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Colt View Drop Down
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Direct Link To This Post Topic: Solar panel recommendations
    Posted: 24 Jan 2020 at 7:12pm
Well, you need to calculate your demand in watts times the hours you will use whatever it is.  That is watt-hours  Then, size your panel(s) to generate that many watt-hours during a 6 hour charging day (average hours of full power generation in a day). 

So, if you run 20 watts of lights x 6 hours, 80 watts of TV x 4 hours, 10 watts of ventilation x 8 hours, you have 120 watt-hr + 320 watt-hr + 80 watt-hr = 520 watt-hr.  You need a 100 watt panel (100 watts x 6 hrs), it you set the angles for best generation.  A little higher panel wattage will not hurt.  Your Watts May Vary.

Size the battery bank for your load x 3 days of no sun and double that so the battery doesn't drop below 50% charge.  520 w-h x 3 x 2 = 3120 w-h.  Divide watts by 12.5V and you get 250 Amp-Hr.  That's a lot of battery, but you get the gist.  You may find it acceptable to run with 1 or 2 days reserve and then recharge with your vehicle, or a small, suitcase gas generator.  Two Group 24DC batteries like my 180 has would be 150 a-H, almost 2 days. 
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Direct Link To This Post Posted: 27 Jan 2020 at 9:51am
Originally posted by Colt

Well, you need to calculate your demand in watts times the hours you will use whatever it is.  That is watt-hours  Then, size your panel(s) to generate that many watt-hours during a 6 hour charging day (average hours of full power generation in a day). 

So, if you run 20 watts of lights x 6 hours, 80 watts of TV x 4 hours, 10 watts of ventilation x 8 hours, you have 120 watt-hr + 320 watt-hr + 80 watt-hr = 520 watt-hr.  You need a 100 watt panel (100 watts x 6 hrs), it you set the angles for best generation.  Your Watts May Vary.

Size the battery bank for your load x 3 days of no sun and double that so the battery doesn't drop below 50% charge.  520 w-h x 3 x 2 = 3120 w-h.  Divide watts by 12.5V and you get 250 Amp-Hr.  That's a lot of battery, but you get the gist.  You may find it acceptable to run with 1 or 2 days reserve and then recharge with your vehicle, or a small, suitcase gas generator.  Two Group 24DC batteries like my 180 has would be 150 a-H, almost 2 days. 

Its better to run this calculation in amp hours than in watt hours. This is because if you do it in watts hours you aren't taking into account the losses in the solar module and battery. If you do it in amps and amp hours those losses are taken care of automatically.

Measure each of the loads in amps using a clamp on multimeter. Multiply each by the number of hours of daily operation and add them up to get total load amp hours per day. Divide that number by 4 for a less sunny area or 5 for a sunnier area. Look up the max power current rating  (not the watt rating) of the solar module you're interested in and be sure that its that value or higher. That should run your loads indefinitely on solar in a clear location in spring, summer, and early fall. If you only want to extend a battery charge for a few days of camping you can get a smaller solar module.

You can use the daily amp hour number you calculated to size your battery as discussed in the post above. 
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Direct Link To This Post Posted: 27 Jan 2020 at 1:49pm
Originally posted by offgrid

Its better to run this calculation in amp hours than in watt hours. This is because if you do it in watts hours you aren't taking into account the losses in the solar module and battery. If you do it in amps and amp hours those losses are taken care of automatically.

Measure each of the loads in amps using a clamp on multimeter. Multiply each by the number of hours of daily operation and add them up to get total load amp hours per day. Divide that number by 4 for a less sunny area or 5 for a sunnier area. Look up the max power current rating  (not the watt rating) of the solar module you're interested in and be sure that its that value or higher. That should run your loads indefinitely on solar in a clear location in spring, summer, and early fall. If you only want to extend a battery charge for a few days of camping you can get a smaller solar module.

You can use the daily amp hour number you calculated to size your battery as discussed in the post above. 

I disagree with this philosophy, as you then have to compensate about series/parallel issues. I have for years just converted batteries and loads into watt-hours. Then you can deal with each battery and load as just a "box of watts". As it happens, when battery voltages go up and down, the current (amps) will go down and up. The watts pretty much stay the same.

You will also notice that the industry is moving that way too. The batteries in EVs are listed and used as watts and watt-hours. 

It just simplifies things all over the place.
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Direct Link To This Post Posted: 27 Jan 2020 at 2:21pm
Originally posted by GlueGuy

 

I disagree with this philosophy, as you then have to compensate about series/parallel issues. I have for years just converted batteries and loads into watt-hours. Then you can deal with each battery and load as just a "box of watts". As it happens, when battery voltages go up and down, the current (amps) will go down and up. The watts pretty much stay the same.

You will also notice that the industry is moving that way too. The batteries in EVs are listed and used as watts and watt-hours. 

It just simplifies things all over the place.

There are no series parallel issues on our 12V systems. And, its very easy to measure your loads in amps using a clamp on dc meter. Watts measurements are a lot more difficult. The reason EV's are rated in kwH is for comparison purposes because each manufacturer uses a different operating voltage. That has nothing to do with the simple little 12V power systems in our rPods.

Working in watts actually makes things much more complicated. Batteries and solar modules are not boxes of watts. If you do the calculations in watts and watt hours you have to take into account several derate factors which are built in if you work in amp hours.  Lead acid battery round trip efficiency is about 85% and solar modules never produce their rated watts due to the way then are measured at the factory. They are also designed to produce more voltage at max power than a battery can normally take so unless you are using an MPPT charge controller there is another loss there. Overall, by treating batteries and solar modules as lossless boxes of watts you will overestimate system production by something in the range of 30-35%. 

I've been designing solar modules and systems for 40 years, I assure you that amp hours are much simpler. If you really want to use watts and watt hours you can certainly do so, but you have to build in about an 85% efficiency for the battery and about an 80% multiplier for the solar module(s). You will get to the same answer either way, as it should be. 





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Direct Link To This Post Posted: 27 Jan 2020 at 4:01pm
Sorry OG.. I tend to lean towards GlueGuy on this. Watts are watts no matter what. Amps leads towards voltage levels producing watts.. How many at what voltage.

Not to take away from the OP's question and would love to discuss further with both you and GG. I believe the entire group could learn the benefits of each side of this. One of you wish to start another thread?? I for one am interested in the conversation.
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Direct Link To This Post Posted: 28 Jan 2020 at 1:04pm
Most appliances and devices report watts consumed, not VA.  For most people, using watts and a lowish 12.5V for the system voltage is simple and easy.  That should comfortably account for resistance losses in the system.  This is a design exercise intended to get to a useful system without unnecessary expense and weight from over-kill.  YWMV. 

My PV panels are fixed, but some times of the year, I get a pleasing 85% of rated outout, at the battery.  Azimuth and elevation angles are important to performance, but you can control that easily enough when boondocking. 

A good site for establishing angles. 
https://www.solarpaneltilt.com/
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Direct Link To This Post Posted: 28 Jan 2020 at 4:14pm
Originally posted by Colt

Most appliances and devices report watts consumed, not VA.   For most people, using watts and a lowish 12.5V for the system voltage is simple and easy. 

Oh dang, just when I was gonna tell yall all about the swap here at my place (with three phase) that the electric co-op did, swapped 3 old transformers that were 10 KVA for 3 new transformers that are 5 KVA...

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Direct Link To This Post Posted: 29 Jan 2020 at 3:50pm
VA differs from watts only for AC circuits where the voltage and current wave forms are out of phase.  IOW, VA = watts for DC loads. 

Go ahead and use reported watts on your trailer DC loads if you can find them (and you believe them). Or, you can measure the loads quickly and easily in amps with a clamp on hall effect meter, or with a current shunt if you prefer.

12V batteries are rated in amp hours, so if you work in watt hours you have to convert back to that for the battery sizing anyhow. 

Lead acid batteries have a round trip efficiency of about 85%. But, their coulombic efficiency (amp hours out over amp hours in) is essentially 100%. Their efficiency loss is in voltage. You charge them at 14 plus volts and discharge them at a bit over 12V. Hence the 85%. But, if you work in amps and amphours you don't need to take battery efficiency into account, its taken care of.

Now on to the solar side. If you look at the Vmp (voltage at max power) in the spec of a PV module intended for 12V battery charging you will see that it is generally in the 17-18V range. Why so high to charge a lead acid battery at only 14-14.5V or so? 

Its because of how PV modules are tested and rated. PV modules are tested in the factory at something called STC (standard test conditions). Those conditions are 25 degrees C module temp, 1 kW/square meter irradiance aka "1 sun"), and a specific solar spectrum, which I won't get into here.

PV devices lose about 0.5% output (essentially all in voltage) per degree C.  Modules heat up by about 25-30C at 1 sun so the only time you can get rated power out of a solar module is on a sub freezing day, with a perfectly clear sky and the sun's rays perfectly perpendicular to the module surface. The rest of the time the voltage and power are lower, but the current at max power doesn't change significantly. 

So why are PV modules spec'd and tested that way? Seems like cheating? Because 25C is room temperature and the factories are kept at room temp. The modules don't have time to heat up in the factory flash testers. So, the STC spec was set up for the manufacturers' convenience. 


So if its a typical summer day and its say 86F/30C outside the solar module is going to be at about 55C or higher and you'll have about a (55-25)*0.005=15% voltage loss and your module max power (Vmp) will be around 14.5-15.5V depending on which one you buy. 

14.5-15.5V is still perhaps a bit high for 12V battery charging so there is one more loss that the PV module designer takes into account. That is the voltage drop in the conductors between the solar module(s) and the battery. Generally folks plan for an additional 0.5-0.6V there. So, that is about another 5 pecent loss (0.95 multiplier).

Multiplying all those losses together gives 0.85*0.85*0.95=0.69. So, by all means feel free to use PV module STC rated watts and apply a derate factor of 69%. 

But guess what, if you were to just take 12V/17.5V you get the same 69% number. And since the module spec sheet gives the max power current (Imp) at that 17.5 volts (Vmp) or thereabouts you can just take that Imp rating and then multiply by your average full sun hours (kWh/square meter/day, typically 4-6 depending on where and when) and you have the average daily amphours delivered by the solar system  (battery + module) to the load.   Very simple. 

But you can use watts too if you want, no argument, you just need to account for that 69-ish percent derate factor. Derate by another 5% or so if you roof mount your module and don't keep it clean.

BTW, if you want a great website that will tell you what to expect for average sunhours are your location here it is:


End of PV101 lecture.Geek
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Direct Link To This Post Posted: 29 Jan 2020 at 10:30pm
Originally posted by podwerkz

Originally posted by Colt

Most appliances and devices report watts consumed, not VA.   For most people, using watts and a lowish 12.5V for the system voltage is simple and easy. 

Oh dang, just when I was gonna tell yall all about the swap here at my place (with three phase) that the electric co-op did, swapped 3 old transformers that were 10 KVA for 3 new transformers that are 5 KVA...

Clap




ROTFLMAO!!! How's that workin' out for ya? 
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Direct Link To This Post Posted: 29 Jan 2020 at 10:40pm
At least the part of what offgrid wrote that I read is correct.  But, offgrid, you miss the point.  This isn't Physics class, it's handbook engineering, because people want a quick, easy answer and there's bills to pay. 

Setup the panels and let's go fishin' (or huntin', shootin', whatever). 
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