Been doing some math, and it doesn't look so good

[Gump]

I've been doing some math to study how much power I can get at the summer solstice and at the winter solstice, and from my calculations it appears that I'm only getting 0.59 of my solar panels power output in December at my latitude. Combined with the change in the length of the day, the number of amp hours I get ranges from 106.8 in June to only to 47.5 in December. And that's with a 300 watt, 8.5 amp panel. Granted, my refrigerator use will be much lower in the winter, but that seems... extreme. I'll go from easily having enough power to just plain not having enough.<div><br></div><div>Does this seem about right from other peoples experiences?</div><div><br></div><div>Here are my calculations.&nbsp;<a href="" target="_blank"></a></div><div><br></div><div>From this I gather that a panel mount that can be angled is a must.</div>

 

[Gump]

And that graph that shows power falloff as the panel angle to the sun increases is just a guess. I was reading that flat panels can lose up to 25% of their potential in North America, so I just made a linear equation extending that out. The actual line is probably a lot more complex than that.

 

[sl1966]

I'm not sure if you've already done these things or not but just in case. To cut down power use switch internal incandescent bulbs to LED. In the winters run your fridge off of propane (if it's dual fuel) since it won't have to fight the summer heat to stay cool.&nbsp;<div><br></div><div>I've plans to use two panels at 200 watts and a friend commented that that's a lot of power. Your spreadsheet is a bit more complex than my understanding is of this stuff, but I've linked you to the panels I'm buying in case you had more questions about them.</div><div><font face="verdana, arial, helvetica, sans-serif"><span style="font-size: 14px; line-height: 21px;"><br></span></font></div><div><a href="http://www.amsolar.com/home/amr/page_30/om100_solar_panel.html" target="_blank">http://www.amsolar.com/home/amr/page_30/om100_solar_panel.html</a>&nbsp;<font face="verdana, arial, helvetica, sans-serif"><span style="font-size: 14px; line-height: 21px;"><br></span></font><div><br></div><div><br></div></div>

 

[VanTrekker]

<p style="margin: 0px;">What kind of panel(s) and charge controller are you using?&nbsp; I'm consistently getting 10-13 amps out of my 235 watt panel by using the MPPT&nbsp; charge controller even though the panel is only rated at 8 amps.&nbsp; That's with no tilt.</p><p style="margin: 0px;">If I was using a straight PWM controller, I'd never see more than the rated 8 amps - it wouldn't utilize anything above about 14 volts - much of the panel's power (30V, 235 Watt) would go to waste... </p><p style="margin: 0px;">&nbsp;</p><p style="margin: 0px;">&nbsp;</p><p style="margin: 0px;">I've noticed that the biggest thing that kills efficiency is cloud cover.&nbsp; It's a problem in April around here - our rainy season.&nbsp; It was overcast and I was lucky to get 1 or 2 amps all day long...'</p><p style="margin: 0px;">&nbsp;</p><p style="margin: 0px;">VT</p>

 

[Gump]

Interesting... how could you be getting more power than the panel is rated for? That may mean I need to get a new charge controller... the one I got is limited to 15 amps.<div><br></div><div>It's all theory at this point for me. Don't have my panels or charge controller yet. Yes, it's MPPT.</div><div><br></div><div>This is the panel I was doing the numbers for:&nbsp;<a href="http://www.wholesalesolar.com/products.folder/module-folder/SolarWorld/sw-255.html" target="_blank">http://www.wholesalesolar.com/products.folder/module-folder/SolarWorld/sw-255.html</a><br><br>With all possible volts used, it's 8.5 amps, but it's a 255 watt panel. Surely it can't put out more than 8.5 amps?</div>

 

[VanTrekker]

<p style="margin: 0px;">You are right - the panel is only putting out about 8 amps or so... but that's 8 amps at 29.6 volts... Power equals current times voltage...&nbsp; 8 amps times 30 volts&nbsp;is about 240 watts.</p><p style="margin: 0px;">&nbsp;</p><p style="margin: 0px;">The problem with straight PWM chargers is that they clip off everything much above 14V...&nbsp;&nbsp; before considering tilt angle...&nbsp; The extra 16 volts times the 8 amps of current would never be used.&nbsp; That's automatically 108 watts down the drain even before you start.</p><p style="margin: 0px;">&nbsp;</p><p style="margin: 0px;">MPPT chargers use the whole 30 volts or so... Similar to how an inverter changes DC to AC, an MPPT charge controller can harness the power that would be been lost by the PWM charger.</p><p style="margin: 0px;">&nbsp;</p><p style="margin: 0px;">I shouldn't say the panels are putting out that much current - it's the charge controller that's doing it because it's sort of recycling what would have been wasted by.</p><p style="margin: 0px;">&nbsp;</p><p style="margin: 0px;">Even though I'm only getting 8 amps at 30V, the charge controller converts it to around 12 or more amps at 14.2V...</p><p style="margin: 0px;">&nbsp;</p><p style="margin: 0px;">I would say that a 15 amp controller is a little small for a panel that size.&nbsp; I'm using a 25 amp for 235 watts.</p><p style="margin: 0px;">&nbsp;</p><p style="margin: 0px;">If you're already using MPPT then you've got the most efficient way of regulating the higher voltage solar panel output.</p>

 

[akrvbob]

Hey Gump, I took a look at your calculations and couldn't understand a word of it. Can you walk me through how you arrived at your calculations. I think you have drastically, out-thought solar power. It simply is not that complicated. <br><br>It looked like you are 35 degrees lattitude, can you just tell me where you are? I am at 35 degrees lattitude and I have&nbsp; 190 watts of solar. That more than meets all my needs even on Dec 21. During the day I use all the power I want and still can't bring the batteries down past 12.6. If 300 watts doesn't meet all your needs, you are using a huge amount of power that I can't even imagine. <br><br>As long as you have an MPPT controller, you divide 300 by 12 to determine watts. Thats 25 amps, not 8. It doesn't matter the voltage of the panel, the MPPT controller will step it down to 12 volt to charge the battery. Whenever you step down volts, you have to increase the amps, which the MPPT controller does automatically. If you don't have a mppt controller, then you are right, you will loose a bunch of power, but most 300 watt panels are high voltage so you MUST use an mppt controller.&nbsp; <br><br>If you have any questions, feel free to ask. Bob<br>

 

[Gump]

Oh geez. That changes everything. I thought amps was fixed based on the ratio of watts to volts, and without an MPPT charge controller you'd be getting less than 8.5 amps for that panel. My average power use that I calculated was only about 60 aH, so at 25 amps, even if I just get 5 hours of charge a day, that's massively above my needs. Hurrrrrr I feel like an idiot and may downsize my system.<div><br></div><div>Also looks like I'll be returning that charge controller.</div><div><br></div><div>Damnit. Should've asked you guys first.</div>

 

[VanTrekker]

<p style="margin: 0px;">Don't feel bad... I think we all have been through it too... </p><p style="margin: 0px;">&nbsp;</p><p style="margin: 0px;">There is a learning curve.&nbsp; Some things really don't even have an answer so there will be some rework - guaranteed.&nbsp; I held off for almost a year to watch prices, learn, and then did the solar thing - it was a home run but&nbsp;much of the credit is due to&nbsp;the people on here and the wisdom they imparted.</p><p style="margin: 0px;">&nbsp;</p><p style="margin: 0px;">Geez, I blew a lot of time and money on some things I regret.&nbsp;&nbsp;&nbsp;Ibought 460 amp hours of battery when 230 would have sufficed.&nbsp; I bought a 2500 watt inverter when 1000 would have been way oversized.&nbsp; Poop happens.&nbsp; Then a person looks back when finished and realizes how much he or she had grown and learned and feels proud.</p><p style="margin: 0px;">&nbsp;</p><p style="margin: 0px;">It sounds like you've got plenty of insight and drive.&nbsp; You'll have a hell of a finished project with all the thought and energy you are putting into it.</p><p style="margin: 0px;">&nbsp;</p><p style="margin: 0px;">P.S.&nbsp; Here's a "redneck nerd's" solution to the tilt angle vs. efficiency thing...&nbsp; I just bought a bigger panel so I didn't have to tilt it.&nbsp; </p><p style="margin: 0px;">&nbsp;</p><p style="margin: 0px;">Have fun!</p><p style="margin: 0px;">&nbsp;</p><p style="margin: 0px;">V.T.</p><p style="margin: 0px;">&nbsp;</p><p style="margin: 0px;">&nbsp;</p>

 

[sl1966]

I bought a 2500 watt inverter when 1000 would have been way oversized.&nbsp;

<div><br></div><div>What are you powering? I mean assuming this was peak usage on a normal day for you.</div><div><br></div>

 

[Ricekila]

<P>

I just bought a bigger panel so I didn't have to tilt it.

</P><br>Ya-know -- if you let the air outa da tires on the sunny side --- thats another 3% more sun <IMG src="https://vanlivingforum.com/images/boards/smilies/crazy.gif" align=absMiddle border=0><br><br>( I looked it up <IMG src="https://vanlivingforum.com/images/boards/smilies/comp.gif" align=absMiddle border=0>&nbsp<img src="/images/boards/smilies/wink.gif" border="0" align="absmiddle"><br>

 

[akrvbob]

Solar is one of those deals where MORE really is BETTER!! You will never regret having too much solar. Solar panels do create a small amount of power even during cloudy and rainy days, so the more panels the better. If the sun pops out for a even a few hours you will be glad you have all that power. <br><br>The key to making use of the power you have is a large enough battery bank to store it for overnight and for rainy days. You need a minimum of 4 golf cart batteries and 6 would be better. That is a lot of battery storage. if you are frugal with power, that would last most of us for a week without sun. That's a good thing as far as I am concerned. <br><br>If you are in the market for a new MPPT controller, I recommend the BZ 500 mppt controller. I know several people with them and they are very happy with them. It is what I will buy when I upgrade my system. Here it is for $200:<br>http://thesolarstore.com/charge-con...se&amp;cvsfa=3594&amp;cvsfe=2&amp;cvsfhu=3632<br>

 

[Seraphim]

<p style="margin: 0px;">Gump</p><p style="margin: 0px;">&nbsp;</p><p style="margin: 0px;">Don't feel bad - I've been trying to figure thos out for months, and still can't get my head wrapped around it.</p><p style="margin: 0px;">&nbsp;</p><p style="margin: 0px;">Quite a blow to my ego....</p>

 

[Gump]

&gt;<span style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: 13px; text-align: left; background-color: rgb(255, 255, 255); ">If you are in the market for a new MPPT controller, I recommend the BZ 500 mppt controller.</span><div><span style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: 13px; text-align: left; background-color: rgb(255, 255, 255); "><br></span></div><div><span style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: 13px; text-align: left; background-color: rgb(255, 255, 255); ">Oh.</span></div><div><span style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: 13px; text-align: left; background-color: rgb(255, 255, 255); "><br></span></div><div style="text-align: left;"><font face="Verdana, Arial, Helvetica, sans-serif" size="2">My.</font></div><div style="text-align: left;"><font face="Verdana, Arial, Helvetica, sans-serif" size="2"><br></font></div><div style="text-align: left;"><font face="Verdana, Arial, Helvetica, sans-serif" size="2">God.</font></div><div style="text-align: left;"><font face="Verdana, Arial, Helvetica, sans-serif" size="2"><br></font></div><div style="text-align: left;"><font face="Verdana, Arial, Helvetica, sans-serif" size="2">That's the one I got. I remember now, I was looking at one less than half that size, and then I saw that one for almost a hundred dollars cheaper and with a much larger capacity and read some reviews on it. So I got it, figuring I might get more PV someday.</font></div><div style="text-align: left;"><font face="Verdana, Arial, Helvetica, sans-serif" size="2"><br></font></div><div style="text-align: left;"><font face="Verdana, Arial, Helvetica, sans-serif" size="2">Apparently idiot uneducated past me made a good decision.</font></div><div style="text-align: left;"><font face="Verdana, Arial, Helvetica, sans-serif" size="2"><br></font></div><div style="text-align: left;"><font face="Verdana, Arial, Helvetica, sans-serif" size="2">So um... this works out well.</font></div>

 

[sl1966]

What do you guys think of this one ...&nbsp;<a href="http://www.blueskyenergyinc.com/products/details/solar_boost_3024il/" target="_blank">http://www.blueskyenergyinc.com/products/details/solar_boost_3024il/</a><div><br></div><div>It came highly recommended.</div><div><br></div>

 

[SoulRaven]

@Gump... It really is past time to <u>Stop</u> all the calculations and <u>Start</u> real world field testing... Start with weekend get away's and start shaking out all the bugs that real world field testing will expose... Find out what is and what is not working to include monitoring your solar package and the power consuming attachments...<br><br>When you do this set-up your complete camp as you envision... You will be amazed that over time with corrections how much easier it becomes due the changes you make, till it all finally comes together for ya... It is also very important to go out in all weather conditions...<br><br>