monocrystalline vs polycrystalline debate – if you really want to, there are 100s of articles and opinion pieces on this, and also some excellent scientific papers. For the average camper the differences are a few percent, and make very little practical difference at all. But maybe a good topic for discussions around a campfire…
Solar Panels and Temperature
Something however that does make a difference, especially to us in Australia, is temperature. The panel voltage drops about 0.4% per °C and the power by about 0.5% per °C. The STC we mentioned before measures the panel output at 25°C and any increase above that is going to drop the voltage and power output of our panel. This is important for a number of reasons.
In summer 40°C is not uncommon at all, and in the outback that might be a nice average summer’s day (hello Marble Bar!). For our solar panel this can mean about 65°C on its surface, maybe more – if you have a pyrometer, you can measure it, just don’t touch it! At 65°C this is a 40°C increase in temperature above STC (25°C), so our power will drop by 20% (0.5% x 40°C) and our panel voltage by 16% (0.4% x 40°C). This obviously means a decrease in charging current to the batteries but there is another effect that has important practical implications.
PWM and MPPT:
Every solar panel needs a regulator, to make sure we don’t overcharge the battery when it’s full, and to give it everything the solar has got when the battery is low. You will probably have heard of the two types – PWM and MPPT – pulse-width modulation or maximum power-point tracking. Now this might get a bit technical, but hang in there, we’ll take it slowly, and the results are important.
The Differences
Let’s take the PWM first. When the battery voltage is low it needs maximum charging, so the PWM basically connects the panel straight to the battery and gives it everything the panel’s got. In doing so the battery pulls the solar panel down to its voltage, let’s take a typical 12.5 Volts for the battery voltage.
The diagram shows a typical IV-curve for a 60W solar panel which plots the behaviour of its voltage (horizontal axis) and current (vertical axis left). The blue line also shows Power output in Watts (vertical axis right).
Now let’s see where that PWM regulator has placed us on the curve. If we draw a line straight up from 12.5 Volts (dotted red line) then it cuts the IV line (black) where the red dot is. To know what current this corresponds to, we draw a line across to the current axis and we have a current not far off the short-circuit current (Isc), just under 3.8 Amps.
Ok, now what would the MPPT do in the same situation? The blue line shows the power available from the panel and the MPPT will look for the maximum power point. This is pretty easy to spot as it’s the highest point on the blue line (power). At this point the panel will operate at its peak-power voltage and its peak-power current, for this panel the figures are Vp=17.273V and Ip=3.510A. The MPPT technology is able to down-convert the higher panel voltage to the battery voltage (in this case 12.5V), and in so doing it also boosts the current. So for a 100% efficient MPPT the current would be (17.273V/12.5V) x 3.510 Amps = 4.85 Amps which looks like a nice gain over the PWM’s current of 3.8Amps – right?
Well maybe, maybe not.
Factor in the Temperature:
Remember that stuff about the temperature? In summer that panel’s peak voltage will be 16% less, which brings it down to (17.273V x 84%) = 14.51 Volts. Then there’s the regulator efficiency – most are around 90-96% at their best, so let’s be generous and say 93% efficient on average. So if we factor that all in, we have (14.51V/12.5V) x 3.510 Amps x 93% eff = 3.79 Amps. So we’ve paid a price-premium for the MPPT but it’s giving us virtually the same current as the less expensive PWM in our Australian summer conditions!
For the PWM its output current is not affected by a drop in panel voltage – it’s still well above our battery voltage of 12.5V. The shape of the IV curve simply changes a bit and its operating point stays pretty much the same.
So that’s why MPPTs make more sense in Europe and North America where it’s cold, because the panel voltages are higher. Here in Australia rather stick with a good PWM – some of the best ones are Australian made too.
Higher Voltage Panels
Now if you have a panel with an open-circuit voltage (Voc) higher than about 22 Volts, then you don’t have a 12 Volt panel. So if you want to charge a 12 Volt battery, there is little choice but to go with the more expensive MPPT option. The MPPT will then down-convert the voltage to suit your 12 Volt system and boost your charging current accordingly. Just check that the regulator can handle the open-circuit voltage of your panel – you’ll find that in the regulator’s specifications as Input Voltage.
Higher Battery Voltages:
Also, if your system has higher battery voltages, say 24V or 48V then a good MPPT might well make sense, because the difference between battery and panel voltages is greater, and the MPPT can turn that voltage difference into more current for your batteries. And I must stress that it needs to be a good MPPT regulator. Fortunately most of the good forums & blogs have now wised up to cheap eBay regulators that claim to be an MPPT but when you open them up there’s just a simple PWM circuit inside – and a bad one at that! Rather just stick with well-known and tried-and-tested brands.
Bargain?
While we’re on that topic, if you’re looking at buying a solar panel that seems “too good to be true” then the simple calculation of efficiency that we did before should give you a rough guide. Calculate the area of the panel (in m2) and then divide its wattage by this area to see if you’re in the ballpark of 15 – 20% efficiency, or a bit less after adjusting for the practical factors we described before. If not, then that panel is claiming it does more than we can do with real-world science and physics, and you’re probably paying for something you’re not getting…
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16 Comments
Such interesting reading for the layman,
I’ve taken notes on may things, but still lost when it comes to choosing a solar panel.
I would obviously go as big as space and money allows. I am leaning towards 250w panel for a little caravan.
The prices can vary from $150 to $800 or more. I know you get what you pay for. But what are the obvious tell tale signs of what you are looking for to get a middle of the road?
In the same note, lithium batteries are the same, a big variation of prices – how to chose?
Thank you again
Andre
Hi André,
Good to know you’re enjoying the 12Volt Blog!
Hmmmm – solar panels can seem so simple but as you say, making a choice can be tricky.
First off, 200 Watts is about as big as 12 Volt panels go, and staying with 12V will allow you to use a less expensive PWM regulator. Going bigger than 200W means a higher voltage, which needs an MPPT regulator (and if the price is under $100, it’s not a real MPPT). So I’d stick with a 12Volt panel.
And then some panels say they are 200W but aren’t actually. To see how “believable” the panel’s specs are, we can check the efficiency (see “Practicalities and Panels” above), and that should be around 15% or so. For instance I found one 12V panel online that says it’s 200W but measures 1620 x 657mm – that’s 1.064 square metres, which means it’s claiming nearly 19% efficiency. Another panel, also 12V 200W is 1580 x 808mm, which is 1.277 square metres, with a claimed efficiency of just over 15%. In this case the second one is much more believable, so you’re most likely getting what you paid for.
Prices do vary quite a bit, so does quality – and that goes for panels as well as batteries. Buying from a retailer means you can take it back if you’re not happy, which also means they’re much less likely to sell dodgy stuff. Buying online is a big risk (unless you’re buying a well-known brand), and I’m aware of a few shady online stores based in Australia as well.
It always helps to ask around too – camping and caravanning forums are great for that – people are always happy to have a yarn about their 12Volt setups!
Cheers,
Alistair
Hello Alistair. Thanks for this site.
I have an off road van and one of its two solar panels was shattered, i think by a rock.
1 – Should I replace both panels in order to have even performance from both panels? I think they are either 100 or 110 watt units, but I am uncertain. I have not removed them from the roof of my van as yet so I don’t have specs on them yet.
2 – What panel(s) should I buy?
My setup is a Morningstar Sunsaver 20L SS-20L-12V controller, which is 12V and can handle 20 amp. My batteries are two 130 AH (real good quality AGM).
Thanks in anticipation. I am in Melbourne.
Hi Las,
Glad you’re finding the 12Volt Blog useful – and yeah, a panel usually comes 2nd in a fight with a rock – bad luck mate.
Anyway, the good news is that you don’t have to replace both panels. Given your regulator is PWM, the panels will be in parallel, so any 12Volt panel can be paired with the one that’s still good. Very nice regulator too, by-the-way, and if your panels are 110 Watt or thereabouts, the regulator has some good spare capacity too. In fact that regulator will happily handle 2 x 150W panels, so if you’re looking to increase the panel size then this might be a good time. Your batteries are also nicely sized, so they’d be able to store some extra solar, plus the fact that Melbourne gets about 20% less than most capital cities in Australia, so increasing your solar is good from a number of angles.
So, losing a panel is not nice, but it might also be an opportunity!
Cheers,
Alistair
Hello Alistair. I’ve just fitted a 160W panel to the top of a caravan. Full sun it was producing 24v and in the shed 17v. Once connected to the PWM regulator with max input 30v I get nothing from the connectors to the battery. Once connected to the battery, I get a reading of 12.6v but am thinking this reading is from the battery as the battery voltage doesn’t change with or without connection to regulator. There is also another set of wires off the regulator for a light and there is no current there either. I’m thinking I have a faulty regulator. Do you think this is an accurate assumption? Cheers, John G…
Hi John,
Good question – and all your measurements makes sense too. An open-circuit voltage of 24V tells me you panel is fine, and a battery voltage of 12.6V means it’s about 80-90% full. In the shed you’ll get just about zero charge, but with full sun around midday you should be getting close to 10Amps from that panel. In this case you should see the battery voltage rise slowly, but you may need to be patient, and it should then rise into the 13s and top out just above 14V, before dropping back to 13.5V or so.
In terms of measuring nothing at the regulator terminals, that doesn’t actually mean the regulator is dead – it only wakes up and starts charging once it sees a battery, so that’s quite normal. The kicker will be if you sit looking at it for an hour, in full sun, and the voltage hasn’t moved – then it’s time for a new one!
Hope this helps.
Cheers,
Alistair
Hi Alistair,
Thanks for a good series of articles.
I’ve noticed the advertising is now using “German panels = 25% efficiency” however reputable companies are more conservative at around 16% efficiency. The best panels in 2020 are around 22.5% so perhaps a good guide to your readers is 25% = bulls$$t. I have two panels for camping and both perform at around 15% x area of panel.
With just a fridge, lights and electric blanket and 280Ah of battery I can go off grid for 5 days without sunlight (solar panel failed) and indefinitely with a 160Watt panel. The biggest challenge has always been a shady campsite and the 30metres of cable to the open sunny spot for the panel.
Thanks again, Phil
Hi Phil,
Glad you’re finding the blog articles useful – that’s what it’s all about!
I looked for this marketing claim of 25% for German panels and came across a claim of “25% better” efficiency. When I looked at the actual brochure, that claim seemed to be based on what they call an “energy boost” that would somehow increase the output by up to 25% under “optimized conditions”. But they don’t explain what those conditions are, or how you or I could achieve them.
Interestingly, the brochure also measures the efficiency under STC as 16.7% – so on a level playing field (= real-world measurements) the results are suddenly believable – amazing!
(;-)
Thanks for a great question.
Cheers,
Alistair
Thank you Alistair, can you recommend a good MPPT Regulator ? There is so many on the market and some claim to be MPPT when they are not. Again, thanks for the advice,, very helpful,, Cheers Norm.
Hi Norm,
I normally shy away from promoting specific brands on the 12Volt Blog but I can happily recommend Morningstar – the downside is they are really expensive in Australia. As a good second, Victron do a range of MPPTs with solid specs. Some of them have bluetooth which allows you to see what the regulator is up to on your phone – personally I like that, but each one to his own. To handle the Voc of your panel, the 75/15 model is just fine – it can handle up to 75V input which is a nice safety margin, and it’s capable of 15Amps output which should allow you to use all of the 190W from your panel. I came across a few places online that carry Victron in stock, so it should be readily available in Australia.
Hope this helps mate – happy camping!
Cheers,
Alistair
G’day mate, I’m new here. I have a old N.E.S.L. Panel190 watts, open circuit voltage is 46.2 volts. Puts out about 5 amps,, I think. What can I do with it to make it useful for camping, (Charging 12 volt) ?
Hi there Norman,
Yep, 5 Amps looks about right for this panel. Unfortunately it’s a 24Volt panel, so using it to charge a 12Volt battery is not ideal, but it can be done! There are two ways to go here.
One way is use an MPPT regulator to bring the voltage down to 12Volt, but a good MPPT is over $100, so that might hurt a bit. The good news though, is that it’ll put 10 Amps into your battery (if we halve the voltage, we double the current).
The other way is a bit cheap-and-cheerful – you could use a simple 12V PWM regulator but you’d only get 5 Amps doing that – plus you’d have to check that the regulator could handle the panel’s open-circuit voltage of 46.2Volts.
So, can be done – and you’ve got the panel for free, so maybe the MPPT is the way to go – and 10 Amps is a decent charging current too.
Hope this helps!
Cheers,
Alistair
Thanks Ziggy, for sharing such pertinant info.
I am on a 42′ yacht and struggling in keeping batteries
at 50% in the AM.
I have just added what I thought was a 24 volt Amppair wind generator
however sadly it is a 12Volt 10 amp model.
I am considering a stepup transformer of say 9-20 volts imput and 28 volts output.
The other option is feeding it via a MPPT converter direct to the Batteries $x 250A Calcium.
plus 2 high crank amps start batteries.
Which option is the best? Will the MPPT scavenge from the 12 to 17Volts and step up the voltage with resultant amps loss ? to assist in charging on cloudy/ windy days?
Can Ziggy or anyone assist on feedback ?
Hi Bazz,
Welcome to the 12Volt Blog!
First off – going with a windgen is a savvy addition – it charges at different times to other power sources like solar or the boat’s alternator – so that’s a great start.
Stepping up from 12V to 24V is not going to be easy though – it can be done, but it’s a bit complicated, and expensive.
The first option is a step-up MPPT – like the GSL BMPPT-150 (http://www.gsl.com.au/bmppt150.html) but those are designed for charging 24V batteries from 12V solar panels, so that might not work with a 12V windgen. You’d have to check that with GSL themselves.
The other step-up option is a 12V to 24V battery charger (boost charger) but again, this is designed for charging from a 12V battery, so you’d need to add a 12V battery for the windgen to charge, and then feed this into the boost charger to charge the 24V batteries.
The simplest may well be to swap the 12V windgen for a 24V one. There may be logistical or other implications, but at the end of the day this might be the best option.
So, a windgen is definitely a great addition – stick with that – just depends how easy it is to get your hands on a 24V one.
Hope this helps.
Cheers,
Alistair
A good read. Thanks.
Re PWM vs MPPT… isn’t the latter supposed to be better if there’s a bit of cloud around?
And the temperature effect…. 40 C might be a reasonable assumption for the Northern half of the country in summer but the Southern? In winter where I live and in which I do most of my camping apart from the far North, I’m happy to see 25.
Hi Ziggy,
Glad you enjoyed reading the Blog, and your questions are both spot-on.
First, if there’s cloud around then the panels will be cooler and the panel voltages higher, so the MPPT will be able to convert this into more current at 12Volt. Also, according to the guys at the Solar Centre in Alice Springs (DKASC), in cloudy weather we get a “scatter effect” on the sun’s rays that can increase the panel’s output. So yes, the MPPT will give us more output in cloudy conditions than a PWM.
And second, if you’re in a region where 25 deg is the high side of temperatures, then again, the panel voltages will be higher and the MPPT can turn this into more current for our batteries.
I guess the upshot is that an MPPT will seldom perform worse than a PWM regulator. But I have a scottish surname, so cost tends be high on the list for me, and that’s where the PWM wins. And the money saved by getting a PWM instead, can be spent on extra solar which more than makes up for the gains of an MPPT.
But if you’ve already invested in a good MPPT – no problem at all! – it will always maximise the battery charging current.
Cheers,
Alistair