Solar Power math explained and simplified

• Author: DigitalMind
• Filed under: Solar Power and Off-Grid Living, Uncategorized
• Date: Oct 19,2008

As far as Solar Power goes, 80% of everything comes down to this. People try and figure out how many batteries and solar panels they need before they know how much power they use. This is the most basic part, and one of the longest. However once you have that number the rest is fast and easy. This article contains the steps you need to take to figure out how many solar panels you need, and how many batteries you need for your renewable energy system.

Step 1 : Figure out how much power you use daily.

I know that it’s a pain, but it’s inevitable.  Make a list of all the electrical appliances you use, figure out how many watts it takes, and estimate how many hours per day you use it.  Multiply the total watts of the appliance with the hours used and that’s your total for that particular electrical product for one day. Do the same for all your electrical things and add them all together.  Now you know how many watts of power you need to generate per day for everything.

Tips :

• Voltage * AMPS = Watts (Lots of power adapters only list the voltage and current, so you need to do the math yourself to get the total Watts it takes)
• Watts are measured on a 1 hour basis.  (So a lightbulb that takes 60 watts uses 60 watts PER HOUR it’s on)
  
• Some appliances are VERY hard to figure out the total power consumption.  If you can call the manufacturer’s they often have these specifications.  (Usually measured in KiloWatt hours per YEAR) Just divide it by 365 to get the number of kilowatt’s in takes per day.  Multiply that by 1000 to get the number of Watts it takes per day on average.
• Use a Kill-A-Watt meter to measure the exact power consumption that something uses. (These are AMAZING and not expensive : Especially the find out the power consumption of things like fridges and computers)

Step 2 : How many hours of sun do you get per day ?

Figure out the average minimum of sunlight that you get per day.  For me, here in Quebec, I figure 4-5 hours of sun per day.  This is the average all winter long.  We get much more in the summer, but I think it’s best to measure based on the minimum average you’re getting.  This means that I’ll have power to spare in the summer.

Step 3 : Figuring out how many solar panels you should get to meet those needs.

Divide the total amount of watts you need per day by the amount of average sun hours you get per day.  This will tell you how many watts of power you have to generate per HOUR in order to have enough power.   SO :

Solar panel wattage needed = total watts used per day / total sun hours per day

So lets say you use 2000 watts per day, and you have 5 hours of sun per day. 2000/5 = 400

That means you need a total of 400 Watts worth of panels at the minimum to get enough power everyday. Solar Panels are rated in Watts.  Just like all appliances, that number is a PER HOUR amount. So a 100 Watt panel generates 100 Watts per hour of sunlight that hits it.

A quick note about buying solar panels :

Solar panels usually come in one of two voltage ranges : the 12 volt range that is used to charge a 12 volt battery bank. These panels usually output about 18 volts to charge your 12 volt battery bank.

Other panels come in 24 volts so these are used to charge a 24 volt battery bank.  These panels will output over 30 volts when in the sun.

You can connect 2 12 volt panels in series in order to make it like a big 24 volt panel to charge a 24 volt battery bank. However using a 24 volt panel to charge a 12 volt battery bank is not as easy. So if your battery bank is going to be 12 volts, make sure you get the right panel !! (Usually panels over 100 Watts automatically come in 24 volt configuration.)

Step 4 : Sizing your battery bank.

Your battery bank should never be discharged more than 50%.  (And that’s already pushing it because others say not to discharge more than 30%) So using the number you got in step 1 (Your total, daily power consumption) we will figure out the size of the battery bank we need.

Battery bank size in watts = daily power consumption * 2  (Or 3 if you have money to spare)

Batteries are labeled with the voltage and Amphour capacity.  Simply multiply the 2 together to get the total WATTS that the battery can generate.  So if you use 2000 Watts per day, you need a battery bank with a capacity of atleast 4000 Watts.  (The less you discharge your batteries, the longer they will last, so it would be better in the long run to have a battery bank of 6000 Watts, but not necessary)

Quick Notes autbout battery banks :

• Battery banks are usually setup in 12 volt or 24 volt configurations. This is done by connecting batteries together in series or in parallel.
  
• When batteries are connected together in series, the voltages of the batteries being put together add up but the amp/hour capacity stays the same. (So : 2 6 volt batteries rated at 225 amphours connected in series = a battery bank of 12 volts 225 amp hours)
• When batteries are connected together in Parallel, the voltage stays the same, but the amphour capacity adds up. (So 2 6 volt batteries rated at 225 amphours connected in parallel = a battery bank of 6 volts, 450 amphours.)
  
• Click on this link to see my post about what kinds of batteries are used for renewable energy systems.

Step 5 : Get to work !

Now you know how many solar panels you need, how many batteries you need, and how much power you use on a daily basis.  You’ve done some of the hardest work already ! What’s left now, is choosing an inverter for your system, and getting the right electrical cable to go between the panels and the battery bank. This cable can be very expensive (It’s not regular power cable, it needs to be a higher gauge in order to not lose power) so keep the panels as close to the battery bank as possible !

Start shopping and go green !

If you’re looking for LOTS of information on solar power and wind power systems, you might consider buying these sets of e-books that are filled with tips and tricks for solar and wind power.

Feel free to leave any questions or comments below !