Determine Your Needs
This is the most important factor in considering your solar system because this is the step that determines how much and what type of electrical energy you end up with. Step back and evaluate what you need, or want, to do with your solar generated electricity.
If all you need is light, entertainment, small pumps and fans you may be able to meet your needs with a 12 volt DC (direct current) system. If you want to buy solar to reduce your current electrical bill in your current home or business you may need a solar intertie system. (Look at your utility bill or click here for cost savings estimate
To determine your needs for independent solar systems, you can just list all the items you want to run, how much power they take, plus how long you want to use them. Here is an example:
DC Device 12v amps Hours/Day Daily Amp/ Hours
Radio 1amp X 5 = 5 amp/hrs
TV (12v 9") 3amp X 3 = 9 amp/hrs
Light 1.5amp X 4 = 6 amp/hr
________ _______ __ _______
sub total 20 amp/hr
If your needs or wishes include 120 volt alternating current appliances you can, look at your old utility bills (watts/volt=amps)) or, you can list all the appliances, their power requirements, and how long each is turned on. So you make another list:
AC Loads DC amps(AC amps X 10) Hrs/Day Amp/hrs/day
Microwave oven 125 amps X .25 = 31.25 amp/hrs
Computer 10 amps X 4.00 = 40.00 amp/hrs
Hair Curler 5 amps X .5 = 2.5 amp/hrs
DSS Satellite 2 amps X 4.00 = 8.00 amp/hrs
_________ ________ ______ ____________
sub total 87.75 amp/hrs
So then you add the two subtotals together : 20.00 amp/hrs
total 107.75 amp/hrs
So this is how many amp/hrs you want to use daily. Your solar panels will have to generate a little more than this to make up for battery and inverter inefficiencies. So multiply the total by 20% or .20 to allow for these.
107.75 amp/hrs X .20 = 21.5 amp/hrs
So we need to generate 107.75 amp/hrs for our loads and 21.5 amp/hrs for inefficiencies for a total of.
107.75 amp/hrs + 21.5 amp/hrs = 129.25 amp/hrs
Our solar system in this situation needs to generate 129.25 amp/hrs each day to meet our requirements. The sun shines more in some areas than others, and solar panels produce more during mid day than early or late in the day, so a standard was developed called "average peak sunlight hours " in order to calculate total amount of sunlight for different areas and seasons. For example early in the morning a 4 amp solar panel may only be producing 1 amp, then midday it may produce 4 amp, and again in the afternoon 1 or 2 amps again. In southern California we use the factor of 5 average peak sunlight hours. In other areas it could be more or less like on the coast it may be a factor of 3.5 or 4 for the average peak sunlight hours. So in California that 4 amp solar panel would produce 4 amps time 5 average peak sunlight hours for a total 20 amp/hrs.
So if we divide the total amp/hrs we want 129.25 by Southern CA average peak sunlight hours. 5 we get 25.85 amps of solar panels that we need.
Four 7.1 amp solar panels would give us a little hedge or about 28.4 amp/hrs/day . That will give us a little extra to make up for cloudy days. This gives us the total number of panels needed to give us independence and to keep our system running effortlessly, economically, quietly, for years to come. Now we just need to size the inverter, figure the number and size of batteries, then we have a solar system tailored to your needs.
The size of inverter for your solar system depends on the total amount of electrical appliances we run at a particular time. Your inverter needs to be able to handle all the things you are likely to turn on at the same time, plus a little extra to account for the extra power some loads need to get started. Inverters are usually rated in wattage continuous and surge. The continuous rating tells us the wattage that an inverter is able to run for long periods, the surge rating is what it can put out for long enough to start demanding appliances like motors , compressors, or water pumps. For example:
Appliance Watt Surge Watt
Microwave 1200 1300
Toaster 1200 1200
Washing Machine 480 1700
Lights 120 120
Totals 3000 4300
In this example we would recommend a 3000 watt inverter with a 4300 watt surge capacity or better. Of course we might want to watch TV and maybe check out a few web sites out on the computer so we may choose a larger inverter to take care of these extra loads. At any rate you get the idea. The more things we want to turn, on the bigger inverter we need.
Choosing the Type of Inverter You need:
The type of inverter you need again depends on your particular application. A modified sine wave inverter is less costly. This type of inverter works great for most small to medium sized systems. ( 50 to 3000 watt systems)
You might choose a pure sine wave inverter if you an audiophile and want to do sound recordings or listen to your music without static. Pure sine wave inverters are also better at starting motors under load like compressors or water pumps. You will also need one if you choose to feed your extra power back into the grid. This is called a solar intertie system.
There are two types of solar intertie systems. Solar intertie, and solar intertie with backup power capabilities. A basic solar intertie system can be sized without regard to your load. If you choose this type of system you do not need to calculate all the appliance wattage in the example above. These inverters just need to invert the power from you solar array into the voltage of your utility feed. In these systems the utility provides the extra power you need. When the sun is shining the solar modules convert the sun into DC (direct current) current) electricity the inverter changes the DC to AC (alternating current) synchronizes it with the utility power, and if you are making more than your house is using your power meter will run backwards. When you turn on a heavy load like air conditioning or at night, you use the utilities power. Your power bill at the end of the year is reduced by the amount or energy you fed back into the power grid. The basic solar intertie systems are becoming more popular all the time because you can enjoy the benefits of solar energy without the extra cost of a huge inverter or inverters and the cost of purchasing and maintaining large battery banks. The downside of these systems is that if the electricity grid is interrupted then your system needs to shut down also so that you stop feeding the grid to protect electrical personnel working on the disruption. If you need power even when the grid fails you should consider the solar intertie with backup capability.
The solar intertie with backup is a system that you can utilize to keep essential electrical appliances running in the event of a utility power interruption. When the electricity is disrupted the inverter still disconnects from the grid but continues to operate your critical loads. So with these systems you can continue to run your computer, refrigeration, water pumps, communication equipment, etc. We will need to figure out the total wattage and surge wattage of all these loads. Your inverter will need to be large enough to handle the combined power requirements of all of them. It is also often necessary to install a sub panel including all these essential circuits that will be run by the inverter in emergencies. Due to the need for larger inverters capable of handling your loads and the complexities of installing sub panels battery banks and solar charge controllers these systems cost about 20% more than the basic solar intertie systems. Next we need to calculate the length of time we expect to need to run these things in order to determine the size of battery bank we require.
Battery Bank Sizing:
In off grid solar systems and utility back up systems your solar energy is stored in batteries. The battery bank is used to provide the energy during the times when the sun isn't shinning. Off grid systems in general require a larger battery bank than a utility intertie backup system. In general we size off grid systems large enough to run your typical loads for five days. A typical backup system may be sized to handle from several hours to a couple of days. ( long enough for the utility to get their stuff together).
The battery storage capacity of an off grid system is dependent on your weather. If you frequently experience weather patterns of several days without the sun then you will need a larger battery bank than if you live in an area that is mostly sunny. Usually we size systems to be able to store enough energy for five days without sunlight. This means that we need to look at the number of amp/hrs we use in a day and then multiply by five.
In order to size a battery bank for a utility intertie backup system we need to figure the total amperage of the loads we are backing up and then decide how long in hours we need to back up and multiply. This is the size of battery bank we need for backing up our critical loads in the case of power failure.