How many watts of solar panels?

[ThankYouJerry]

Can anyone please help me determine how many watts of solar panels will be sufficient for my needs? I'm planning on using a flexible solar panel affixed to my Thule Box and another flat mounted flat on my roof if necessary. I've found 50W, 100W, and 120W flexible solar panels to choose from (HQST, Renogy, and King Solar).

Typically, I will be camping from May-Sept on the West Coast. Mostly sunny but sometimes coastal clouds and/or partial shade. Of course, I understand that aiming/direction/cooling of the panels affects their performance.

Right now, with just my 2 aux bats (100ah each), I can sit for 48-72 hours max before they need charging depending on ambient outside temps, power usage, etc. I'd like to be fairly autonomous and not have to rely on driving or shore power to recharge my aux bats every 48-72 hours. However, I also don't want to spend more on solar panels than I have to. I've been hoping that 100-150W of solar panels will be enough.

Here are my parameters:

Reserves:
Aux Battery Bank (independent from a separate starter battery):
2 DieHard Platinum AGM (100ah each)

Draws
Propex 2211
Nominal voltage: 12V d.c.
Operating range: 10.0V d.c. min. to 15V d.c. max.
Maximum current d.c. gas operation: 1.7 Amps (0.01Amps on standby)

ARB50 Fridge
1.40 amps per hour

Plus:
1-2 LED interior lights 1-2 hours per day
In dash stereo (used rarely 1-2 hours per day)
2 iPhones charged 1x day each
1 iPad charged 1x day
2 Goal Zero Mini Lanterns charged 1x/day each - Cell Capacity: 11.1 Wh (3.7V, 3000mAh)


Which controller PWM or MPPT?

I've read that PWM controllers are best for systems under 200W and that sizing the PWM controller to the system is less important than with MPPT controllers. However, I've also read that MPPT controllers deliver more amps especially on cooler and/or overcast days.

Final (Dumb) Question...
Because I'm such a noob... Does the solar panel stay connected through the controller to the batteries even when I'm driving (charging with the alternator) or do I have to throw in a switch that keeps solar panel charging independent from alternator charging?

 

[IdaSHO]

Your location could help.

It helps determine average hours of useable sun per day.


But my rough calcs generally use 6 hours of good sun per day.

Figuring your daily amp consumption, you look to be in the 50-75 amps range, depending upon just how discharged your devices are before recharging.

75 amps / 6 hours of sun = 12.5 amps you need to generate per hour of sun every day to replenish your usage.


12.5 amps @ 12v = 150 watts

So your in the ballpark.

 

[jonyjoe101]

my recommendation is the biggest panel you can fit on your roof. But at minimum 120 watts. Thats what I use to have when I had a 12 volt edgestar fp430 fridge. As long as the sun was good it kept up with the fridge, in your case with your 2x100ah batteries it will let you go longer during the cloudy days (i only had a 75ah battery).

Below is what kind of output you get with different panels/controllers this is real world from my use on my van, panels flat on the roof.
120 (21 volt) watt panel will give you 6 amps of charge power with either pwm or mppt.
240 (36 volt) watt panel will give you 6 amps with pwm and about 12 amps with mppt.

mppt only gives more power with the high voltage panel (above 30 volts) , thats why they recommend it on the larger panels. Myself I use an ecoworthy 20 amp mppt (cost 102 dollars) , its been running 24/7 on my 240 watt panel for almost 3 years. I also used it on my older 120 watt panel (it didnt extract any extra power from it) . No need to spend too much money on a mppt controller for a small system. For pwm you can get a 20 dollar 20 amp wincong controller, thats what I used before I got the ecoworthy. It did the job, it even worked on my larger 240 watt panel.

The most power (amps) you can get from a panel, you divide size of panel and divide by 14.4 volts. example 120 / 14.4 = 8.3 amps. 14.4 is the minimum volts you need to charge a battery. In real life from a 120 watt panel you you might get 6 amps and only when the sun is overhead.

You can keep the solar charge controller hooked up to your aux battery 24/7, even when it is being charged by the alternator. The charge controller measures the voltage of the battery and if its full, it doesnt do anything. Voltage/amp wont go back up to controller and damage it.

 

[4x4junkie]

120W is about what entered into my mind also as a minimum as I read the OP. 150-170W would give a small "buffer" for if you encounter less-than-ideal conditions for awhile (high temps that cause your fridge to run more / cloudy days that lower energy harvest / etc.).

Below is what kind of output you get with different panels/controllers this is real world from my use on my van, panels flat on the roof.
120 (21 volt) watt panel will give you 6 amps of charge power with either pwm or mppt.
240 (36 volt) watt panel will give you 6 amps with pwm and about 12 amps with mppt.

mppt only gives more power with the high voltage panel (above 30 volts) ,

That may be true with a panel that has no ventilation under it that is very hot from the sun beating right down on it... However if the panel is cooler (air is allowed to flow along it's underside, on partly-cloudy days, etc.) you will usually see a gain of 10-25% with MPPT, even with 17-18 Vmp panels. This is because a panel's Vmp rises considerably when it is cooler (while current output remains essentially the same), which a PWM controller is not able to take advantage of this.
I have witnessed this on my own setup also, so I know it to be true.

 

[IdaSHO]

+1 on the remark about 17V vs 30V


And yet another reason to stick with rigid panels, mounted with an air gap beneath as apposed to adhesive flexible panels.

You want to squeeze every available ampere out of those panels as possible :ylsmoke:

 

[Bbasso]

My current setup is close to yours and at 200 watts and in Florida, I still think I need more...

 

[ThankYouJerry]

Thank you for all the replies so far. Most appreciated.

 

[Mobile Solar]

In principal the solar array has to match the load and system losses during a 24h period.

We manufacture and sell camping solar kits, www.flexopower.com. There is a correlation between load demand, duration and available sun to determine the size the solar array. Based on hundreds of client's practical experience with our kits, the following seems to work here in Southern Africa (this also assumes you have a dual battery system and the batteries are fully charged when arriving at the camp site):

1) load is 1 fridge, LED lights, small consumers for 1-2 nights in camp: typically a 79W or similar suffices. This solar size will not fully replenish what your load consumes, but will extend the batteries charge and typically by the time you pack and leave camp, your battery is still >50% charged. This does not leave margin for an overcast afternoon, but sufficient for short stays.

2) load is 1-2 fridges, small consumers for >2 nights: typically a 158W or similar is sufficient. Some clients reported autonomy, i.e. their solar system produced more than what they consumed.

3) extra heavy load such as several fridges, ice maker, AC equipment and inverter for >2 nights: 237W or higher.

The US West coast has less available sun than Southern Africa and I would double up the above to make up for the lower insolation.

You also mentioned MPPT or PWM. Personally I am using a PWM in my 237W system, as the MPPT gains from such a small solar array are little and not worth the extra cost for me. A way to increase efficiency of your solar system is to eliminate battery losses. Instead of charging the battery first and from there run the load, you have an option to connect the load to the DC output of the solar regulator. This feeds DC from the panel directly to the load. Any surplus charges the battery. If there is not enough current from the PV, the balance will be supplied from the battery. At sunset and sunrise the regulator automatically switches between PV and battery. By simply wiring the DC load to the solar regulator eliminates 15% wastage of solar power in the battery.

 

[goodtimes]

I had a single 100w CIGS flexible panel glued to the roof of my camper.

I ran a ARB 50 fridge continuously, a couple LED lights intermittently at night, and charged a couple I-phones & camera batteries at night.

With a single 100ah house battery, I never ran out of power. Once I hooked up the solar panel, I never needed to hook the house battery to the truck to charge it. By mid-day, the solar had brought it to [near] 100%.

We favor the deserts (S/W US & N. Mexico) over the mountains, so the panel virtually always had a clear view of the sky - but even with less than perfect conditions (IE: Overland Expo on Flagstaff last spring), I had plenty of power.

 

[craig333]

100w was enough for me in ideal conditions. However I just don't experience enough of those so I upgraded to 250w. I'll get back to you after some miles on it but I'll be surprised if it isn't sufficient.

 

[Mwilliamshs]

...
Here are my parameters:

Reserves:
Aux Battery Bank (independent from a separate starter battery):
2 DieHard Platinum AGM (100ah each)

Draws
Propex 2211
Nominal voltage: 12V d.c.
Operating range: 10.0V d.c. min. to 15V d.c. max.
Maximum current d.c. gas operation: 1.7 Amps (0.01Amps on standby)

ARB50 Fridge
1.40 amps per hour

Plus:
1-2 LED interior lights 1-2 hours per day
In dash stereo (used rarely 1-2 hours per day)
2 iPhones charged 1x day each
1 iPad charged 1x day
2 Goal Zero Mini Lanterns charged 1x/day each - Cell Capacity: 11.1 Wh (3.7V, 3000mAh)
...

You give hours of use per day for the LED lights (but not power consumption) and stereo (no power consumption for it either) and the same info can be figured for the recharging of iphones, ipad, and lanterns but the hours of use per day for the Propex and ARB50 will be needed to give you a real idea of your needs. This can be calculated from their duty cycle, that is the number of minutes they run (blower, fuel pump, compressor, etc) in each hour. Absent that information, the power needs of recharging your 100ah battery bank from 50 to 100% can be calculated thusly: 100ah = 1 amp for 100 hours, 100 amps for one hour, etc but since we expect 4-6 hours of sun on a good day we'll say it's between 100/4 = 25 ah and 100/6 = 16.6 ah per day. I assume your DieHard Platinum AGM batteries with 100ah are these Group 31s. They are manufactured by Odyssey and in their literature (page 15) Odyssey recommends a charge voltage of 14.7. So you need sufficient solar panels to generate 25 amps at 14.7 volts to fully charge a half-drained battery bank of 200ah. P = VI tells us you need 367.50 watts of solar to do that, in a perfect world with 100% efficiency. Batteries aren't perfect, nor are solar panels, nor chargers or the cabling that connect them all. Figure you'll lose 30% (20% is a standard solar efficiency loss number, leaving 5% each for the charger and batteries to lose on their own) which is somewhat conservative and you can expect a 200ah battery bank, discharged 50% to be fully recharged repeatedly by 525 watts of solar panels in 4 hours of full sun. I'm not saying that's what you need, just that it's what is required mathematically to replenish (in 4 hours of full sun) your 2 100ah batteries if discharged 50%. In other words, that's the most solar wattage you could make use of unless you're running loads while the sun is shining as that would use some of the available power and result in batteries charged to a lesser degree.

To calculate what you actually need on a daily basis, you have to determine what you'll actually use.

If your fridge has a 50% duty cycle, meaning its compressor runs 30 minutes of every hour, 24 hours a day, that's 12 hours x 1.4 amps = 16.8ah per day. This will likely vary with ambient temperature, the temperature setting you select, how often the door is opened, etc.
If your Propex has a 50% duty cyce, that 12 x 1.7 = 20.4ah per day. This will also likely vary with ambient temperature and weather conditions, thermostat setting, how often your doors are opened, etc.
LED light fixture power consumption rates vary GREATLY. Get yours.
I found listings of a 1,570 mAh battery in the iPhone 5S, 1,440 mAh battery in the iPhone 5, and 1,507 mAh battery in the iPhone 5C. An iPad battery appears to be 1,900mAh. The voltage matters some but since you're converting 12v into whatever it takes, I'd just stick with that and add these mAh to your overall daily ah needs, same for the 6000mAh of the two lanterns, just add 6ah to your total. (Yes, 6000mah = 6ah). These numbers would be accurate for fully recharging a dead battery but do not include efficiency considerations so either add 20% or whatever makes you comfortable.

Experimentation or research would be needed to get these figures closer to real-world.

 

[JCTex]

Those who posted above who have actual systems in use are basing their answers on what worked for their needs. However, there are some electrical physics involved here. The first one is physical space. It sounds like you are limited to the "small" voltage panels. The possibles you mentioned are that. Attaching a flex panel flat is a bad idea for heat, etc.; but if that's what you need to do, do ahead on. Just be aware you must monitor their production regularly. When they start to fail--which will b MUCH sooner than a ventilated panel, replace the before a cell melts and maybe fires.

So, confining yourself to the 100W-ish world, look at their specs. The Renogy Eclipse is new, very efficient, and my rec. But it's a solid panel. King is your route if you are determined to go flex. Look for the Imp. This is amps you should expect per hour in perfect conditions. For this size, expect to see 5-6A Imp. Figure 4 hrs/day in great conditions for max solar input. Add an hour on each side at 50% input. That's 5hrs/day. 5.7Ah Imp x 5hrs = 28.5Ah/day per panel. Multiplied x 12V (versus 14.4V to allow for losses in system) x 2 panels gives you ~684W, and that's not on cloudy-rainy days. This is the most you can count on with a PMW controller on a good day to replenish your battery bank.

Your bank is 200Ah AGM deep cycle. You can get away with a 60% DoD. You'll be cutting your recharges from ~1,000 top 500 lifetime; but at the cost of two batteries, who cares? 120Ah x 12VAC = 1,420W. So, with only 2 panels at 60%, you won't refill your bank. Same is true for 50% DoD at 1,200W. However, that's what you've got. It is what it is. Adding more battery storage won't help unless you add more panels. Sometimes physics can be a hard taskmaster.

Do an energy audit of your uses expressed in Watts. You will find which things you can use depending on what your monitor tells you is left in the bank.

Of course, a good isolator-combiner, like the HellRoaring will help your alternator fill the bank when moving. So will buying a 2,000W Yamaha or Honda PSW generator. Also, plugging into your starting battery via a DC receptacle will steal some amps (watts) for 1-2 day stays. I know some people find room to pack fold out panels to prop up near the vehicle and augment what their permanently mounted ones are doing. In your case, you'd need another 2-100W's.

I concur PMW is all that's necessary for a controller. IMO, you must have a good monitor to tell you what's going on, especially since you are living on the edge. I recommend the TriMetric TM 2030; their SC 2030 controller is great, too. You want a breaker that acts like a switch, too, on both sides of your controller. I didn't see if you have a main battery cutoff switch on your pos battery cable. You also need a main fuse or breaker before the pos post. The amperage of that disaster fuse depends on the most amps will ever flow thru that cable.

The pos and neg lines from the controller, the isolator-combiner, the inverter, the charger, the AC main breaker and your DC main breaker should IMO all connect to a double row main buss bar with separate lugs for ease of working on. The neg cable (larger) goes from its lug on that buss to the shunt for your TM 2030 and on to the neg post. The pos cable (larger) goes from its buss lug to the main cutoff switch, on to the catastrophe fuse or breaker, and on to the pos post. I am not an electrician. Follow this at your own risk. But this is how I wired mine.

 

[Ozarker]

Comment to batteries, get away from using automotive group batteries for solar applications, much better performance, capacticy and life out of 2 6V golf cart batteries or even better, RE type batteries built specifically for solar applications, RE stands for "renewable energy". Look up Trojan T105 RE, prices is comparable to the wrong kind of batteries mos end up starting off in RE projects. And don't be concerned with "cranking amps" it's Ahr that matters. As mentioned, it's best to have a gen set and solar for extended periods.