some newbie help

JustinAdams · April 2009

I'm interested in buying a Solar panel for camping. I'm looking at powering some led rope lights (7-10 watts) and a small radio (14 watts). I figured if I ran them 6 hours a day with 7 hours of sunlight, I'd need a 20W Solar panel

The 20W panel is a little out of my price range. I was hoping I could use a 5W panel or something that size. If I used a 5W panel, would that mean I'd have to charge it for 4 times as long as I would the 20W panel?

Also, I have a 400w Inverter that I plan on using. And looking at getting the EverStart 27DC-6 Deep Cycle Battery from Wal-Mart.

If anyone can help me out I'd appreciate it.

icarus · April 2009

Re: some newbie help

I don't know how you have figured, but here is a little back of the napkin rule of thumb.

Take the name plate rating of our solar panel,,, multiply that number by the number of hours you get GOOD sun,, divide that number by 2 and you will get a pretty good number of watt/hours that you can draw out of a solar powered battery system. This accounts for panel efficiency, charge controller ef, battery charging ef, and inverter ef. One note,, cheap, modified sinewave inverters are VERY inefficient so this number will be worse.

So in your case,, 20 watts X 6 hours= 120 wh X .5 (ef)= 60 watt hours,, enough to run your 14 watt radio for ~4 hours,, or your rope lights for ~6 hours. You can see that a 5 watt panel will not even keep up with the self discharge of most batteries.

As we say too often here,, do the math,,, on both the load side as well as the PV side. Avoid the ready fire aim,, figure out your loads and how to reduce them,, and then figure out how much PV you need.

As an aside,, most people over estimate the hours of GOOD sun they get,, and underestimated the loads they will draw. (think of the efficiency factors I spoke of above!) Also if you are concerned with battery longevity you should size the battery to only draw down to a maximum of capacity (some of us think that drawing down to only 70% is a better idea). A battery that is left not fully charged for more than a day or three will be permanently damaged quite quickly,, so in your "load" calculations you have to figure out how to keep the battery full on an every three day basis,, either from the PV or from shore power.

Good luck,

Icarus

BB. · April 2009

Re: some newbie help

Regarding the 5 watt solar panel... Assuming that it is a 12 volt 115 Amp*Hour battery... The 5 watt panel is basically just a trickle charger for the battery when in storage. It really does not provide very much energy.

Lets say 3 rope lights at 10 watts, and a 14 watt radio for 6 hours a night:

6h*10watts + 6h*14watts = 144 watt*hours (units for how much energy you used)

I would doubt that you will give "7 hours" of sun unless you are in a very sunny area, and perhaps have a solar tracker. Lets assume for now that you have 6 hours of sun in the summer. That you batteries are 80% efficient and you are not using the inverter. And, the solar charging system is ~77% efficient (panel derating, charger losses):

144 watt*hours / ( 6 hours of sun * 0.80 * 0.77) = 38 watt solar panel

So, just to power the loads above--roughly you would need a 40 watt panel.

Before I go on--questions/comments?

-Bill

mike95490 · April 2009

Re: some newbie help

Few places have 7 hours of good, charging sun, more like 5hr in general, unless you manually track the sun 4x a day to keep the panel aimed.
And you have battery recharge efficency, of 80% for flooded cell, 90% sealed AGM. So you need to replace 10 or 20% MORE watts than you take out.
Most panels are marked 20% higher than what the end user actually sees. (Just like car mileage stickers)

some newbie help

Comments

Categories