Real world solar charging

[N147JK]

Looking at the 100 watt portable suitcase solar chargers. What I'm reading is to expect on average 30 amp-hours of charge per day. For those already using these systems, is this a realistic figure? How many amp hours are you actually getting per day? - Thanks

[folivier]

These are probably rated at 7 amps/hour in direct full sunlight. My experience is to figure on 5-6 amps/hour for about 5-7 hours. More if you can move them around so the sun is directly on the panels.
But the other question is how many amps do you use in 24 hours?

[N147JK]

Originally Posted by folivier

My experience is to figure on 5-6 amps/hour for about 5-7 hours.

But the other question is how many amps do you use in 24 hours?

Ok thanks, so the 25-30 amp hours/day is about right.

12V fridge + CNG furnace (winter time only) + misc lighting I think worst case consumption is about 75 amp hours/day. So one of these panels would never keep up. I assume that is under ideal solar conditions. On a cloudy day in the winter what is the output? Figure 15 amp hours/day or less?

[mkguitar]

variables such as temperature ( less efficient above 85º for many) angle of sun and etc.


I have a small solar briefcase which supplies a constant about 1 amp ( it's small) which is equivalent to the added fans on my fridge ( if all are on) which are my highest draw when camping


with meager electrical use ( led lights) and solar supplement I can last longer than my water supply


increased efficiency if your controller is at the battery end rather than at the panel end of wiring. DC doesn;t travel long distances well. most controllers are wasting power to regulate to 12~14 volts... if controller is at the end of a 20' wire you may end up with 11 volts at the van.


I may get a larger folding panel set up, as long as it fits in the space I have ( where the shower pan used to travel)

[N147JK]

Hmmm one of the big draws of the portable systems is that they allow one to park in the shade, then position the panel out in the sun. But that requires a wire run, which is bound to penalize the available charge power.. kind of defeats the purpose?

Well thanks for the good info. Interesting note about the proximity of the controller to the battery.

[mkguitar]

Originally Posted by N147JK

But that requires a wire run,

most panels will put out somewhere around 18 volts ( +/-) regulate that close to your battery to avoid a meaningful voltage drop

[booster]

Originally Posted by mkguitar

most panels will put out somewhere around 18 volts ( +/-) regulate that close to your battery to avoid a meaningful voltage drop

That all depends on if you have an MPPT or PWM changer. If it is a PWM it just cuts down voltage and maintains the amps so the power available from 14.4v and above is dissipated by the charger, at least how it was explained to me. With an MPPT, it will use all the power received regardless of voltage to put out maximum amperage at the set charge voltage.


In both cases, however, a small system may or may not be able to hold rated output voltage from the panels because the batteries will accept more than that. When the voltage gets pulled down to nearer the battery voltage acceptance at it's state of charge, the PWM will probably be a bit more efficient under those conditions.


So voltage loss from the wire lengths on the suitcase styles can be significant as I think I have seen numbers of over 2 volts or more.

[@Michael]

Originally Posted by N147JK

But that requires a wire run, which is bound to penalize the available charge power. kind of defeats the purpose?

Amperage is small, so longer runs are feasible with moderate wire gauge. I'm using 30' of 10ga. for 200 watts of portable solar (2 x 100 in series), with minimal power loss.

The less your daily consumption, the more the portable solar will help keep you powered.

[N147JK]

Originally Posted by @Michael

I'm using 30' of 10ga. for 200 watts of portable solar (2 x 100 in series), with minimal power loss.

That is excellent - thanks for the input.

[@Michael]

FWIW - you can do the math on energy loss for a given length of wire at a given current, but consider that even with some loss on the wire, a 100w panel in the sun is generating significantly more power than if you had no panel.

Here's a writeup on what I built a couple of years ago.

https://janke.photography/2020/07/12/portable-solar/

Since then I've made a few modifications - one portable panel is now mounted on the roof. I've installed 300Ah of lithium battery and more-or-less abandoned the AGM battery bank, so the connection from the portable panels to the battery bank is simpler than it was.

Also, with 300Ah of lithium battery, I can last 2 - 3 days with minimal sun, at which time I'll likely have full tanks and be ready to move on anyway, so I'm less dependent on solar than I was two years ago.

[booster]

I put on a thread a number of years ago that covered how to calculate the harvest with any angle of panel and any latitude at any time of the year at any time of day. Yes, there are automatic calculators for that, so pretty easy do except the calculator didn't totalize for a whole day. You can do that manually though with a bit of effort. I am sure the angle thing changes are not consistent between installations as some panels are much better than others are less than perfect angles.


If anyone is interested, I will try to dig it out, hopefully the calculator link is still good.


One of the biggest issues with the smaller solar systems, especially with larger battery banks or high acceptance like lithium batteries is that the panels get dragged down in voltage off of their maximum power point, which on most good panels these days is somewhere around 17.7 volts for a 12v nominal panel. Once off the max power point the watts drop pretty quickly, especially when the panels get dragged to very near the battery voltage which is pretty common with high acceptance battery banks. Both PWM and MPPT controllers have similar issues this way. With the low output from being dragged down to lower wattage they often will never get to the full absorption voltage even as there aren't enough good sun hours a day. This is also where any voltage loss in the wiring becomes and issue.