System Posts: 2,511 admin
Hello all; Thanks for this great forum.

I have been camping for two years now with no hydro, just a generator. North of Toronto, Ontario.
Recently I got my hands on 4 broadband 12 volt gell batteries, slightly used for phone or cable power backups. They look like $200 each and I got all 4 for that. I allso just purchased 2 30 watt folding solar panels from Sunforce. Looks like another great deal at $150 each. I built a stand (looks like a sign with the 35 decree angle) out of 4X4 posts and plywood, in my most sunny spot. Haven't had a decent week of sun yet. LOL

I have just a couple of questions now.
I heard that solar panels will work on cloudy days and even moonlight (I don't think so, That's why they are called Solar panels) True or False?

Combining smaller wattage panels with larger panels, will make them all smaller wattage. or do you just add up the total wattage ? (combine 2-5 watt with 2-15 watts equals 4X5=20 or 40 watts?)

I think I read here or elsewhere that 3-5 watt panels equals 10 watts, you lose wattage on charging battery load?

A good point I want to add is; when testing solar panels make sure your multi-meter is set to 100-200 volts DC, not 12-20 volts like I did. I had mine set to 20 volts thinking it will read 12-15 volts, but in direct sun they read 24 volts and overloaded the meter to read nothing.


  • BB.
    BB. Super Moderators, Administrators Posts: 33,402 admin
    Re: Camping

    Solar panels really generate useful amounts of energy in in full sun and on bright overcast days. They will not give you much power in shade, or under moonlight. There was an article that did say they work under starlight--but that is simply not true.

    And under cloudy sky's you will only gather around 1/2 to 1/10 of that under clear and sunny conditions.

    When you add panels together--generally the watts just add up... two 30 watt panels will output 60 watts, etc.... However, the devil is in the details... For "standard" (typically less expensive charge controllers) you can only connect the panels in parallel to efficiently add power to your battery bank. If you have a more expensive "MPPT" type controller, you can add panels in parallel or series efficiently (and, as always there are limits that you must understand and not exceed).

    And, you need to get some basic electrical theory under your belt.
    I think I read here or elsewhere that 3-5 watt panels equals 10 watts, you lose wattage on charging battery load?

    There are Watts and there are Watt*Hours...

    Watts are watts... 3x 5 watt panels will output 15 watts (peak).

    However, you also need to know for how long that 15 watts is being generated.

    So, now you need Watt*Hours... The panels will only generate ~15 watts around 11am-1pm or so... But, they will generate useful power from 9am-3pm (or even longer, especially up north with long summer days, and moving the panels to track the sun during the day).

    So--you may get "5 hours of sun" (equivalent to full sun) out of your 12+ hour day.

    5 hours of full sun * 15 watts = 75 Watt*Hours per day.

    But, as you state, there are losses... Solar panels rarely output 100% power (closer to 80% of rating on warm days). And you have losses for charge controller, battery charging, and inverter (if you use 120 VAC).

    Take the 75 Watt*Hours you "think you gathered"--and you will find:

    75 WH * 52% = 39 Watt*Hours per day in summer.

    And, what can you do with 39 Watt*Hours....

    A 100 Watt Light Bulb:

    39 Watt*Hours / 100 Watts = 0.39 hours = 23 minutes of light

    A 13 watt CFL:

    39 WH / 13W = 3 hours of light

    So--you need to understand your loads, and how much power you will "need". And for bad weather, you probably will want a small genset to charge your battery bank (unless you can live with near 0 watts power usage until the sun come back).

    Unless you just need some power to recharge your digital camera, you may find that 30 watts of solar panel is not a lot of power and hauling around solar panels, battery, and inverter is more trouble that it is worth (just bring some extra batteries instead).

    Also, the quality of the solar panel does matter... Many of the small/mass market solar panels loose 20% of their output in the first 6 months of sun (and stabilize at lower output)--and may have early failures.

    But the "good" solar panels are still made from thin tempered glass panels--they are sort of strong--but will break like any glass window if not handled carefully.

    In the end, decide on how much power you need from your solar RE system, then work backwards to design it (panels, batteries, charge controller, inverter, etc.). Otherwise--you probably not find it real handing for weekend camping.

    A small Honda eu1000i or eu2000i genset with a gallon or two of gasoline can deliver much more power and is more useful/smaller to pack. Of course--who wants to bring their own "smog" when camping.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Camping


    Welcome to the forum.

    First, Solar panels,, like batteries can be wired in series or parallel arrays,, or even series/parallel arrays. 2 12 volt 100 watt (nominal) panels wired in series would put out 100 watts into 24 volts. The same panels wired in parallel would put 100 watts into 12 volts,, but at double the amperage (current) Remember,,, when you double the voltage you 1/2 the current and vise versa.

    I suggest you read the following links regarding gel cels,, as they are pretty finicky about charge currents and voltages:

    Solar panels put out close to zero under cloudy conditions.

    As for your "bargain" 30 watt panels. I suspect that they will prove to be no bargain. (That doesn't mean they won't work,, just that for the same $$ per watt you probably can get better panels) Right now, name brand panels can be had for as low as ~$3/watt in small quantities,, a bit less if you buy by the pallet load. If I am reading you correctly you paid $150 for 30 watts,, or $5/watt,,, no real bargain sorry to say.

    As you go forward, please read and absorb all you can. The more you learn, the less likely you are to fall into the classic "ready, fire, aim!" syndrome that plagues too many solar newbies.

    Do your math,, consider your loads and your charging capabilities. You will need a charge controller going forward to prevent over charging your expensive gel-cells. Also it is very likely you will you will underestimate your loads and over estimate your charging ability. Read the battery FAQ's and it will give you a better idea of how much you can expect to get out of any given system.

    My rule of thumb is that if you take the name plate rating of your array (in this case 60 watts) divide it in half to cover charging loses, inverter losses, wiring loses and charging efficiencies, (30 watts,,,) multiply this by the average number of hours of "good" sun on an average day (4-6max in most climates)=120 watt/hours,,, enough to run a 15 watt cfl bulb for ~ 8 hours.

    You also have to build in a reserve for your batteries. We like to consider the three day rule. That is have enough battery for 3 days without sun before we have to run a generator for fully recharge the batteries. You also have to decide how far you are willing to discharge the batteries during any cycle. Most people recommend never getting below 50% state of charge. I personally don't like to go below 80% state of charge. Check the specs for your batteries for better details.

    Finally,,, you will in all probability destroy at least one set of batteries as you begin to learn how to use them with solar.

    Good luck, and shout when you have other questions.

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Camping

    A bit of anecdotal info here:
    Under a full moon, my charge controller will actually click on and try to charge the batteries - but there isn't enough power so it shuts down again. The moon can actually cause enough rise in the open circuit voltage to trigger the controller. This may be where rumors like "works in moonlight" get started.

    I'm always leery of used batteries; there's so much you can't tell with even a hydrometer and load-test meter.