how to size your solar power system if money doesn't matter

[willprowse]

Hey guys! 

I noticed that a lot of people here have had trouble figuring out how much solar they need. Traditionally we calculate the load, then calculate how much power the solar panels will produce, what the charging efficiency of solar charge controller and battery is, wire loss etc etc..... super boring, yawnn

But there is a much easier way! 

When you are building a solar power system on a moving vehicle, you will have a limited roof space. So build your system around this huge constraint. 

Step 1: Fill the roof with as many solar panels as you can (example: 4x 100 watt solar panels can safely fit on your roof)

Step 2: Purchase a solar charge controller that can handle the power produced by the solar panels. It is best to buy a solar charge controller that is larger than what you need. This way you can scale your system later if you need to. (example: for 400 watts of solar, I would stick with a 40 amp mppt charge controller)

Step 3: Choose a battery large enough to work with your solar panels. Typically, for vehicle mounted solar power systems, for every 100 watts of solar power you have on your roof, you should have 75-100 amp hours of sealed lead acid batteries. (example: 400 watts of solar on the roof means a 300-400 amp hour sealed lead acid battery). Be sure to check with the battery manufacturer to see how many solar panels they recommend, or find the datasheet and determine the "safe charge rate" in amps.. (or check out my website)

Step 4: Buy the largest inverter you can afford. Most people do well with a 1500-2000 watt inverter. Pure sine wave inverters are ideal and worth the money. Spend extra money to buy some thick inverter cables.

Step 5: Build your system and use it! If it powers your loads, you are good to go. If you need more power, scale your system (not easy to do if you filled your roof to the rim with solar panels, so you may need more batteries instead)

I have never had someone tell me: "oh geez Will, I have too much solar on my rig". Not once. Fill that roof with solar panels. If you have a tiny vehicle, or aerodynamics is an issue, use flexible panels and a small, fast charge rate battery. If you are in this for 5+ years, buy a lithium iron phosphate battery. 

I also have other methods of determining system size (depending on application), but the steps above should help some of you. I also have some "solar system blueprints" on my website, of pre-calculated system sizes. No thinking involved! Just pick the one that works best for your life and/or vehicle.

This post obviously does not deal with wire gauge size, or any of the other important aspects of solar system design, but this should really help some of you guys get a good idea of what you need!

Let me know what you guys think I hope this helps some of you!

 

[Motrukdriver]

I've always said build a spreadsheet of your uses and how often you use this or that and that will determine your amp hours needed over a 24 hour period. It sounds simple but you need to do the math of actual use of each, how many hours each thing uses for your daily use, multiply that by 2 so you never discharge your batteries over 50% and then multiply that by the number of days you can go without sunshine. This will closely tell you how much solar you need. The spreadsheet will also tell you if you need more solar should you have to replace a device or add another use, or if you can get by with what you already have. Remember, this site is more geared towards cheap rv living and I'd hate to buy more solar than I actually need when I could have used that money for something else like tires or food. I should probably build a downloadable spreadsheet where folks can plug in what they have, etc..

So, for my needs, I'd put as many solar panels on my TT roof as I need being careful not to put them where there is a possibility of shade that I can prevent. Roof fan covers are a good example and a good tape measure is your friend. If for some reason I need more panels than my TT roof can hold I can always put a couple more on the Jeep Cherokee roof and maybe those can be tilting for added solar charge. I 100% agree on the charge controller being sized up a tad and I'd also add buy a well known name brand (Morningstar comes to mind) and not some flashy no-name with bells and whistles you don't need or can't get serviced if something goes wrong. I'd also include a Trimetric battery monitor so I can monitor my battery health and see what is actually happening. I will be purchasing 6 volt flooded cell golf cart batteries so I can keep an eye on acid level and use a hydrometer to add another health check on my batteries. I'd have no use for an inverter since nothing I'll be toting will be 110vac. Others may and again buy a good quality one if you need one. I used them in my big truck when I drove but I needed them then. I'll do everything I can not to use them again. If it all goes smooth I'll have exactly what I need and not spend more than I have to when I get back to the US... Come on Jan 22nd!!!!

 

[frater secessus]

Step 4: Buy the largest inverter you can afford. Most people do well with a 1500-2000 watt inverter. Pure sine wave inverters are ideal and worth the money. Spend extra money to buy some thick inverter cables.

This gets so many newbies into deep shizz.  They assume that having a giant inverter means they have the power to run it.

My advice is the opposite:  don't buy or use an inverter unless you have to.

 

[UptownSport]

Step 1: Fill the roof with as many solar panels as you can (example: 4x 100 watt solar panels can safely fit on your roof)

Not helpful.

Buying what may not be needed is just wasteful and glutinous- whether you can afford it or not.
Website purports to be 'Cheap' living.
This is another thinly veiled way attempt to sell lots of stuff people don't need, no different than anywhere else.

 

[Weight]

I have 4 Renogy panels on my roof. I could fit 1 more but left space for the roof vent fan. The panels cost $400. What the panels do for me makes them the bargain of the vehicle. What Will posted is what I have found to be. Size the battery to the solar, not the solar to the battery. This is for the final van build. We can start with one panel, with a starter battery if funds are x tight. Then at least have a light, a small fan, and charge our lap top or tablet.

 

[lenny flank]

I use only 100w. I suspect many people don't use much more than that.

For me, it'd be an enormous waste of money to put up a big huge system that I simply don't need.

 

[frater secessus]

I agree about this being Cheap RV living and not buying things that are not needed.  I am notoriously cheap frugal.

And although I am a big fan of what can be done with small solar installs, excess panels themselves aren't wasted in the normal sense.  Too much lead-acid bank?  Heavy and a problem to keep charged to 100% SoC.  Too much controller?  Self-power consumption goes up.  

Panels excess to absolute need will:

• start collecting usable power earlier in the day, and keep collecting usable power later in the day
• get the bank through Bulk faster
• allow for significant real-time loads like coffee makers, rice cookers, water heaters, etc. 
• reduce the multiple of "days of reserve" required in the battery bank calculation, since the panels will be making power under marginal weather conditions (overcast, rain, smoke, etc)
• collects more power when insolation is poor:  higher latitudes, winter.   Allows one to boondock longer in the PNW, for example.
• is able to collect more power in short periods, as when parked in a forest, slot canyons, in a mountain's shadow, behind a rock windbreak, etc.  This frees one up to camp in more spots.  
• eliminates problems of finding matching panels later if need increases

I am definitely not encouraging folks to spend money they don't have.  I am saying that if one has the cash and finds a good deal, extra panels won't actually go to waste.

 

[kllcbosmetris]

Step 1: Fill the roof with as many solar panels as you can (example: 4x 100 watt solar panels can safely fit on your roof)

Step 2: Purchase a solar charge controller that can handle the power produced by the solar panels. It is best to buy a solar charge controller that is larger than what you need. This way you can scale your system later if you need to. (example: for 400 watts of solar, I would stick with a 40 amp mppt charge controller)

Step 3: Choose a battery large enough to work with your solar panels. Typically, for vehicle mounted solar power systems, for every 100 watts of solar power you have on your roof, you should have 75-100 amp hours of sealed lead acid batteries. (example: 400 watts of solar on the roof means a 300-400 amp hour sealed lead acid battery). Be sure to check with the battery manufacturer to see how many solar panels they recommend, or find the datasheet and determine the "safe charge rate" in amps.. (or check out my website)

Step 4: Buy the largest inverter you can afford. Most people do well with a 1500-2000 watt inverter. Pure sine wave inverters are ideal and worth the money. Spend extra money to buy some thick inverter cables.

Step 5: Build your system and use it! If it powers your loads, you are good to go. If you need more power, scale your system (not easy to do if you filled your roof to the rim with solar panels, so you may need more batteries instead)

Questions:

- Step 1: Consider you are going for urban stealth and have to use flexible panels. What is the largest capacity flexible panel out there? Are they all limited to 100w-135w?

- Step 2: Can you explain what 40amp (in 40 amp mppt controller) does or stands for? What is the function of 40 amp there or [X] amp. If you had 600w solar on your roof, for example, would that number change?

- Step 3: You said for each 100w on the roof, the general rule is ~100 amp lead acid battery. Is that ratio different if you are buying lithium batteries? Does that have anything with usability rate (50% vs 80%)

- Step 4: If you'd like to be able to run 1800w A/C appliances, would a 2000w inverter be enough?

 

[jimindenver]

Panel is cheap, the least expensive parts of the systems, use that to your advantage. You will appreciate it when the sun is low in winter, the nights are long and a few cloudy days in a row can easily munch a bank designed to meet your needs on long sunny summer days. Refilling that bank AND meeting your needs can be a chore especially in the winter.

Covering the roof also removes the "if I just added one more panel" syndrome.

Now I do contest the biggest inverter that you can afford concept. A microwave is the largest load most of us employ and if you need it, you need it. Should you not need it then larger quality inverters are huge, cost a lot and suck a lot of juice running or in stand by. So much so that some advocate having the big inverter but running it only for the big load while using a smaller inverter that can be left on to run small loads. A work around to the stand by draw is to have a load sensing inverter that will shut itself off when not needed.

 

[regis101]

Thanks Will.  It's a nice outline to help get a mind set.  And as I surmise the caveat, there is room for custom tailoring.  
One hun + pages in the forum alone and I noticed pretty much the same questions being asked.  I'm guilty as well.  
Personally I lean more toward the 2:1 ratio but , eh, moot point.
Thanks again

 

[tx2sturgis]

Will, that whole process assumes that the newcomers all have unlimited funds and maximum power needs.

It's a 'shotgun' approach that never even considers what the person needs to power. 

Your solution is at one end, and one corner, of a sliding 3 dimensional scale and not as useful for others in different parts of the country, with different power needs, limited budgets, restricted building and wiring skills, and do not yet know whether they want to camp in the desert, or in the forest under a canopy of trees, and with or without refrigeration.

What we try to do here (in the threads about solar power), is focus our advice to the specific needs, wants, abilities, and budgets of the people who are not yet familiar with solar power and its benefits and limitations. In these threads, I learn as well as help.

And while I agree that a roof filled with panels is a beautiful thing in the desert on the top of a boondocking RV, it might not be needed (or wanted) by an urban camper in a minivan parked under some trees most of the day, or a beginning part timer who only needs to recharge a laptop or a power pack, or keep the vehicle starter battery topped off.

I think you're trying to help everyone solve everything in one post, and it just won't happen. We are running an information marathon here, not a sprint. 

 

[Ballenxj]

Will, that whole process assumes that the newcomers all have unlimited funds and maximum power needs.

It's a 'shotgun' approach that never even considers what the person needs to power. 

This is what I'm seeing as well. Your subject line that says "Sizing a Vehicle Mounted Solar Power System" is misleading. People have different power requirements that are not addressed in your post.

 

[willprowse]

Wow you guys sure are specific! I go over the traditional way to size a system in my book, and the 5 step method I mentioned actually kicks butt. Your roof is not getting larger and building your system around that is a great way to go. And most people rarely use what they calculate. Typically they will use their electricity till the batteries are low. So might as well build the largest system possible. Sure you can make micro systems for specific needs, but if you are living full time in a vehicle, might as well build it big!

How do I change the title of this thread?

And I will try to respond to everyones comment to argue every single point they made. I am amazed how specific you guys are. This method actually works really well!

 

[willprowse]

I've always said build a spreadsheet of your uses and how often you use this or that and that will determine your amp hours needed over a 24 hour period.  It sounds simple but you need to do the math of actual use of each, how many hours each thing uses for your daily use, multiply that by 2 so you never discharge your batteries over 50% and then multiply that by the number of days you can go without sunshine. This will closely tell you how much solar you need.  The spreadsheet will also tell you if you need more solar should you have to replace a device or add another use, or if you can get by with what you already have.  Remember, this site is more geared towards cheap rv living and I'd hate to buy more solar than I actually need when I could have used that money for something else like tires or food.  I should probably build a downloadable spreadsheet where folks can plug in what they have, etc..  

So, for my needs, I'd put as many solar panels on my TT roof as I need being careful not to put them where there is a possibility of shade that I can prevent.  Roof fan covers are a good example and a good tape measure is your friend. If for some reason I need more panels than my TT roof can hold I can always put a couple more on the Jeep Cherokee roof and maybe those can be tilting for added solar charge.  I 100% agree on the charge controller being sized up a tad and I'd also add buy a well known name brand (Morningstar comes to mind) and not some flashy no-name with bells and whistles you don't need or can't get serviced if something goes wrong.  I'd also include a Trimetric battery monitor so I can monitor my battery health and see what is actually happening.  I will be purchasing 6 volt flooded cell golf cart batteries so I can keep an eye on acid level and use a hydrometer to add another health check on my batteries.  I'd have no use for an inverter since nothing I'll be toting will be 110vac.  Others may and again buy a good quality one if you need one.  I used them in my big truck when I drove but I needed them then.  I'll do everything I can not to use them again.  If it all goes smooth I'll have exactly what I need and not spend more than I have to when I get back to the US...  Come on Jan 22nd!!!!

Yeah sure and so do I, but your roof isnt getting larger, and might as well build the largest system you can. Most people use their system till the batteries drain, then turn off appliances. Might as well make the system as large as possible. And considering all the possible losses in a system, you wont know how much power you have till you build your system and actually use it. So make it large, make it scalable, and use it. If it doesnt work, add more panels and batteries. 

Cheap rv living implies frugality, but these are cheap. solar panels are cheap. you can buy refurbished forklift batteries for dirt cheap. what do you mean? I have a full video where I build a huge system for under 400 bucks. I know people here are spending more than that on gas every month. And expense is relative. We should cater to all income levels here. 

Oh gosh why flooded? Why not sealed? That sounds like a pain in the butt honestly. and sure, good name brand and a tape measure is pretty obvious. I dont know why someone wouldnt do that. And thats great about not needing an inverter.

 

[willprowse]

This gets so many newbies into deep shizz.  They assume that having a giant inverter means they have the power to run it.

My advice is the opposite:  don't buy or use an inverter unless you have to.

No thats not true at all. Buy the largest one possible because the discharge rate is high and most large loads are not powered for that long at all. Tell the newbies to get the largest inverter possible, and watch the voltage of their system. Easy as pie. And inverters have low voltage warning/shut off system. So what you said does not matter at all.

 

[willprowse]

Not helpful.

Buying what may not be needed is just wasteful and glutinous- whether you can afford it or not.
Website purports to be 'Cheap' living.
This is another thinly veiled way attempt to sell lots of stuff people don't need, no different than anywhere else.

Are you joking? Your roof is not getting larger and over-paneling is awesome! Especially if your system can handle it. 

Yeah its cheap living and this stuff is cheap. Panels from china are pennies. My first panel was 120 watts and cost 600 bucks. Now adays panels are 100 bucks for 100 watts, and you can buy refurbished forklift batteries.

And people can afford to build a nice solar system. We all spend more on gas in a few months than we do on a solar system. They can easily afford it. 

And not even! Its not about selling people more stuff than they need. I have lived off grid for 8 years and electricity is the most important. Build it as big as you can. And having limited roof space means that you cant build big anyways. Its realistic advice.

 

[willprowse]

I use only 100w. I suspect many people don't use much more than that.

For me, it'd be an enormous waste of money to put up a big huge system that I simply don't need.

100 watts?? What are you powering?? That is so tiny. I would die if I had to live off a single 100 watt panel. Unless I had a big alternator and high charge rate lithium battery. Is there any reason you choose to have such a small system?

 

[willprowse]

I agree about this being Cheap RV living and not buying things that are not needed.  I am notoriously cheap frugal.

And although I am a big fan of what can be done with small solar installs, excess panels themselves aren't wasted in the normal sense.  Too much lead-acid bank?  Heavy and  a problem to keep charged to 100% SoC.  Too much controller?  Self-power consumption goes up.  

Panels excess to absolute need will:

• start collecting usable power earlier in the day, and keep collecting usable power later in the day
• get the bank through Bulk faster
• allow for significant real-time loads like coffee makers, rice cookers, water heaters, etc. 
• reduce the multiple of "days of reserve" required in the battery bank calculation, since the panels will be making power under marginal weather conditions (overcast, rain, smoke, etc)
• collects more power when insolation is poor:  higher latitudes, winter.   Allows one to boondock longer in the PNW, for example.
• is able to collect more power in short periods, as when parked in a forest, slot canyons, in a mountain's shadow, behind a rock windbreak, etc.  This frees one up to camp in more spots.  
• eliminates problems of finding matching panels later if need increases

I am definitely not encouraging folks to spend money they don't have.  I am saying that if one has the cash and finds a good deal, extra panels won't actually go to waste.

Cheap rv living still applies to this thread because your roof isnt getting bigger, and solar panels are cheap. And yes, batteries are heavy. Totally agree. But people can easily carry them in a rv or large van. Its not a huge issue. If you have a tiny van, then sure, make a tiny aerodynamic system. But if you are living full time in a vehicle, why would you not fill the roof with panels? They are just so cheap.

 

[tx2sturgis]

 I am amazed how specific you guys are.

Everyone's needs here are different. 

In the year 2000 I bought my first 'large' solar panel for my Class A RV. It was 64 watts and $400! Back then, large solar panels were expensive. I powered the entire RV house systems that way for years, using propane for refrigeration and heating. I had no need for hundreds of watts of solar. 

And, now, for example, on my van, I have one 45 w panel to balance the parasitic loads while parked for days on end. Another, 120 w portable panel to power ham radio gear and the battery for it. The roof on the van only has room for the smaller panel, with vent fans, awnings, and luggage rack up topside. 

My cargo trailer, on the other hand, has a LOT of roof real estate available, but a 200 watt folding portable panel is plenty to power a multi-transceiver ham station and internal LED lighting, and I can aim it at the sun to get the maximum output, and cleaning is much easier. I also own 5 generators so I can choose which unit to bring depending on what I need for that trip. 

My situation is WAY different from say, a person on limited funds, in a small SUV, who needs to run a rice cooker and a cpap, and keep their medicine and fresh veggies cold in the middle of nowhere for 3 months. 

Just saying....

 

[willprowse]

Questions:

- Step 1: Consider you are going for urban stealth and have to use flexible panels. What is the largest capacity flexible panel out there? Are they all limited to 100w-135w?

- Step 2: Can you explain what 40amp (in 40 amp mppt controller) does or stands for? What is the function of 40 amp there or [X] amp. If you had 600w solar on your roof, for example, would that number change?

- Step 3: You said for each 100w on the roof, the general rule is ~100 amp lead acid battery. Is that ratio different if you are buying lithium batteries? Does that have anything with usability rate (50% vs 80%)

- Step 4: If you'd like to be able to run 1800w A/C appliances, would a 2000w inverter be enough?

Sure, my recent setup was 200 watts of flexible solar. And yes I havent really found many panels larger than that. 100 watts is great for mobile roof solar arrays. 

40 amps is how much current the solar charge controller can handle. It is a great metric to use to refer beginners to a solar charge controller, but it doesnt tell much. Each solar charge controller is quite different in design and some 40 amp solar charge controllers can handle a lot more power (total wattage of solar panels going through it) by having a larger input voltage. This will require putting the panels in series/parallel arrangement until they reach the ideal voltage required by the solar charge controller. So that means that when a solar charge controller is rated for "40 amps", it is not telling use everything we need to know. If you are new to this, and everything I said did not make sense, follow the manufacturer recommendation for how many solar panels to connect to the solar charge controller. I could easily hook up 500 watts of solar panels to a 20 amp high voltage input mppt charge controller. most beginners would not know how to do this, so I recommend getting a 40 amp controller so they dont have to think about it. it is just easier that way. 

Yes absolutely! Almost any size lithium will work with any size of solar panels. Lithium *typically* have a higher charge rate than lead acids. That means that you should buy as large of a lithium battery as you can afford, and as many solar panels as you can fit on the roof of your vehicle. Unless your solar system is being used for a very specific application. 

Depends on the loads. Induction loads, such as large motors and microwaves, require a large amount of electricity just to get started. So typically, it is best to double the size of your largest induction load, when sizing your inverter. If you have a 1800 watt load, buy a 3600 watt inverter. Even if you do not plan to run a large load, you should buy as large as possible. I have seen so many inverters turn themselves off when powering loads they were rated for. The biggest cause for this is using inverter cables that are too thin. Especially with large inverters. You need cables as thick as quarters if you are powering large loads with inverters (0 gauge at least for 2000 watt inverters). This is because we are working with 12 volts. If you had a 48 volt system, the inverter cables would be much smaller. But then you would have to buy a specialty inverter which would cost much more. So typically you are fine to use common, large 12 volt inverters, just be sure the wires are bigger than needed. 

I just woke up so I hope all of these responses made sense haha let me know if this helps you