How Many Solar Panels Do I Need?
One of the most asked questions when it comes to Solar Energy is;
“How Many Solar Panels Do I Need?”
This seems like an easy question but when we go to answer it the question becomes much bigger and complicated. The answer to this question depends on a lot of variables but the simple answer is;
We need enough solar panels to supply enough electricity to power all of our stuff.
So how many is that?
First we need to know how much power we are consuming. If you are paying a power bill, all you have to do is look at the power bill and it will tell you how many kilowatt hours of power you are using each month. If you are planning on moving to a homestead and you are going to take all of your electrical devices with you then the number of kilowatt hours will remain the same.
For instance I am still on the grid so I get a power bill each month. Last month my bill told me that I used 370 kilowatt hours of power. Or to put it another way I used 370,000 watts of power for the month.
The first things we need to understand is that electricity consumed and produced is measured in Watts and 1 Kilowatt is 1000 watts. So the power generated by our solar panels is measured in Watts and our devices consume that power in Watts.
If you don’t get a power bill then the answer to our question takes a bit of work to figure out. To get our answer we need to go through several steps.
Step 1 List All Of Your Devices
Make a list of all of the devices that you plug in. Your list will need several columns. Start in one room and make a list of all of the electrical devices that you have plugged in or plug in on a regular basis. For instance your TV is likely always plugged in and you curling iron is only plugged in when you are using it. If you are making a list for a 4 bedroom house, you will have a much longer list than someone living in a tiny house.
Step 2 Amount Of Power For Each Device
Once you have a list of all of the devices, you need to know how much power each device uses when it is plugged in. To find out how much power each device uses, look on the identity plate where the serial number and part number is. You should see a number that is says clearly the number of watts or you might see a number with the letters VA after it. That is the wattage number you are looking for.
In some cases the tag won’t show a wattage number but will tell you how amps the device uses. If that is the case you need to convert that number to watts. The way to do that is to multiply the current number by the supply voltage. The supply voltage for North America is 120 volts. There are also a couple of devices in just about every home that run on 240 volts. These are your Dryer and your kitchen range. So keep this in mind when you are making your list of devices. So our formula for watts is current multiplied by voltage.
V X I = P
V = The voltage supplying the device
I = The Current that is being drawn by the device
P = The number of Watts being used by the device
Step 3 How Long Are Things Plugged In
Now that you know how much power each device uses you need to add a time factor. List the amount of time each of your devices is on. For instance the electric clock on the wall is on 24 hours per day. Your cell phone is likely only plugged in for a couple of hours a couple times per week. Your lights are only on for a few hours in the evening.
So if you have a device that is plugged in to a regular electrical outlet, draws 2 amps of current, and is plugged in for 8 hours per day, your calculation would be;
2 amps X 120 volts X 8 hours = 1920 watt hours= 1.92 kilowatt hours.
Step 4 Calculate Your Total Power Needs
Calculate all of the kilowatt hours for each of your devices and total them all up to get a total number of kilowatt hours. This is the amount of power that your solar panel array will have to generate each day to power all of your devices. At this point this number probably looks pretty scary and very expensive.
This is also the point when most people start thinking about reducing that number. Reducing the amount of power that you consume can save you a lot of money over the long term. In the short term the reduction in watts consumed can be a bit expensive. For instance new appliances use a lot less electricity than old appliances. Using a gas range and a gas hot water tank will significantly reduce the amount of electricity that you use in a month.
Now that we know how much power we need, we can start figuring out where that power comes from and how we are going to produce it.
Where Does Our Power Actually Come From?
Most people when they think about off grid power only think about the solar panels, windmills and other devices that gather the power. Most of the time we don’t really think about where the power for our devices actually comes from. In all renewable energy systems, the power for our devices actually comes from a storage battery. The solar panels or other renewable energy system is what we use to keep the batteries charged.
So since solar panels only produce power when there is light, and windmills only produce power when the wind blows, it is more important to be thinking about how many batteries we need to supply our power needs and then figure out how we are going to charge our batteries.
So the real question is “How many batteries do I need?” Most batteries that are used for renewable energy systems store about 100 amp hours of power. Make sure that when you are buying batteries for your system that you are buying deep cycle batteries and not automotive batteries.
Deep cycle batteries are designed to be discharged repeatedly to 25% of their capacity and can be recharged to full charge with a small trickle charge. Automotive batteries are designed to discharge quickly to around 90% of their capacity and be recharged quickly by brute force from an alternator or battery charger. It is very difficult to consistently recharge automotive batteries to 100 % capacity using solar energy.
So to find out how many batteries we need to power all of our devices. We need to divide the total power needed that we already calculated, by the power that can be stored in the average deep cycle battery.
If an average 12 volt deep cycle battery used for renewable systems stores 100 amp hours that is 1200 watt hours or 1.2 kilowatt hours.
The calculation is number of amp hours X the voltage of the battery = watt hours divided by 1000 = kilowatt hours.
So now all we have to do is divide our total number of kwh needed for our devices by 1.2 to get the number of batteries we need to produce the power we need.
So in my case I am using 370 kilowatt hours per month or 12.33 kilowatt hours per day to power my house so I would need 10.28 or 11 batteries to run my house.
370 kilowatts per month divided by 30 days equals 12.33 kilowatt hours per day.
370/30=12.33
12.33 kilowatts per day divided by the number of kilowatts in a battery equals number of batteries.
12.33/1.2 = 10.28 batteries = 11 batteries.
Now we know how much power we need and how many batteries we need to charge to maintain our power needs.
The Answer To Our Question
Now we can calculate how many solar panels we need to recharge our batteries. Solar panels come in many different power outputs and are rated in watts. For our calculations we are going to use a solar panel that generates 150 watts. This rating is based on the solar panel being exposed to direct sunlight. A solar panel will generate power as long as it is daylight but solar panels put out the most power when looking directly at the sun.
Since our solar panel in this instance is stationary and is not going to track the sun lets assume that the average output of our panels is only 100 watts. Now we need to figure out how many hours of the day our solar panels are going to be exposed to the sun and producing power.
For our calculations we are going to assume that our panels will be producing power for an average of 8 hours per day. This number will of course vary by the seasons, weather, and tree growth.
So if each panel produces 100 watts of power for 8 hours per day that means that each panel produces 800 watt hours or .8 kilowatt hours per day on average. In our example we are charging 11 batteries and we need 11 times 1.2 kilowatt hours per day so we need our solar array to produce a total of 13.2 kilowatt hours.
Our solar panels are going to produce .8 kilowatt hours per day so we need to divide the number of kilowatts we need by the number of kilowatts we can produce for each panel.
13.2 kilowatts needed divided by .8 kilowatts produced by panel = 16.5 or 17 panels.
So in order to power my house and move completely off the grid I would need 17 solar panels, 11 deep cycle batteries, a charge controller capable of transferring all of current from our panels to our batteries, and we will need an inverter capable of producing enough power to run the maximum number of devices that are turned on at one time.