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Determining your solar power requirements and planning the number of components.

Important Initial Considerations
The following information is a general guide for sizing, but not intended for more critical applications or remote sites requiring very high reliability. These types of systems require extensive analysis of regional climate history, site specific data, expert understanding and selection of system components and should be designed by professionals. For example, among other things we use highly proprietary performance analysis software and climate histories of 30 or more years when planning for applications requiring very high or no-fail reliability to paneles solares

In sizing an electric system using solar power the first two factors we consider are the sunlight levels (insolation values) from your area and the daily power consumption of your electrical loads. Orientation of a solar array is best at true south. True south is slightly different than a magnetic reference or compass south. The more an array is situated off of true south the less the total insolation value. A quick way to determine solar south is to divide the span of time between sunrise and sunset in half. The position of the sun at the resulting time would be true solar south.

The angle of the solar array can be anywhere from your latitude plus 15 degrees to latitude minus 15 degrees for a yearly fixed mount position. Your latitude offers the best year-round position. By biasing the array “latitude plus 15 degrees” you will get slightly more insolation during winter months. A “latitude minus 15 degrees” will bias the array to summer months.

Insolation, or sunlight intensity is measured in equivalent full sun hours. One hour of maximum, or 100% sunshine received by a solar panel equals one equivalent full sun hour. Even though the sun may be above the horizon for 14 hours a day, this may only result in six hours of equivalent full sun. There are two main reasons. One is reflection due to a high angle of the sun in relationship to your solar array. The second is also due to the high angle and the amount of the earth’s atmosphere the light is passing through. When the sun is straight overhead the light is passing through the least amount of atmosphere. Early or late in the day the sunlight is passing through much more of the atmosphere due to its position in the sky. Sun tracking devices are available and can help reduce reflectance, but cannot help with the increased atmosphere in the sun’s path.

Because of these factors the most productive hours of sunlight are from 9:00 a.m. to 3:00 p.m. around solar noon (solar south). This is different than 12:00 noon. Before and after these times power is being produced, but at much lower levels. When we size solar panels for a solar power system, we take these equivalent full sun hour
figures per day and average them over a given period. You can quickly refer to Solar for U.S. Major Cities, and then come right back here. Just close the new window that appears.

For a view of global solar insolation values (peak sun-hours) use this link: Global Peak Sun-hour Maps
Then, you can use [back] or [previous] on your browser to return right here if you want to.

In most locations in the United States winter produces the least sunlight because of shorter days and increased cloud cover, as well as the sun’s lower position in the sky. Usually, we work with a yearly average, a June – July average when insolation is highest, and a December – January average when insolation is lowest.

The diagram above illustrates the path of the sun over varying seasons. Remember when selecting a site for your solar power panels to pick a spot that is clear of shade from a minimum of 10 A.M. to 2 P.M. on December 21st. Even a limb from a deciduous tree will substantially reduce power output.

Many solar sites are quite uncomplicated in terms of shading and aspect. You may already have a good idea of where the sun appears in the morning and disappears in the evening, as well as how low it swings in the winter sky. If your site is partially shaded, it may be necessary to determine exactly where the best placement of solar panels will be. If you need a more sophisticated site analysis, please contact us. We also have world-wide insolation data as well as more local data that can be useful for your particular location.

Nominal DC System Voltage
Since solar panels charge your battery and these are both typically low voltage DC items, it’s best to decide up-front what your nominal DC voltage will be. The decision of which DC voltage to use is often dictated by the distance between the various components. For example, with solar panels wired at 12 volts charging a 12 volt battery it is difficult to “push” the 12 volts very far, so if the solar array is going to be more than 75 -100 feet from the batteries it would be advisable to have 24 volt nominal charging since 24 volts will push farther than 12 volts over the same wire size. Rather than increase the wire size to the thickness of your thumb as in a AWG#0000 (4 ought) cable to carry the 12 volts efficiently, it’s usually advisable to use 24 or 48 volts and keep the wire sizes between components much smaller. For further reference click the link below or contact us for assistance.

Wire Loss and Ampacity Tables – to determine voltage drop and wire size for various distances


1. Load Calculation Work FormThis worksheet determines the total amp hours per day used by all the AC and DC loads in your system.
CLICK HERE – Print out a form  to apply your own data.

Step 1 Calculate your AC and DC loads.
List wattage and hours of use per week (or other period) for all loads in the spaces provided. Multiply Watts by Hours/Week to get Watt-Hours per Week (WH/Wk.) for each load. Then add up all the watt hours per week to determine total Watt Hours Per Week. For total home systems that have a grid-connected electrical history you can simply use the kWh per month from your bill and convert this into a weekly figure, where the monthly kWh (X) divided by 4.3 times 1000 equals your average weekly watt-hours per week.

The form requests weekly totals,
but you can change weekly watt-hours to daily or any period which applies to your particular situation by simply modifying the time period that you’re working with…as long as you establish Ah/day in Line #10.

Note: Wattage of appliances can usually be determined from tags on the back of the appliance or from the owner’s manual. If an item is rated in amps, multiply amps by operating voltage to find the watts. Another way to more accurately calculate your AC loads is to use a power meter. We sell various power meters that simply “plug in” and you read the actual wattage. These are very handy for planning a solar power electric system, but also very useful to have around after you get your system up and running. These power meters start at $99, but can often save you by more accurately calculating your actual loads for specific items. Contact us for more information on the power meter.

Wire Loss and Ampacity Tables – to determine voltage drop and wire size for various distances.


Inverters are rated in continuous wattage and surge watts. Continuous watts is the total watts the inverter can support indefinitely. So a 4000 watt inverter can power up to 4000 watts continuously. Surge watts is how much power the inverter can support for a very brief period, usually momentary. So a 4000 watt inverter rated at 7000 surge watts can handle up to 7000 watts momentarily while starting such loads as motors – which usually require more than normal power to get started.

Go to Inverter Terminology
to better understand equipment specifications
(opens a new window – use [X] to close)

To select the appropriate inverter size, refer back to the LOAD CALCULATION WORK FORM and add up the wattage of your specific items which will (or potentially can) operate simultaneously to determine the minimum continuous watts you need. Then, also look at the potential surge of the specific items to determine the minimum surge wattage you’ll need. Usually, you’ll need 1.5 to 2 times the continuous rating. Some deep well submersible pumps can require 3 times the surge protection. We can assist you with this if you have any problems determining either continuous or surge requirements.

Finally, if any of your specific items operate at 220-240 volts you’ll need either a step-up transformer – which will also give you the 220-240 volts for one or more items, or you can “stack-interface” two inverters to produce both 120 and 240 volts. We can assist you with this if you’re not sure which way is better for you.

3. Solar Array Sizing Work Form

This type worksheet helps figure the total number of solar modules required for your system.
CLICK HERE – Print out a form  to apply your own data.

To find average sun hours per day in your area (line 3), check local weather data, or go to the Solar Energy Maps page. If you want year-round reliability, it’s best to use the lowest of the figures or “smooth” the data. The peak amperage of the module you will be using can be found in the module specifications. You can also get close enough for this basic understanding if you divide the modules wattage by the peak power point voltage, usually (17 to 18.5.