minimum amount of solar

To the OP. I have camped for weeks on end with a fifty watt panel and a single group 27 battery very successfully. My usage is similar to yours. "Minimum"? Probably pretty close. Fun? Yes.

TomG2 , the min is ZERO but you knew that.

The "experts" strike again.

and then later

(And your derogatory comments.)

hmm... :B

Deleted by member.

Hi TomG2,

I suggest you use lamp cord to replace the power cord on your RV. It will 'work' sorta.

TomG2 wrote:
The "experts" strike again. The OP asked about "minimum", not "maximum", not even "optimum". Why wouldn't the 100 watt setup he mentioned provide the "minimum" needed to gain some additional dry camping time as well as provide him with an introduction to the realities of solar? Or is it more fun to confuse the novice with acronyms and recommendations costing a couple thousand dollars?

agree, the 100 watt setup is a great starting point. It helps keep from running the battery down quickly.
Its good for putting "Something" back in the battery. Great for charging phones, will take the "Parasitic Draw" off the batteries or battery when producing solar.

I started with a 100 watt panel, went "dry camping" and it helped allot, after three days of battery use (heater at night and water pump) I would need to run my generator at dinner time to re charge the batts. But during the day I could run the heater, charge phones and play the on board radio.
Installed another 100 watt for 200 watts total and now we can go out for about five to six days before the need to run the generator.
We camp at the coast often so sometimes there is clear sky's and we get full solar, sometimes we don't but at least it still puts something back in!

The "experts" strike again. The OP asked about "minimum", not "maximum", not even "optimum". Why wouldn't the 100 watt setup he mentioned provide the "minimum" needed to gain some additional dry camping time as well as provide him with an introduction to the realities of solar? Or is it more fun to confuse the novice with acronyms and recommendations costing a couple thousand dollars?

babock wrote:

BFL13 wrote:
Actually, a typical 100w panel has an Isc of 6.2 amps, which is what you get to the battery bank with PWM when the panel is aimed at a high sun.

Depends on the voltage of the panel. A Renogy panel will have those specs. Other panels that are closer to 18V or 20V Vmp will have less current.

Yes, although the specs on a panel might not mean much. I have two different brand 100w panels that have way different specs, but in the sun, they have the same 6.2 amp output.

One is a Coleman (Sunforce) which advertises its output to be 5.8 amps. However that is its spec Imp, so that is not its output. The other is a panel made in India with a higher Vmp so it has a lower Imp of 5.3, but also meaningless with PWM. It does 6.2 amps output.

Specs on panels are (Coleman/India):

Vmp- 17.1/18.8
Imp- 5.8/5.3
Isc- not shown/5.8
Voc- 21.5/22.7

Both panels are exactly the same size and shape and in real life have the same measured (by me) Vocs and Iscs.

Some folks on here insist on using Imp with PWM, which drives me crazy, but seems unstoppable. I have done many "field" measurements with panels first taking Isc, then connecting and taking amps to the battery, and they are the same number of amps every time.

You should use Isc as your "expected amps" for how big of a controller you need (and add 20% for margin in case of getting higher than Isc). You use Voc for your controller's input voltage limit (no margin there--go over that and it will fry, no second chances)

Vmp is just for MPPT ("bulk stage" only) and Imp is only useful for calculating needed wire gauge from panel to MPPT controller. Imp has nothing to do with amps to the battery. With MPPT that is what you get by dividing controller output watts by battery voltage.


Whatever, you have to start somewhere with estimating "how much" solar you should get. In real life, you get "some solar", find out what it does for your own camping (if you can even measure that), and then maybe get "more solar" if you need it (or just want it! ) If you have enough for summer it is not enough for winter, and enough for winter is too much for summer. So you just have to pick something and try it out.

BFL13 wrote:
Actually, a typical 100w panel has an Isc of 6.2 amps, which is what you get to the battery bank with PWM when the panel is aimed at a high sun.

Depends on the voltage of the panel. A Renogy panel will have those specs. Other panels that are closer to 18V or 20V Vmp will have less current.

Basically the more solar wattage to batteyf capacity ratio, the longer the batteries are going to last, total accumulated cycle wise.

Solar is a bit addictive. if you go small as you currently do not need much, well at least run oversized wiring which will not be stressed if you later decide to use more electricity.

While i used to have 3 group 27 marine batteries and 130 watts then 200 watts of solar, I am getting far better service with a single AGM-27 cycled deeper and charged at higher rates.

Flooded marine 27's are not very good batteries to deep cycle in my opinion. They seem to require ridiculous times in absorption to maximize specific gravity and regular EQ cycles when deeply cycled to get a few hundred deep cycles from. More solar allows earlier absorption to be reached and thus more time to be held.

But your usage is light and any solar is better than no solar. I'd just recommend allowing room for adding more solar wattage to whatever you initially install, by using thick wiring from panel to controller capable of carrying more current without excessive voltage drop. Running additional or thicker cable later on is more expensve and more work.