How to Size your Solar Power System

today we're gonna learn how to size a solar power system in the most simplest way possible I am going to give you guys a certain watt hour load and we're gonna figure out what size battery we need with a certain amount of days of autonomy or how many days of backup and we're gonna figure out what size solar array will charge that battery in a decent amount of time so let's say we have a laptop and it takes 120 watts and you want to run it for five hours a day all you have to do is multiply the consumption by the hours that you are using it for and so 120 multiplied by five is 600 and so what we need is a battery that can supply 600 watt hours for our given load every single day but we want to have enough time for backup or days of autonomy so typically a good figure is three to five days of autonomy so what this means is if you have a cloudy or rainy day you will have enough backup power to power your loads during that duration of reduced power output of your solar system so all you do is multiply this by three so 600 times three is 1800 watt hours in 1800 watt hours is the total size of our battery but this is the usable size so understand that if you have a thousand watt hour lithium battery it will deliver 1,000 watt hours typically the recommendation for lead acid batteries it's 50% of the discharge that means you can only use half of the battery's rated capacity so if you have a lead acid batteries that is rated for 1000 watt hours you can only use 500 watt hours so for this example a battleborn lithium iron phosphate battery will deliver 1200 watt hours so you could say that you need at least two of them to fulfill this recommendation so this system to be able to power our laptop consistently without any problems and with three days of backup will take two battleborn batteries or you could say they will take 400 amp hours of lead acid batteries at a standard depth of discharge of 50 percent and you'll notice that two battleborn batteries will deliver 2400 watt hours so typically you want to round the number up and also you can't stick with one battery so you have to round the number up if you want to fulfill the low requirement with three days of autonomy you live in a very sunny location you don't need this large of a battery you could probably get by with this small like here in Las Vegas I could easily power this load with a 600 watt hour battery because I know I'll never really need that many days of backup power and even on the bad days I'll still be able to generate enough power to power this load so it really depends on your local weather conditions and other factors such as your latitude so for this laptop to run it for five hours a day with three days of backup you want to battle born batteries or for 100 amp power sealed lead-acid batteries sorry about my handwriting you guys get the idea alright we're just trying to make it easy so now we need to figure out what size solar array will charge these two battleborn batteries in one day and so we need to be able to charge 2400 watt hours in one day so how do we determine the size of the solar array typically the recommended figure in the United States is that you get five hours of good sunshine and so this makes our life very easy if you are using an MPPT and standard test conditions solar panels all you need to do is take the size of your battery in watt hours and divide it by five because overall most of the United States you will get five hours of sunshine and that's how much power in watts it will be required to charge up this battery in one day so 2400 divided by five is 480 watts so we need an array that is 480 watts in size and because solar panels are cheap you're gonna want to bump this up to 600 watts for sure and that's a pretty simple recommendation to belabor batteries or for lead acid batteries it is 600 watt array now that we know the battery needs to be this size and the solar array needs to be this size we're going to figure out what size solar charge controller we need to charge a 12 volt 2400 watt hour battery with a 600 watt solar array and this is super simple you just take your solar array size and you divide it by the battery voltage so if this is a 12 volt battery in there to bowell borns in parallel we just take this number and divide it by 12 and that will give us 50 and this is the amps required by our solar charge controller to be able to support this solar panel array at this voltage because the wattage divided by the voltage will give you the amperage and so this is the rating at the output if you are sizing a solar charge controller for a system you typically want to make it larger than necessary and typically there in 20 amp increments at this size so I would definitely be going with a 60 amp controller and that's also for efficiency reasons and so using a 60 amp controller will support our 600 watt solar panel array and because we have a 2400 watt hour battery let's say we want to power a microwave for 30 minutes a day we could also throw that into the equation let's say we're not using the laptop and we want to power microwave for 30 minutes a day let's say the microwave uses 1000 watts continuously so that means it will use 500 watt hours in 30 minutes so what we can say is that this battery can easily support our microwave and laptop depend on how we use it depending on how many days of backup we'll determine our use if it's sunny every day for the next week you can use a lot more power than our recommended 120 watts so in this instance with this large of a battery we have a lot of power to work with if it's sunny outside if it's cold and cloudy you can't use the bigger appliances and you need to reduce your load so that you have that full three days of backup but for most people this size system with this battery in this solar array in this solar charge controller will be able to power a lot of good stuff and when it comes to solar you want to make it as big as possible solar panel arrays are very cheap solar charge controllers are cheap but batteries are very very dreadfully expensive unfortunately I mean solar panels now you can give them for $0.50 a watt or cheaper used you can even buy Sun Power Cells for dirt cheap from like San Tan solar so I mean this is something that you want to bump up as much as possible I've actually done it in the past in sunny locations I would cut this number in half and have like a tiny battery and I'd have such a large solar panel array that I could reach origen very quickly and for this size system this 600 watt array could charge this in less than a day and that's typically good for it like a battery that's a lead-acid you need to cycle it once a day or keep it at a high state of charge at the end of every single day so that it doesn't have too much degradation if you if a lead-acid battery is kept at a low state of charge for a prolonged duration you will have increased internal damage on that battery so you want to keep it cycled once a day and then a high save charge as long as possible but yeah that's pretty simple you know for that laptop I mean this is super simple math that anybody can do now let's make this more fun let's take this 600 watt array and divide it by 24 and figure out what size controller we need so 600 divided by 24 is 25 so we need a 25 amp controller that can handle a 24 volt battery but think about how much cheaper a 25 amp controller is over a 50 or 60 amp controller this one of course we would want to bump it up to 30 amps but you're getting a huge cost savings by using the 24 volt battery and this is why you can save literally hundreds and hundreds of dollars by bumping up your voltage of your battery to 24 volts I have other videos that cover the pros and cons and how to do that and how to use converters so you can still use 12 volt appliances but keep that in mind when you have a large array in a large store charge controller and you bump up the battery voltage it will decrease the cost of the system substantially also the cost and wire which we need to make another video about that can decrease substantially with a higher voltage system because if you're using a 12 volt system with this sized battery and you're pushing large loads the wires are going to be very thick and copper is not cheap if you're pushing large loads and you have a 48 volt or 24 volt system it will be a lot smaller and wire which means reduce cost but again I have a hole in there video that goes much into deeper details on that topic so let's say we want to charge this battery with a DC to DC charger let's say that you drive your RV or van every single day and you have a small solar power system and you want to power the solar battery with your alternator safely so let's figure that out so let's say we take the Rena g1 and they have at one a 40 amp one let's say that we buy you the 40 amp one and we want to know how fast it can charge this battery well what we need to do is figure out how many watts this is pushing so we have 40 amps as the output from the alternator with the DC to DC charger and the battery voltage is 12 volts so 40 times 12 and the answer is 480 and so those two numbers divided by each other give us five so that means it will take five hours for the DC to DC R energy charger to charge up our two Bell born batteries it will take five hours of driving most people are not going to deeply discharge it all the way typically you know 50% depth of discharge with these would make more sense so it would take like two and a half hours of driving a day for most people to charge up their battery and so as you notice we're just messing with watts watt hours kilowatt hours to figure out how long it takes to do certain things or how to store power or how to you know charge up a battery in a specified amount of time for our given conditions if this video lost you and you don't understand why two volts and amps please check out my beginner videos you need to read those or check out my book that's probably the easiest way to do it it's like step by step I show every single thing that you need to know and it makes it very easy to understand this might be a little confusing because I'm rushing through it because it's a video another thing to consider is in my book I have rules of thumb so that you don't have to use math or you have to use very little math and I also have pre calculated packages on in the book and on the website that I have so you guys can check that out as well but if you want you know power a specified load for a certain amount of time this is the best way to do it now that we've done a simple example let's do a more difficult example let's say we have an air conditioner and it's a small window air conditioner and it takes 500 watts to power let's say we want to power this for 24 hours a day because out here in the desert I need to power one for that much time so I'm kind of doing the math for that right now so what you need to do is take this 500 watts and multiply it by 24 and this will give us 12,000 because 1,000 watt hours equals one kilowatt hour we can say that this is 12 kilo watt hours and now what we want to do is multiply it by three and this is the days of autonomy so they'll give us their six kilowatt hours is the size of our battery bank to power this air conditioner with backup redundancy and that is a very big battery pack all right you guys and so that's like 30 battleborn batteries so we're talking like $30,000 dollars here yeah $30,000 Wow so now we need to figure out what size solar panel array will charge this size battery in one day in this recommendation of three days of autonomy does not apply to some people you guys might be looking at that thinking oh my gosh that is so expensive but if you live in Alaska and you're trying to power large loads you need to over panel your array you are gonna need this size of a battery to consistently power your device 24 hours a day 365 days out of the year so that might look scary but that's the true cost of powering a load with actual redundancy it's expensive and so just like we did before we just need to take this and divide it by five it will give you seven point two thousand so that means that we need a seven point two kilo watt solar array to be able to charge this up if you are actually building the system you want to bump this up to like nine or ten I would say ten kilowatt array for that size of a battery would be great and now let's say that this solar array is charging a 48 volt battery let's figure out what size in amps we need to use for a solar charge controller it's a 10,000 divided by 48 for our battery being voltage will give us 208 amps this is the size of your solar charge controller if your solar charge controller has over current or over panelling protection you could actually instead use like a 200 amp controller or you could use two 100 amp controllers and have two separate arrays going through each controller so it really depends on what you want to do in your budget it's funny though through Chinese controllers you can actually get a better deal instead of finding like an 80 amp you can actually do better off with 240 amp controllers so it really depends on how much money you want to spend but if you can spend $30,000 on this battery bank and then considering 10 kilowatt hours and 50 cents per watt you could say that that's 5000 of solar panels so we're looking at like 35,000 plus the copper plus the mounting the permits and stuff like that safety permits for mounting that large of a grounder right we're looking at like a forty thousand dollar system right here which is not bad considering what we're powering we're powering a pretty large load 24 hours a day I mean think about if we use this to charge up a Tesla in my battery pack in the Tesla is 60 kilowatt hours it would only take two days to charge that thing and that's with quite a bit of safety Headroom so that would give us actually 72 kilowatt hours in my battery is only 60 but considering the charging losses in conversion losses and also large losses of depend on what kind of battery we have and all the other losses throughout the system you could say that I could charge my Tesla in two days with a forty thousand dollar system which that is pretty darn expensive that's why it's not smart for most people to do a truly off-grid system for extremely large loads you're better off using the grid as a load dump or as a battery and just having a huge solar panel array but this is a very large system if you just cut this system in half like a twenty thousand dollar system you could charge up a Tesla no problem given most people's daily driving habits so that's pretty cool and in my personal experience after living off-grid for ten years I've noticed that 5,000 watt hours for me has been great well five thousand watt hours can easily supply a normal person's living electrical needs it's pretty easy to power a life with that um a lot of times also over paneling your system is pretty fun to do if you live far away from the equator you're gonna have to bump up the size of your brain um it's unfortunate because when you're far from the equator and you want to over panel you also have to deal with cold temperatures and with cold temperatures especially in the morning you have an increase output in the voltage and you can get a voltage spike that can burn out the solar charge controller so when you do over panel it what I like to do is have current protection so that if you do get that burst of power current wise you'll be protected and you have to calculate that for your solar array and then also the voltage if it spikes you want to make sure that there's Headroom usually it's like 25% it really depends on what kind of control you're using because some of the ones that are ul listed have safety Headroom for a voltage over voltage on built-in the cheaper ones use cheap capacitors with lower voltage radians and you will burn them out and I've done that a couple of times so be very careful some of them you can hit that limit and keep it there for years and then of the cheaper ones it will just destroy itself instantly so that's where you know buying ul listed devices and you know calculating it all out everything works out perfectly but sometimes the cheaper stuff it'll say it's a 40 amp controller you push 40 amps and it burns it out in a couple weeks so it really depends on what you're working with but yeah this is like a very large system in the last example was a small system so I hope you guys found this video useful I thought was fun I get these questions a lot and it's so simple if you know how large your load is and how long you want to power it for you can calculate any size system instantly I mean I do this all day long whenever I see new batteries and different prices and stuff I just plug in these numbers if you do not understand these watt hours and watts please check out the beginner stuff and it's only a couple equations it's volts times amps equals watts and once you know that you can mess with the division you can calculate any size system grid tire off-grid it's super simple so yeah I hope you guys found this video useful and let me know what you guys think please leave a comment below and yeah I'll talk to you guys later bye

Channel: DIY Solar Power with Will Prowse

Views: 1,784,818

Keywords: off grid solar, will prowse, diy solar, rv solar, sizing a solar power system

Id: TJBGbufexEM

Length: 16min 52sec (1012 seconds)

Published: Thu Oct 03 2019

Reddit Comments

[u/YOUREABOT]

Have u done a clip about tying a wind generator with solar. I also wonder about If I need 1 or more.