Solar strategy in the east

Hi CA,

It would be more correct to say the battery voltage output set point limits the PWM input voltage. However that doesn't affect the amps (much) for 12 volt panels. The higher the voltage on the panels the more the pendulum swings to a (good) mppt controller.

MPPT does about 8% better over all than PWM. It may (or may not) start charging sooner in the day, and continue charging later in the day. There are too many variables and so far no one has made two identical systems where one is pwm and the other is mppt.

CA Traveler wrote:
PWM can only charge at the panel rate and loses significant power due to the voltage drop.

Hi BFL13,

Not only trees--but Forestry creates more wealth in Saskatchewan than farming.

BFL13 wrote:
PT, you mean there are trees in Saskabush? Who knew? :)

CA Traveler wrote:
PWM can only charge at the panel rate and loses significant power due to the voltage drop.

Let's not get confused. Voltage drop is normally a term reserved for post-controller wiring, and is not relevant in shading issues. In this context, voltage drop affects PWM same as MPPT - battery is kept at lower voltage than output voltage of controller terminals, and this is compensated (in good controllers) by voltage sense wires. On a short wire and small PV wattage this drop is not important.

CA Traveler wrote:

BFL13 wrote:
Nerd here. When a PWM 12v loses voltage from 21 volts (12v panel) it still has plenty to be over 13v so amps don't suffer. MPPT (37v "24v" panel) makes its amps from voltage so it does suffer if voltage goes down.

Wow - lets look at reality using your example. The 12V panel loses 7V or 7/21 of its power. So a 100W panel can only deliver 67% of its power with a PWM controller. The same panel with a good MPPT controller would lose only about 8% of its power.....

.

The 100w panel cranks out 6.4 amps until battery voltage reaches about 13.5 then amps drop slightly till battery voltage reaches 15v and then amps crash (IV curve)

So at full amps at say battery 13v, watts is 6.4 x 13 = 83.2w So who cares if the panel is not producing all of its 100w, it is still cranking out full amps. (it's amps you want into the battery)

That is why PWM can "afford" to lose some voltage; you still get the amps. MPPT can't afford that. Look what happens to amps with MPPT when the panel loses voltage in the heat of the mid-day sun. Ouch! Meanwhile Isc (which PWM uses for amps) goes up in the heat. :)

BFL13 wrote:
Nerd here. When a PWM 12v loses voltage from 21 volts (12v panel) it still has plenty to be over 13v so amps don't suffer. MPPT (37v "24v" panel) makes its amps from voltage so it does suffer if voltage goes down.

Wow - lets look at reality using your example. The 12V panel loses 7V or 7/21 of its power. So a 100W panel can only deliver 67% of its power with a PWM controller. The same panel with a good MPPT controller would lose only about 8% of its power.

What you're missing is that PWM can only charge at the panel rate and loses significant power due to the voltage drop. MPPT on the other hand coverts the amps and voltage of the panel (ie the watts) to the higher amps and lower voltage of the battery charge. Hence the battery charge amps with MPPT is higher than the panel amps. It's all about power conversion and both amps and voltage are part of the equation.

PT, you mean there are trees in Saskabush? Who knew? :)

Hi BFL13,

Why would I want to downgrade to something that doesn't work in the shade? lol

pianotuna wrote:
Hi BFL13,

I was under the impression that 12 volt panels were a nominal 17 volts--not 21.

17 is the Vmp which means nothing to a PWM. Voc is 21-22v.

Actually amps at Vmp (Imp) is less than what you should be getting with PWM, which is the Isc

PT, once you upgrade your solar from MPPT to PWM all this will make more sense when you see it in action! :)

BFL13 wrote:
You can get an amp or two in the shade instead of your proper x amps, but so what? 1 amp for 8 hours is 8 AH. Whoopee do.

I agree with everything else in the above post. And almost agree with this last statement, but ...

8 AH is almost nothing, yes. However, if they cover all the roof with panels - even with crystalline panels, ~600W array will generate 2-3 amp under leafy shade, and this is 18-20 AH. Still looks like nothing if your daily draw is over 80 AH. What if somebody's daily draw is 40-50 AH? LED lights, pumps, DC guts of LP frig and small electronics will hardly draw more than 45, and it's realistic to limit it to 30-35 AH on those days when you are not watching TV or laptop. If they cover all the roof with amorphous panels, it could collect more than 30 AH, this is pretty close to what they need then.

Not to advocate MPPT over PWM - no sun is no sun, no matter what - just to note that camping in the shade without generator is possible. Though, I wouldn't be willing to try if it's a dense and permanent shade where panels are not exposed by at least 50% of area for at least an hour a day. Not with crystalline panels for sure. With amorphous - don't know, so few people are using them. Might be the ticket for shade, yes.

200W portable array of regular crystalline panels might work as well (sans microwave), if they stay for at least a week at that place. Otherwise it's too much trouble schlepping them back and forth every time.

Some are 21V PT. The Kyocera 140s are 21V.