Stand Alone (or Off Grid) Solar Design is a very large and complex area. It is also the one area of solar design that most people try to do themselves, with that said I thought I might go through the design process step by step to help anybody who may be having difficulties.
I thought the best way to tackle this is to approach it in the same way I approach the system design:
Before you can do anything you need to understand how much energy is required each day, and how quickly that energy needs to be delivered. I always work in kWh (kilowatt Hours) for the amount of energy needed and kW (kilowatts) for the rate at which the energy needs to be delivered.
LOAD: this refers to something that uses electricity. For example a toaster is an electrical load, its quite a big load drawing close to 10A at full swing. An iphone charger is also a load, it is a small electrical load.
Lets use an example:
In my holiday cabin I want to run the following electrical loads.
2 x 100W incandescant light bulbs. I will run them between 6pm and 11:30pm every day.
0.2kW x 5.5hrs = 1.1kWh
1 x Electric jug. I am a big coffee fan so I would boil that jug at least 5 times a day. I have a tendancy to only boil enough water for the coffee I am about to drink so it only takes 60 seconds to bring the water to boil.
1 x 1000W = 1kW
0.08hr (5min) x 1kW = 0.08kWh
1 x 300W plasma TV. I know plasma TV's are really inefficient TV's but I dont want to get rid of it. I would normally like to watch about 5hrs of TV in the evenings because I like to have it on as background noise when I am working.
1 x 300W = 0.3kW
5hrs x 0.3kW = 1.5kWh
So for the purposes of our example I am going to assume all of the electrical loads can and probably will be running at the same time. So this means my Off Grid Solar system needs to be able to supply 2.7kWh (round up from 2.68) worth of energy during the day and it needs to be able to supply a peak load of 1.5kW.
Our key design parameters:
OK.. this topic is not for the faint hearted, but I will do my best to keep it simple...
Based on our estimated energy requirements we will use 2.7kWh of energy each day (and we can assume its all going to be used at night).
Based on the diagram above we can see the inverter has the job of converting stored battery energy to AC electricity. Like any conversion of power from one form to another, this cannot be done without some sort of loss. In the example of the outbacks its safe to assume a loss of 10%. There are many inverters that claim a better than 90% conversion rate, but I have not seen better than 90% efficiency, so dont always believe the marketing material.
Back to the battery sizing. If the Inverter needs to deliver 2.7kWh of energy to the loads it must have used 3kWh of battery power...
Quick Check... 3kWh in. 10% lost = 0.3kWh, so total energy out is 3kWh - 0.3kWh = 2.7kWh.
OK at this point we know we will be drawing 3kWh of energy from the batteries every day, but before we can continue lets look at a couple of concepts. The first will be Days of Autonomy, the second is Daily Depth of Discharge (DoD).
We know from our previous calculations, we need 3kWh every day from our batteries.. But what happens if we get 3 days of rain.. how much energy do we need stored in our batteries... The answer is simple (kind of), we need 3 days x 3kWh = 9kWh. But what happens if we get 5 days of rain and misery? then we will need 5 days x 3kWh = 15kWh.
The accepted practise is to provide enough stored energy for 3 days of rain and misery, if there is no sunshine at all after 3 days then a generator or some other source of power needs to be found.
In some climates 3 days is not enough, yet in others 3 days is too much, however the general rule is 3 days, and that is what we are going to base our calculations on.
So based on 3 days of autonomy we will need a battery bank that can store 3 days x 3kWh = 9kWh of energy.
This normally takes a bit to get your head around.. in the world of solar batteries we talk about how much energy has been taken, not how much energy is remaining. The Depth of Discharge refers to what percentage of the batteries stored energy has already been taken. For example if the a battery is capable of storing 30kWh of energy, and I have been told the Depth of Discharge (DoD) is 10%, I know that 3kWh has already been taken from the batteries.
For standalone solar systems the Daily Depth of Discharge is an important metric, it tells us what percentage of the batteries available capacity the system was designed to use each day.
Lets assume we are using Gel Cell batteries. I know that 100% of the C100 rated energy can be extracted from the battery without damaging it; so this means at the end of 3 days of no sun the batteries can be safely discharged to 100%, so each day I can take 30% of the batteries capacity. therefore the Daily Depth of discharge will be 30%.
The above calculation is different for flooded cell solar batteries because if you discharge them past the 80% mark you will damage the batteries.
So what does all of this mean... well it means our daily energy requirement (3kWh) can only represent 30% of the total batteries capacity.
So as before if we are using Gel Cells we need a battery bank that can store 9kWh of energy; if we had of chosen flooded cell batteries, our battery bank would need to be a lot bigger because our 3kWh could only represent 25% of the batteries capacity.
ok, to summarise so far.. We have to be able to delivery 2.7kWh of electrical energy to my holiday cabin; because of the inverter efficiency we need to draw 3kWh from our batteries each day. We have planned to allow 3 days of autonomy in our energy storage, and we are going to use Gel Cell batteries, so this means out total battery capacity needs to be 9kWh.
Now, you are probably scratching your head wondering why you cant find a kWh rating on any batteries, thats because there isnt one. Batteries will have a voltage and Amp Hour(Ah) rating, Im not going to go into the details of that in this discussion, if you want more information read this The ins and outs of Batteries for Solar Systems.
Battery Amp Hours (Ah) can be converted to kWh by multiplying Volts by Amp Hours. For example if my battery bank is a 24V battery bank and my batteries are 200Ah batteries then there is 24Volts x 200Ah = 4,800Wh = 4.8kWh of energy storage.
So back to my holiday cabin.. I personally do not like 12V battery systems because there is to much current draw, I like to design 24V and 48V battery banks, for my holiday cabin lets assume we will use a 24V battery bank. So the required Amp Hour capacity of my batteries is (9kWh x 1000) / 24V = 375Ah.
So the conclusion to this very long process is I need a 24V, 375Ah battery bank.
Just a few notes... from an engineering stand point it is very poor form not to build in some safety margin, the above battery bank (24V@375Ah) is the absolute minimum required, in reality I would want to add at least another 20% to the 375Ah to ensure the system will function as expected.
Another reason why we try and only take 30% out of the batteries is because it extends their life. The life of a battery is extended the less you discharge out of it. For example, if you discharged your Gel Cells to 100% each day you will only be able to do that about 1400 times (assume 1 dischatge each day) thats only 4yrs of life. If you discharge them only 30% of the way down you will be able to do that well over 3500 times, which give a battery life of 10+ years.
The other important note is most peoples battery capacity is dictated by their wallet, not by what is required. Having said this it is still important to know what the actual solar battery capacity should be to support the loads, then you can work back from there; there is a lot that can be done to reduce the battery size and still get the job done, but you need to know what the storage discrepency is before starting on this road.
Before you begin on the solar array take a look at the info on estimating solar yields ( Estimating Solar System Yields ), particularly the part on Peak Sunshine Hours (PSH).
In summer you will typically see around 5.5 Peak Sunshine Hours (PSH), and in winter you will get roughly 3 PSH (remember Peak Sunshine Hours are the number of hours of sun assuming every hour is 1000W/m2 of solar power, so you may have 8-10 hours of actual daylight but those hours can be compressed into 5.5hrs during summer and 3 hours during winter).
As a general rule of thumb we always size our solar systems to function on the worst possible days of the year, knowing that it will easily function during the best. So in our case we want to use the figure of 3 PSH.
Remember in the last section we decided we needed to have a battery bank 24V@375Ah, this equates to 24 x 375 = 9kWh of stored energy, of that 9kWh we are designing the system to only use 30% of that each day which is 3kWh of energy. So very simply we need a solar array that will store 3kWh of energy into our batteries; this calculation is simple 3kWh / 3PSH = 1kW. So we need a 1kW solar system.
Unfortunately life is not that straight forward. We need a solar array that can store 3kWh of energy into the batteries, but lets look at the steps needed to store the energy (remember every step will represent a loss of energy).
The diagram above shows the typical charging losses through the system.
As you can see accurate solar design can be a bit of a challenge; but the key to the whole thing is to accurately predict what kind of loads you are going to run, unfortunately most people will wildly under estimate the amount of energy they are going to need... so be realistic. In my holiday cottage example we only wanted to run 2 lights, a TV and a kettle, now that is what I call unrealistic.
The other important thing to finish on is to mention all of the above is a generalised view of how things run; for example I have sized enough solar panels just to charge the batteries, but what about if I want to do some washing during the day, so the solar array will not only have to charge the batteries, but it will also have to run 2 loads of washing during the day. Every system is a little bit different and careful consideration needs to be given the each of the components.