Determine Your Solar Panel 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, and RV Solar Panel systems, you can just list all the items you want to run with solar electricity, how much power they take, plus  how long you want to use them. Here is an example for determining how many RVsolar panels you need for and RV solar panel solar electric system:

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 electric 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 amps.   That will give us a little extra to make up for cloudy days. This gives us the total number of solar modules needed to give us energy independence and to keep our solar electrical system running effortlessly, economically, quietly, for years to come. Now we just need to size the DC/
AC inverter, figure the number and size of batteries, then  we have a solar electrical system tailored to your needs.

Inverter Sizing:

The size of DC/AC Inverter for your solar energy 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 solar 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 DC/AC 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 solar 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 an electrical 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 30% 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 solar battery bank we require.

Solar 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.

Join the The Sun Works mailing list 

Energy Efficiency and Renewable Energy Network (EREN). This is a good primer on photovoltaic systems and includes sizing information.

Million Solar Roofs.  US Department of Energy site giving all sorts of Solar information.