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Determining Your solar power System Requirements
for your solar power electric system.

Links to view or print calculation areas below
1. Load Calculations
2. Inverter Selection
3. Solar Array Sizing
4.Wire Sizing
5. Battery Sizing


We encourage you to contact us for assistance.
We can often size your system in about 5 minutes on the phone, rather than you spend hours and hours trying to learn what you need to know to do it yourself. We'll explain all determinations so you'll understand it. But, if you just want to do it yourself first, then read on...however, please contact us so we can go over your determinations before you order your equipment.

In sizing a solar power electric system 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 Insolation 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 Map
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.

  1. Load Calculation Work form

CLICK HERE - Print out a form  to apply your own data.

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 shortening the time period
that you're working with.

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 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 $149, but can often save you by more accurately calculating your actual loads for specific items. Contact us for more information on the power meter.


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.

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 in form), check local weather data, or go to the Solar  Insolation for U.S. Major Cities  or   Global Peak Sun-hour Map   pages. 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

5. Battery Size Work form
This type of worksheet helps determine what size batteries are required for your system.
CLICK HERE - Print out a form to apply your own data.

Battery size is measured in AMP-HOURS. This is a measure of battery capacity.

All lead-acid batteries have a nominal output of 2 volts per cell. Actual cell voltage varies from about 1.7 volts at full discharge to 2.4 volts at full charge. 12 volt lead-acid batteries are made of 6 separate cells in one case. 6 volt batteries are made of 3 cells in one case. Industrial 2 volt single-cell batteries are also used in a series for larger applications. Series connections are where the positive terminal of one battery is connected to the negative terminal of another, resulting in increased voltage. Putting battery cells in parallel (positive to positive/ negative to negative) increases (amps) amp-hour capacity, but does not affect voltage.

This worksheet helps you figure the total number of solar modules required for your system. If you want year-round reliability, it's best to use the lowest of the Peak Sun Hours or "smooth" the data for seasonal applications. The peak amperage (maximum power) of the module you will be using can be found in the module specifications.
1. Total average amp hours per day from the System Loads Workform, line 10.
2. Multiply line 1 by 1.2 to compensate for loss from battery charge/discharge.
3. Average sun hours per day in your area.
4. Divide line 2 by line 3. This is the total solar array amps required.
5. Optimum or peak amps of solar module used. See module specifications.
6. Total number of solar modules in parallel required. Divide line 4 by 5.
7. Round off to the next highest whole number.
8. Number of modules in each series string to provide DC Battery voltage:
DC Battery Voltage# of Modules in Each Series String
9. Total number of solar modules required. Multiply line 7 by 8.
NOTE: There are other factors that will influence the actual solar array sizing that are not refelected here. This form is intended as a courtesy to the general public, for your basic sizing understanding and close approximation of the PV array, but does not replace the more sophisticated system design input used by us when directly assisting our customers.

Event Horizon Solar & Wind LLC

Battery Information

Battery Size The size of the battery bank required will depend on the storage capacity required, the maximum discharge rate, the maximum charge rate, and the minimum temperature at which the batteries will be used. When designing a power system, all of these factors are looked at, and the one requiring the largest capacity will dictate battery size. Our System Sizing work forms take many of these factors into account.

One of the biggest mistakes made by those just starting out is not understanding the relationship between amps and amp-hour requirements of 120 volt AC items versus the effects on their DC low voltage batteries. For example, say you have a 24 volt nominal system powering a load of 3 amps, 120VAC, which has a duty cycle of 4 hours per day. You would have a 12 amp hour load (3A X 4 hrs=12 ah). However, in order to determine the true drain on your batteries you have to divide your nominal battery voltage (24v) into the voltage of the load (120v), which is 5, and then multiply this times your amp hours (12 ah). So in this case the calculation would be 60 amp hours drained from your batteries - not the 12 ah. There are other factors for determining the full extent of the battery drain, such as temperature, start-up factors, etc., but this should help you get a more complete picture on how to size your low voltage batteries when powering 120/240 volt loads using an inverter. Our System Sizing workforms take many of these factors into account.

Temperature has a significant effect on lead-acid batteries. At 40°F they will have about 75% of rated capacity, and at 0°F their capacity drops to about 50%.

The storage capacity of a battery, the amount of electrical energy it can hold, is usually expressed in amp hours. If one amp is used for 100 hours, then 100 amp-hours have been used. A battery in a solar power system should have sufficient amp hour capacity to supply needed power during the longest expected period "no sun" or extremely cloudy conditions. In wind systems allowance for "no wind" days should be included. A lead-battery should be sized at least 20% larger than this amount. If there is a source of back-up power, such as a standby generator along with a battery charger, the battery bank does not have to be sized for worst-case weather conditions.

Series Wiring refers to connecting batteries to increase volts, but not amps. If you have two 6 volt batteries like the Trojan L16 rated at 350 amp hours, for example, by connecting the positive terminal of one battery to the negative terminal of the other, then you have series wired the two together. In this case, you now have a 12 volt battery and the rated 350 amps does not change. If you were to series wire four L16's you'd have 24 volts at 350 amps, and so on.

Parallel Wiring refers to connecting batteries to increase amps, but not volts. If you have two 6 volt batteries like the Trojan L16 rated at 350 amp hours, for example, by connecting the positive terminal of one battery to the positive terminal of the other, and the same with the negative terminal, then you have parallel wired the two together. In this case, you now have a 6 volt battery and the rated 350 amps increases to 700 amp hours. If you were to series wire four L16's you'd have 24 volts at 350 amps, and then parallel wire these four to the four other that are in series, then you'd have a 24 volt battery at 700 amps.

Lead-Acid Batteries are the most common in PV systems because their initial cost is lower and because they are readily available nearly everywhere in the world. There are many different sizes and designs of lead-acid batteries, but the most important designation is whether they are deep cycle batteries or shallow cycle batteries.

Shallow cycle batteries, like the type used as starting batteries in automobiles, are designed to supply a large amount of current for a short time and stand mild overcharge without losing electrolyte. Unfortunately, they cannot tolerate being deeply discharged. If they are repeatedly discharged more than 20 percent, their life will be very short. These batteries are not a good choice for a PV system.

Deep cycle batteries are designed to be repeatedly discharged by as much as 80 percent of their capacity so they are a good choice for power systems. Even though they are designed to withstand deep cycling, these batteries will have a longer life if the cycles are shallower. All lead-acid batteries will fail prematurely if they are not recharged completely after each cycle. Letting a lead-acid battery stay in a discharged condition for many days at a time will cause sulfating of the positive plate and a permanent loss of capacity.

Sealed deep-cycle lead-acid batteries are maintenance free. They never need watering or an equalization charge. They cannot freeze or spill, so they can be mounted in any position. Sealed batteries require very accurate regulation to prevent overcharge and over-discharge. Either of these conditions will drastically shorten their lives. We especially recommend sealed batteries for remote, unattended power systems, but also for any client who wants the maintenance free feature and doesn't mind the extra cost associated with these batteries.

Caring For Lead-Acid Batteries Always use extreme caution when handling batteries and electrolyte. Wear gloves, goggles and old clothes. "Battery acid" will burn skin and eyes and destroy cotton and wool clothing.

The quickest way to ruin lead-acid batteries is to discharge them deeply and leave them stand "dead" for an extended period of time. When they discharge, there is a chemical change in the positive plates of the battery. They change from lead oxide when charged to lead sulfate when discharged. If they remain in the lead sulfate state for a few days, some part of the plate does not return to lead oxide when the battery is recharged. If the battery remains discharged longer, a greater amount of the positive plate will remain lead sulfate. The parts of the plates that become "sulfated" no longer store energy. Batteries that are deeply discharged, and then charged partially on a regular basis can fail in less than one year.

Check your batteries on a regular basis to be sure they are getting charged. Use a hydrometer to check the specific gravity of your lead acid batteries. If batteries are cycled very deeply and then recharged quickly, the specific gravity reading will be lower than it should because the electrolyte at the top of the battery may not have mixed with the "charged" electrolyte. Check the electrolyte level in wet-cell batteries at least four times a year and top each cell off with distilled water. Do not add water to discharged batteries. Electrolyte is absorbed when batteries are very discharged. If you add water at this time, and then recharge the battery, electrolyte will overflow and make a mess.

Keep the tops of your batteries clean and check that cables are tight. Do not tighten or remove cables while charging or discharging. Any spark around batteries can cause a hydrogen explosion inside and ruin one of the cells, and possibly you too.

It is a good idea to do an equalizing charge when some cells show a variation of 0.05 specific gravity from each other. This is a long steady overcharge, bringing the battery to a gassing or bubbling state. Typically, we'll recommend an equalization charge at least once a month. Do not equalize sealed or gell type batteries. With proper care, lead-acid batteries will have a long service life and work very well in almost any power system.

Measuring battery condition Connect a voltmeter and measure the voltage across the battery terminals with the battery at rest (no input, no output) for at least three hours. These readings are best taken in the early morning, at or before sunrise, or in late evening. Take the reading while all loads are off and no charging sources are producing power.

The following table will allow conversion of the voltage readings obtained to an estimate of state of charge. The table is good for batteries at 77·F that have been at rest for 3 hours or more. If the batteries are at a lower temperature you can expect lower voltage readings.

You can see that when your voltage reading is about equal to
the battery "nominal voltage" your battery is about 60% discharged.

Battery State of Charge Voltage Table

Percent of Full Charge12 Volt DC System24 Volt DC System48 Volts DC System
The following chart reflects state of charge vs. specific gravity of the electrolyte in each cell.
A hydrometer is used to determine specific gravity.
State of ChargeSpecific Gravity
100% Charged1.265
75% Charged1.239
50% Charged1.200
25% Charged1.170
Fully Discharged1.110
These readings are correct at 75°F


Event Horizon Solar & Wind -
Battery Size Work form

This page should print-out as a single page.
1. Total average amp hours per day from the System Loads Worksheet, line 10.
2. Maximum number of continuous cloudy days expected in your area.
3. Multiply line 1 by line 2.
4. Divide line 3 by (maximum) 0.8 to maintain a 20% reserve after deep discharge period.
To prevent less than a maximum 80% discharge divide by a lesser number in #4 above.
If no special conditions below apply, skip lines 5 through 9 and proceed to line 10.
Special Conditon #1: Heavy electrical load
5. Maximum amperage that will be drawn by the loads for 10 minutes or more.
6. Discharge rate of battery. If unknown, check with battery supplier.
7. Multiply line 5 by line 6.

Special condition #2: High Charge Current
8. Maximum output amperage of PV array or other battery charger.
9. Multiply line 8 by 10.0 hours.

10. Amp hours from line 4, 7, or 9, whichever is largest.
11. If you are using a lead-acid battery, select the multiplier below which corresponds to the battery's wintertime average ambient temperature:
Battery Temperature Multiplier
12. Multiply line 11 by line 10. This is your optimum battery size in amp-hours.
13. Amp-hours of battery chosen. (L16=360, T-105=225 etc.)
14. Divide line 12 by line 13. This is the total number of batteries in parallel required.
15. Round off to the next highest whole number.

Wire Loss Table