Solar panel basics

[scrubjaysnest]

Here are some solar panel basic thoughts and terms.
Voc solar panel open circuit voltage. The highest voltage the system will see and what you need to consider with charge controller, CC, input; breaker and fuse ratings.

Isc solar panel short circuit current; use this for wire size, fuse size or breaker size between solar panel and CC.

Vmp, typical operating voltage of the panel under load. Used to determine panel wattage rating.

Imp, typical operating current of the panel. Varies with battery state of charge, sun intensity and shading of the panel. Used to determine panel wattage rating.

Panel watts = Vmp * Imp or wattage the panel can produce in lab conditions. Varies greatly in real world conditions.

Vmp and Imp are maximum power point values and are what the MPPT, or Maximum Power Point Tracking controllers try to follow to get the most wattage to the battery.

About fuses or breakers, The NEC recommends de-rating by ~20% but nobody to my knowledge can find where this applies to RV's but it can't hurt to err on the safe side. Voc is used when choosing voltage ratings of fuses or breakers. Every solar panel I have seen to date has a maximum fuse or breaker size marked on the panel.

Voltage used when determining charge rates or panel size is typically 14.5 volts for flooded batteries. Typical range is 14.2 to 14.5 volts and is referred to as the absorb voltage. This needs to watched very closely on Gel batteries as they don't like really high voltages.

When a question was posted about running the reefer on dc and panel size I used the absorb voltage in the calculation I posted.

When looking at the discharge side or wattage used from the battery the typical voltage used is 12 volts.

Now that we are on the battery side of things always use a breaker or fuse between the CC and the battery(s). I prefer to use breakers over fuses for two reasons. One is to reduce failures, fuses will fail over time more often then breakers. Two, breakers give you a disconnect means.
One more thought on breakers they must have an AIC, this is an arc in air extinguishing value, of at least 5KV or 5000 volts and need to be marked as such. Most house hold breakers have a 10Kv rating but this isn't a DC rating.
You can use some Square D breakers but be very careful here about Voc exceeding the 48 Volt breaker rating. Yes, there are three ratings for breakers; current, voltage and AIC. For the DIY I can't stress caution here with breakers enough. Same thing applies to fuses and fuse holders.
When dealing with batteries and short circuits a very nasty and intense energy release occurs resulting in a very high fire danger.:eek: Add in the possibility of hydrogen gas being present and look out.

My follow up post to this will be ways to determine solar panel sizing.

@Bill feel free to edit any mistakes of mine.
@rickst29 If you spot anything wrong here send me a PM.

[scrubjaysnest]

The basic rule of thumb found all over the Internet for solar panel sizing is 80 to 100 watts per 100Ah of battery. Not sure how it got started but a little math will help and here it is for a 100Ah battery.

100Ah(battery size) * 14.5 V(charging based on absorb voltage) * (1/0.77) panel + CC derating factor * 0.05 Rate of charge = 94 Wh or a 100 watt panel.

The figure for panel + CC derating appears to come from losses between panel and CC and CC efficiency.
A 5% rate of charge, roc, is a typical minimum value for most battery manufactures.

to clean it up: 100Ah*14.5V*(1/0.77)*0.05= 94 Wh.

Do an energy audit of your usage to get a better guess at the amount of solar you will need. This will save money in the long run even though it is a pain.

See this post on ours: http://www.trailmanorowners.com/forum/showpost.php?p=119946&postcount=1

If you use name plate information from each device your guess will be a little high. If you measure everything it will be closer and may save money in the long run for the project cost. There are several spread sheets and other solar calculators on the Internet.

More math:
Watts = Volts*current, w=E*I and another watts = resistance *current squared, w=R*I*I. This one is losses from using to small a wire and why with DC we use large wire sizes.

A way around this is wire panels in series and a MPPT controller.
Down side cost and shade will have a major impact. To size the CC use Voc times the number of panels in series and the Isc of the series string. Panels in series add Voc but the current remains the same as the Isc of one panel.

Panels in parallel will have higher current and therefore higher losses in the wire. Use large wire sizes and keep wire runs short. Wire is expensive.
On the plus side parallel panels are less impacted by partial shading and you can use a cheaper PWM CC. To size the CC use total panel Isc times 1.25.
For my 2 80 watt panels that is: (5.1 + 5.1)*1.25 = 12.75 amps so use a 15 amp CC.

I suggest 10 AWG wire as a minimum size for panel to CC up to about 15 feet. Over that use 8 AWG. The first makes a nice install for a pair of 100 watt panels on the rear shell, the second makes a nice portable set up for a pair of 100 watt panels IMO.


PWM vs MPPT this is more a budget choice then anything but the rule of thumb break even point is around 400 watts of solar. If you want to run a microwave or hair dryer or other high wattage items and have a 225Ah battery bank, you could be looking at needing 400 watts or more of solar.
The break even point for a 225 Ah battery bank is around 400 watts.
The math for going 3 days to reach 50% soc.

225 AH * 12 volt * 0.85 AC inverter eff * 1/3 days storage * 0.50 maximum discharge = 383 WH per day
This is usage per day and equates to solar panel sizing so around 400 watts of solar should cover for those days you can't get 5 to 6 hours of max sun light.
Note the use of 12 volts since we are drawing off the battery.
per rickst29 and he is quite correct one should use 12.4 volts this would be 395 WattH. At 12.0 volts your battery is about totally discharged and a point you donot want to reach.
For myself I use 12.4 volts and 83% efficiency for the inverter.

A pair of 120 to 150 watt panels on the front shell along with a pair of 100 watt panels on the rear shell will get you a nice solar array for use with MPPT.

[rickst29]

Great post! I wil delete this reply when you modify two things at the very end:

.... Note the use of 12 volts since we are drawing off the battery.
When calculating demand at the battery, perhaps use battery voltage ~ 12.4V. (A battery running at only 12.0 is pretty severely discharged, and being damaged.) For SLA, 50% SOC is about 12.2V (standard conditions).
A pair of 120 to 150 watt panels on the front shell along with a pair of 100 watt panels on the rear shell will get you a nice solar array for use with MPPT. Keep Voc of all panels within 10% of each other.
Right! With Panels in Parallel (for PWM) the lowest voltage among the multiple panels "on the string" will determine the Operating Voltage of the "Parallel String", while the Current of all the panels is combined at that Voltage.

But with Panels in Series, (for MPPT), you want to keep the Imp Current values close: The Voltages simply add together, and widely differing voltage values do not cost power on the "Series String" as whole. But the lowest Current among the panels on the "Series String" determines the performance of the String as a whole, limiting the higher-current panels to the current value of the lowest panel. (That is why a shaded panel degrades a Series String so badly). ;)

My own configuration is now a very good example of "mismatched panels". I've got two 'Renogy' Panels at 5.29 Amds (Imp), 18.9 Volts (Vmp) for 100 watts each. My 3rd panel is rated at 135 Watts, but runs at much higher Voltage: 5.45 (Imp) and 24.9 Volts (Vmp). If I connected all the panels in parallel, the 3rd Panel would only run at 18.9 Volts, for maximum output of just 103 Watts. In Series, as I'm going to use it, it will run at the full 24.9 Volts - but the maximum amps (standard conditions) will be limited to just 5.29, matching the Renogy panels. In standard conditions, 5.29A * 24.9V = 131 Watts, a negligible drop from its "stand-alone" rating.

[scrubjaysnest]

.............snip................


With Panels in Parallel (for PWM) the lowest voltage among the multiple panels "on the string" will determine the Operating Voltage of the "Parallel String", while the Current of all the panels is combined at that Voltage.

But with Panels in Series, (for MPPT), you want to keep the Imp Current values close: The Voltages simply add together, and widely differing voltage values do not cost power on the "Series String" as whole. But the lowest Current among the panels on the "Series String" determines the performance of the String as a whole, limiting the higher-current panels to the current value of the lowest panel. (That is why a shaded panel degrades a Series String so badly). ;)

This is a very important point to keep in mind so I am going to steal it from rickst29

When I suggested the 120 to 150 watt panels for the front shell, these will have an Imp of 7 to 8 or 9 amps where as the 100 watt panels Imp will be around 5 to 5.5 amps. So what rickst29 is saying is you don't want to series a 100 watt panel with a 120 to 150 watt panel.
In this case you would want two series banks to feed the MPPT CC one made up of the 120 watt to 150 watt panels and the other bank the 100 watt panels.
Rule of thumb for series panels, Vmp to battery bank voltage ratio is 2 to 1 and don't go higher then 4 to 1.


@rickst29 keep up the "nitpicks":D

[Padgett]

Mine hase gotten complex enough that I have decided to replace the fuses and all with a small -trailer sized load center. Square D seems to have some dual rated systems.

The main driver for this was the 1200W (continuous) inverter I have and a coffee maker that needs a 770W source. May only be for a minute or two at a time but will pull 70-100A from the GC2s when dry camping, 20 minutes of solar should recover and I only drink one or two cups a day of max strength.

Got used to having the 1KW Freedom/Heart in my RV so could have coffee/toasted bagle without starting anything (had two GC2s and an grp 31 AGM starting battery in there).

Theory is to use the generator only for AC or "extra" and the Honda 2000 should be fine for that (with new circuit even started AC when in ECON mode but had to grunt a bit).

Also doubt that will do much dry camping but at least once a year need for a hot week. Once I start a slow ramble of the US who knows ?

More after I go to Lowes and HD and ramble a bit but expect cost to be between $50 and $100 with breakers. Expensive part will be the 70A or 100A breaker but with AWG 2 wire running to the inverter am not going to just hard wire.

ps the Sharp pulls 980W for Microwave and 1300W for grill bake so about the same as the Fridge. 10A of 120v = 100a of 12 for 10 minutes is about 16AH and will need about 1.2-2 hrs of solar to replace.

Understanding the need for an energy budget better now ?

[scrubjaysnest]

Mine hase gotten complex enough that I have decided to replace the fuses and all with a small -trailer sized load center. Square D seems to have some dual rated systems.

The main driver for this was the 1200W (continuous) inverter I have and a coffee maker that needs a 770W source. May only be for a minute or two at a time but will pull 70-100A from the GC2s when dry camping, 20 minutes of solar should recover and I only drink one or two cups a day of max strength.

Got used to having the 1KW Freedom/Heart in my RV so could have coffee/toasted bagle without starting anything (had two GC2s and an grp 31 AGM starting battery in there).

Theory is to use the generator only for AC or "extra" and the Honda 2000 should be fine for that (with new circuit even started AC when in ECON mode but had to grunt a bit).

Also doubt that will do much dry camping but at least once a year need for a hot week. Once I start a slow ramble of the US who knows ?

More after I go to Lowes and HD and ramble a bit but expect cost to be between $50 and $100 with breakers. Expensive part will be the 70A or 100A breaker but with AWG 2 wire running to the inverter am not going to just hard wire.

ps the Sharp pulls 980W for Microwave and 1300W for grill bake so about the same as the Fridge. 10A of 120v = 100a of 12 for 10 minutes is about 16AH and will need about 1.2-2 hrs of solar to replace.

Understanding the need for an energy budget better now ?

Square D QO's are the only ones they make that are DC rated, they also have a bolt on that is DC rated. MidNight has a nice line and they are rated to 150 Vdc. Check the solar-electric.com site for best prices. Breakers are din rail mount.

[Padgett]

Sent a note to GE about their Q-line. Specs just say the amperage. Just want for safety, don't plan a lot of cycles.

[scrubjaysnest]

Sent a note to GE about their Q-line. Specs just say the amperage. Just want for safety, don't plan a lot of cycles.
Don't believe the GE Q-line is DC rated, major fire hazard if they are not and fail to extinguish the arc if tripped.

[Padgett]

"Extinguish the arc " ? Say What ?

Just ran some tests with a variable load and found that a 30A Q-line breaker was tripping out at 50-60A on 12v DC (actually about 8vdc at that load, was using a jump box).

Very positive trip when it did, just later than expected.

Makes sense that it would, 20A breaker in TM converter panel does not trip in a 44A surge when the AC starts.

Interesting.

[scrubjaysnest]

"Extinguish the arc " ? Say What ?

Just ran some tests with a variable load and found that a 30A Q-line breaker was tripping out at 50-60A on 12v DC (actually about 8vdc at that load, was using a jump box).

Very positive trip when it did, just later than expected.

Makes sense that it would, 20A breaker in TM converter panel does not trip in a 44A surge when the AC starts.

Interesting.
When the breaker trips it opens up but an arc can form. With AC they tend to be self extinguishing. DC doesn't do that and a battery is a very strong short term high energy source. AIC, amperes interrupting current or sometimes called AIR is not the thermal rating of the breaker. It is the rating in Ka for which the breaker can extinguish an arc when it trips due to thermal overload without damage to the breaker.
We normally look at the breaker and say it is a 15 or 20 amp breaker, that is the thermal over load rating.

[scrubjaysnest]

Think this link maybe of some interest to folks on battery SOC:
Solar Homestead (http://solarhomestead.com/battery-state-of-charge/)

It may clear up some confusion or make it worse.:confused:

[Padgett]

Yes, you are correct.