Noel's Muses

In overcast conditions, depending on cloud thickness, you might end up with no (likely Cumulonimbus), little (Stratus, Nimbostratus), or reasonably good (Cirrus) output, the only real way to find out how much Solar is available is with a PV irradiance meter.

When you record 183W/m² as I was lucky (or is that unlucky) enough to get whilst starting to write this article, you might think you're done with such a low reading, but you're not, you're still putting power into your battery, no it's not 183 Watts, my test panel is only 200 Watts, so that 183W/m² is not 183W of actual power, you'll be getting far less than that, think of the 183 as in relation to 1000, it's less than one fifth, so your panel is outputting less than one fifth of its maximum power of 200W.

Just how many amps are being put into the battery depends on your PV panel which usually has a label on the rear with all the information you need, your charge controller, and system voltages, and although losses in cabling also exist, we're not overly concerned about them here because they should be very, very minimal (unless someone stuffed up) to the point you'd never notice.

The charge controller type is a huge part of the equation, and what's concerning is some battery retailers give out some bad information in their blogs on how to calculate the amps, because they seem to copy and paste their advice based on old technology such as PWM (Pulse Width Modulation) controllers, these are pretty much antiques and replaced by MPPT (Maximum Power Point Tracking) controllers a long time ago, if your panel is 12V or one of those portable 100-200W folding types where the Voc (Voltage open circuit - the maximum possible voltage out of the panel when not under any load) is around 22V, a cheap PWM controller might still be acceptable - if you're never going to upgrade.

PWM charge controllers work only at your battery voltage, in a 12V system (most common for campers, caravans, RV's, marine), a 100-200 Watt panel would likely have a Voc of around 20-22V, and typically the PWM charges at about 14.4V being the upper charge voltage of the battery, this means there's about 6V a PWM can't use, it's wasted, and it gets wasted as heat. If you had a 425W panel that operates around 41Voc, the wastage in excess heat in the controller is about 27V - not good! So if you only have a PWM charge controller, any panel above a 22Voc is a waste, both in excessive treatment of the PWM shortening its life, and ultimately a waste of money.

MPPT charge controllers are basically smart DC to DC converters, the excess voltage is converted to more current, so an MPPT is always going to be better, although because of how they work, MPPT's wont actually "kick-in" until your voltage input from the panels is about 5V above your battery voltage, so always opt for panels with a Voc of at least 20v. It is also crucial you pay attention to the limits of your charge controller, the one pictured is a Victron 100/30, in Victron language this in V-in|A-out, meaning the maximum input voltage is 100V (based on panels Voc), and the maximum current it will output to the battery is 30 amps, although this is one of their smaller MPPT's, it's popular with camper trailers, and will work perfectly paired with a 425W panel, using the Power law, 425W / 14V (14V is the mean chargers output value for LiFePO₄ battery, use 12V in this calculation if using a lead acid based battery) which gives us 30 amps peak output, most MPPT's can detect what your battery voltage is, 12V or 24V, just make sure yours is rated for the battery voltage, our Victron 100/30 works with 12V and 24V batteries, but not 48V batteries.

It is imperative you never exceed the input voltage limit on an MPPT controller, you will destroy it, some, including Victron, do have a safety margin allowance on input voltages, but they are very small and temporary allowances for a couple volts over, so just don't exceed it, you will let the magic smoke out at some point.

Let's take a working example, a camper trailer that operates a 12.8V (nominal 12V) 100Ah LiFePO₄ battery, and has one 200 Watt Panel with a Voc of 22V, as I mentioned earlier it is overcast as the thumbnail shows, and my meter has reported 183.3W/m², so let's find out what the real power is and how many amps go into the battery with these conditions using both a PWM and MPPT Solar charge controller.

The formula used is, Power (in Watts) equals metered_Watts divided by 1000, multiplied by the PV panels Voc, in our example

       183 / 1000 = 0.183  *  200 = 36.6

So we only have 36.6W of usable power out of our 200W panel.

Testing our PWM controller, it's important to remember to use the PV panels Voc voltage, in this case, our 200W panel is only producing 36.6W and our Voc is 22V, this calculation is easy

       36.6 / 22 = 1.66

So we have in this low light condition, 1.66 amps going into our battery.

Testing our MPPT controller, the calculation differs, it uses the battery voltage instead of the panels Voc, so in our LiFePO₄ case that's 12.8V, and as above we are only producing 36.6W, but MPPT's will always have a small loss when converting excess voltage to amperes, typically they have a 97% efficiency, so we calculate this value using metered_Watts * 0.97 / 12.8

       36.6 * 0.97 / 12.8 = 2.77 

With an MPPT we are getting 2.77 amps into the battery, nearly double the PWMs output.

Now most campers and RVs have better than a 200W panel, many use house-style panels, which work really well, in fact, that's what I recommend, if you worry about how tough they are, consider the number of hail storms that batter these panels and don't leave as much as a mark when typical hailstone sizes strike year after year, but they do operate at higher voltages, about 41V Voc for our Jinko 425W panel, and at only about 1.7m by 1.2m, they're a perfect size for caravan roofs, RV's, or even A frame campers.

Quickly, using our earlier formulas

       183 / 1000 * 425 = 77.7W 

We now have 77.7W outputting from our panel

A PWM value is determined from

       77.7 / 41 

Giving only a slight improvement over the 200W panel with 1.89 amps.

An MPPT value is determined from

       77.7 * 0.97 / 12.8 

Giving us 5.89 amps, a huge increase over the 200W panel, and a whopping three times that of a PWM charge controller.

Despite the low price tags of around 15-20 AUD for a PWM (as low as 10AUD on Ali Express though), they really should be avoided if you have anything more than a 12V panel, which for a quality panel at around 250-300AUD for 200W, cost more to purchase than house-style panels, like our well known top ranking Jinko 425W panel at around 170AUD. For camping, boating, and off-grid I recommend Victron MPPT charge controllers, the 100|30 costs around 160 AUD at time of writing, and Victron do set the benchmark in quality and reliability, especially for MPPT's.

If you're still wondering if those few amps are really worth it, well you probably wont be cooking dinner with it, but most 12V fridges use about 3 to 4Ah, and the extra spare amp or so might let you have a light or two on when it gets dark, recharge your torch, camping or work lights, portable radio's AA or AAA's, and albeit slowly - your phone or laptop.

Even if you're at home and not out camping, if you've just suffered an event that's caused mass destruction of the grids network like South East Queensland did in 2025 - twice, and left with many overcast days before power's restored, you have the same capabilities without running your genny all the time which will eventually run out of fuel, and mass destruction means mass power outages which means servos cant pump fuel for your genny, so you end up in the dark eventually without a plan C

So is it worth having Solar out in dull skies, in my humble opinion... heck yeah!