Here's my cut-and-paste on solar in RVs... It's long but probably worth reading. A friend called it a "brain dump".
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Question: Anybody out there added solar panels to their motorhome? Is it worth the investment?
Answer: Only YOU can answer that. Read on...
The electrical needs of an RV depend totally on the lifestyle of the folks that live in it.
You won't know if it's worth YOU investing in solar until you determine YOUR power needs / usage and your RV environment. It all depends on what YOU and your lifestyle require.
And in order to make that decision you need to educate yourself as to how solar works... read on...
Remember: you are not going to power your RV from solar! A lot of people forget this.
You power the RV from the house battery (and you may need to expand it) and the solar is just one more source that recharges it.
The biggest problem is that solar charging is variable - it's not like shore power or a generator.
The amount of power depends on the strength of the sunlight (called solar flux) and that varies from zero at sunrise to max at local noon to zero at sunset, and depending on the clouds can cause variances from hour to hour.
The tilt of the earth (winter to summer) changes the strength of the sunlight... A winter boondocker will need more solar than a summer one.
Your latitude (distance from the equator) greatly affects the solar flux - someone in Arizona, Texas or Florida will have a longer solar day than someone in Washington, Wyoming or Michigan (and therefore for the same size system will get get more charge in their batteries per day).
The Texan will need air conditioning - probably the biggest power drain - for 7 months. The Wyoming resident may need it for 2, or maybe none.
The RVer that has a propane refrigerator will need a lot less than someone that has a residential fridge.
The RVer that uses the microwave several times a day will use a lot more that the RVer that boils the water on a propane cooktop.
And someone that lives in their RV year around will need more than someone that is summer-only and stores their RV for the winter.
Electrical power - be it solar or otherwise - is measured in watts. Watts are derived from volts and amps.
Think of voltage as water pressure and amps as the quantity of water flow (gallons per minute). 12 volts and 1 amp is 12 watts.
A 12 volt source delivering 10 amps is half the watts of a 24 volt source delivering the same current. Likewise a 12 volt source delivering 10 amps is half the power of one delivering 20 amps.
Power over time is measured in watt-hours. 12 volts for one amp for 1 hour is 12 watt-hours.
Batteries are rated in amp-hours. 12 volts and 100 AH is 1200 watt-hours.
I was introduced to solar panels when I worked at NASA-JPL in the 1970s. I actually acquired my first solar panels - four "12 volt" panels - as surplus in the early 1980s.
I wired my four panels in parallel and mounted them in a homebrew panel rack.
When I set them up in my back yard I was seeing anything from 5 volts and 1 to 2 amps in the morning (winter vs summer) to 17 volts at solar noon, and peaking at almost 2 amps per panel (in the summer).
"Solar noon" you say? Yes. The sun and the solar panel(s) do not care about time zones or daylight savings time so the term can be defined to be the time when the sun is directly over and perfectly facing and right angle to the face of the solar panel(s).
And it's amazing how little dirt it will take to cut performance especially if flat mounted. So hose the panels off every so often and use a clean sponge mop to get them squeaky clean.
Another variable that you have to consider is the panel mounting method... Most RVs with solar panels have them mounted flat to the roof.
You will see some that have tiltable mounts, especially people that park in one spot for weeks or months at a time... they park their RV broadside to the sun in a location where they can tilt the panels to better present the face of the panel to the sun.
Tiltable panels can increase the power level by a large amount... in my case from around 3 to 4 amps when flat on the ground to almost 8 amps when facing to the sun.
Note that the panel watts numbers you see on solar panel spec sheets or advertisements is for an ideal situation: a cherry-picked panel that is flat facing the sun on a clear day in the summer at solar noon. Those numbers are peak numbers that are only useful to marketing staff to write brochures from. And they always round the numbers up. I always discount spec sheets by 40% to 50%.
Now to answer the big question "How many panels do I need?":
To properly size a system to fit your requirements you need to FIRST quantify your needs before you install the solar...
Find out how much power you actually use... and you only get those numbers by installing a shunt-based battery monitor (like the Victron BMV-712 and the matching shunt) on your existing house battery bank... and logging that info for a worst-case month (or 2 or 3) in the winter and another round in the summer.
A battery monitor will allow you to see exactly how many amp-hours your existing shore power converter is feeding into the batteries, how many amp-hours is going out (i.e. how much power you are using), how fast you are using it, and how much power you have left at any given point.
No, that Victron is not wasted money, you are going to need that battery monitor in your permanent solar system.
Pulling random numbers out of the air, lets say that you used 100AH in 24 hours.
Another random number: you have solar day of 8 hours. That means that in a perfect world of no losses you would need to pump that 100AH (12.5 amps for 8 hours) back into your battery to break even.
But it's not a perfect world - you need to pump in some extra in to actually replace all of what you used. And you will never have a 8 hour solar day in the winter.
One rule of thumb is that under optimum conditions on a middle-of-summer day a 100-watt panel can produce about 25-30 amp-hours when mounted flat on the roof. Give that, and if you know what your needs are, you will have a starting guess as to how much solar generation you will need.
So for a 400 watt array of solar panels (four 100w panels) for 5 hours you can reasonably expect about 2 kilowatt hours per day....
To actually calculate the size of your solar photovoltaic system you need to:
1) Determine your daily KWh energy requirement. You an get that from a shunt-based battery monitor, it's measured in WattHours or KilloWattHours (1000 WH).
Call that number EU for Energy Used. Keeping a weekly (or even daily) log for 3-4 months will result in more accurate numbers. Note that this is number is the energy used in a 24 hour period.
Look here: http: // www.jenericramblings.com/2017/03/30/battery-meters-an-rv-must-have/
2) Determine how many solar hours you get per day. Call it SH for Solar Hours.
And as said above a winter day will have much less solar production than a summer day.
A good rule-of-thumb SH number for the USA is 3.5 to 5 solar hours per day. That number takes into account the lower quantity of sunlight in the morning and evening. If you use 3.5 hours in the winter and 5 hours in the summer your calculations will give you a better idea of how much power you can reasonably expect in "real world" solar results.
Look here: https: // images.unboundsolar.com/media/solar-insolation-map.png
3) Divide your EU by the SH to get the panel output you need. So you are dividing KilloWattHours by Hours and getting KilloWatts. Call that RE for Required Energy.
Now divide the RE by the watts rating of the panels to get the minimum number of solar panels for your system. If you get a fractional number then round up... i.e. use 14 instead of 13.4
However... that RE number is a constant for every hour of the solar day, and the solar power is not constant! Because you don't have full sunlight for the morning and evening you will have to increase that panel count. And because the panels will be flat to the roof you will have to increase the panel count some more. And unless you park in full sun all the time you will have to increase it even more. People like to park their RV under the trees... and solar panels are allergic to shade.
That panel count you actually need will probably be between three and six times larger than you calculated in the step above.
And think about this: What will you do if you have a solid week of overcast days? Or even two weeks? Real life isn't always warm sunshine and 75 degree F / 25 degree C weather. I suggest you keep your generator connected and maintained... a pair of Honda 2000 generators (or similar) can be tied together when you need to run the air conditioners, and when not you can run just one and the second one gives you a backup genny.
Here is a very basic calculation page intended for a sticks-and-bricks home but the concepts and techniques are the same:
https: // www.solarpowerauthority.com/how-to-size-a-solar-pv-system-for-your-home/
The following link is for "Home Power" - a 6-times-a-year magazine that was devoted to home-made power that was published from October 1987 to November 2018 but the index and all of their back issues are available for free download.
There is a tremendous amount of information on solar systems here... however some of the info is dated.
Go to https: // www.homepower.com
Before you start laying out $$$ you should learn about solar panel and charge controller technology (PWM or MPPT), low voltage disconnects, state of charge of your batteries and how to accurately measure it, how wire size (measured in AWG) can make or break a system, battery types (lead acid versus lithium) and a lot more. Many lead-acid battery manufacturers have literature that says that for best life do not discharge below 50% state-of-charge (yes, you need at least twice as much lead-acid battery as you expected).
One manufacturer statement is at https: // http://www.trojanbattery.com/tech-su...ntenance/under
the section titled "Discharging". Yes, you can go to 80% but it will reduce the total number of charge / discharge cycles.
A Low Voltage Disconnect (LVD) saves your batteries - a common lead-acid battery doesn't like being over-discharged, where as a Lithium based battery can be discharged to a much lower percentage, 80% according to one manufacturer. But both still need a Low Voltage Disconnect to prevent battery failure from self-destructive overdischarge. Most commercial lithium battery modules have an internal LVD.
Another charging issue is that the average lead-acid battery bank can only take full charging current for a certain amount of time - usually this is described as an 80/20 relationship. 80% of the charge can be stuffed into the battery in 20% of the total time... But that last 20% of the charge takes 80% of the time! The charge rate of the battery bank is one of the limiting factors in solar - you have to have someplace to put the sunlight-generated power right away, or it is gone...
I suggest that you download and read the writeup called "The 12 volt side of life".
There are 2 versions out there - one version is a single long PDF and the other version is a pair of PDFs titled "Part 1" and "Part 2".
Part 1: The 12volt Side of Life (Part 1) http: // marxrv.com/12volt/12volt.htm
Part 2: The 12volt Side of Life Part 2 http: // marxrv.com/12volt/12volta.htm
Then there's the HandyBob writeups... also well worth reading... they (all of them) should be required reading for anybody that is going to go solar (or has solar).
Go here: https: // handybobsolar.wordpress.com HandyBob has been there and done that for over a decade. And he gives that knowledge away.
Learn two basic formulas: Power (watts) = volts multiplied by amps. Rearrange the formula and you have Amps = watts divided by volts.
A 1200 watt microwave on 120vAC translates to 10 amps drawn from the AC outlet.
That same microwave plugged into a 120v inverter will pull 100 amps at 12v... and you have to figure on some conversion losses, it could be anywhere from 105 to 115 amps depending on inverter efficiency...
That's large diameter wire (read: expensive) from the battery to the inverter (I use oil-proof welding cable). And inverters pull power even at idle/standby/no load... anything from 1/2 amp to 3 amps, depending on brand/model. Different manufacturers refer to that as "overhead" or "idle draw" or "standby power". I've seen inverter disconnect switches installed to eliminate that wasted power, especially overnight.
You DON'T need to run the inverter to charge your cellphone or tablet !
Do everything you can to charge your stuff from the 12v house battery. Install 12vDC-to-USB outlets to power / charge your USB devices. By eliminating their converter bricks, you eliminate double conversion (12vDC to 120vAC and back to DC to charge the device)... I've heard the term “roundtripping” used to describe that.
And some people that have 120vAC refrigerators install a dedicated small inverter to run it. Choosing one with low idle draw will help here. A dedicated unit will allow shutting off the big inverter overnight (and therefore it's idle draw).
As I said above, back in 1978-1980 I had a small system in my back yard... four 2 foot by 4 foot panels totalling 100-150w on a homebrew charge controller and two deep cycle golf cart batteries and a length of welding cable (expensive) from them ot the homebrew inverter... yeah, panels were pretty inefficient 35+ years ago. I later relocated the panels to my garage workshop roof... I learned a lot... but I learned a lot more by reading those documents above.
Another couple of links you may want to check:
https: // www.altenergymag.com/article/2005/08/solar-energy-potential-at-different-latitudes/120/
Northern Arizona Wind and Sun: www.solar-electric.com
Will Prowse's site at www.mobile-solarpower.com
There is a ton of good information there and the folks on the board will help. There is a newbies section. I would suggest you read it first.
DO NOT think that a basic solar system will allow you to live like you are in a sticks and bricks house. A simple device like a 1500 watt hair dryer can pull a LOT more power than you initially expect... Plug it to an inverter connected to a 12 volt battery and flip it on and it will suck over 125 amps! (1500 watts at 12 volts is 125 amps, then you have conversion losses in the inverter... 130 to 140 amps is more like it. A typical 200 ah house battery is going to deliver 100 ah of usable power (remember the rule of don't discharge it to less than 50%?) and 2 minutes is 1/30 of an hour... 130 amps for that time is 4.3ah. Your 100% battery charge just dropped to about 95% in 2 minutes.
And that's not discussing lights, TV, electric blanket, or any other devices that suck electrical energy.
Here's a good comment thread on a new solar system: https: // www.irv2.com/forums/f44/new-solar-system-517041.html
And we haven't touched on the installation... will you be doing it yourself? Or paying someone's labor costs?
We also haven't touched on sourcing the materials... solar panel mounting racks, cabling, connectors, etc... or their costs.
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