When you design off-grid system, you need to size your system, we would like to guide you through the calculations.

In general the system should be big enough to supply all your energy needs for a few cloudy days but still small enough to be charged by your solar panels. Here are the steps to size your system, there are inverters, batteries and solar controllers needed.

To determine the inverter size we must find the peak load or maximum wattage of your home. This is found by adding up the wattage of the appliances and devices that could be run at the same time. Include everything from microwaves and lights to computers and clocks. The sum will tell you which inverter size you need. Don't forget that some appliances take more than their rated power at start-up. The inverter's surge rating should cover these temporary increases. For example: A room has two 60 watt light bulbs and a 300 watt desktop computer. The inverter size is 60 x 2 + 300 = 420 watts

Next find the energy the home uses in a day. Figure out how long each electronic device will be run in hours per day. Multiply the wattage of each device by its run-time to get the energy in watt-hours per day. Add up all the watt-hour values to get a total for your home. This estimate is likely too low as there will be efficiency loses. To get a rough idea of the real value with system loses, multiply by 1.5. This will help account for decreasing performance when temperature increases. For example: Light bulbs run for 5 hours a day. Computer runs for 2 hours a day. 120 x 5 + 300 x 2 = 1200 watt-hours. 1200 x 1.5 = 1800 watt-hours

Now decide how many days worth of energy you want to store in your battery bank. Generally this is anywhere from two to five.

Finally we can calculate the minimum battery AH capacity. Take the watt-hours per day and multiply them by the number you decided upon in step 3. This should represent a 50% depth of discharge on your batteries. Therefore multiply by 2 and convert the kwh result into amp hours (AH). This is done by dividing by the battery voltage. For example: You want the battery bank to last three days without recharging and you use 1.8 kwh per day. As 1.8 x 3 x 2 = 10.8kwh, this is the capacity we need from the batteries. Converting this to AH we have to divide by the voltage of your system. This can be 12, 24 or 48 for commercial application. If we choose to use 48V, the minimum AH capacity is then 10 800/48 = 225 AH. Now if you divide by your battery's rating you find the number of batteries you must use. Careful, this only applies to certain wiring setups.

Then, you also need solar controllers,solar controller sizing is the next step when sizing your system. As you have probably not yet encountered these components we will briefly discuss them.

Charge controllers regulate the power coming from the solar panels to the batteries. They are a key part of any off-grid system and prevent batteries from over-charging. We will discuss two kinds of charge controllers: PWM and MPPT.

PWM (Pulse-Width Modulation) controllers are cheaper than MPPT but create large power loses. Up to 60% of power can be lost. This is because PWM controllers do not optimize the voltage going to the batteries. This limitation makes a PWM controller a poor choice for a large system. However, in smaller systems their low price makes them a viable option.

MPPT (Maximum Power Point Tracking) controllers optimize the voltage coming from the solar panels so that the maximum amount of energy is transferred to the battery bank. The maximum power point, or the optimal conversion voltage, will fluctuate with changes in light intensity, temperature and other factors. The digital optimization process performed by the MPPT controller find and adjusts to the maximum power point quickly. Sophisticated electronics are needed in MPPT controllers to do this, which explains their high price. There is a significant pay-off though: MPPT controllers are 93-97% efficient in converting power.

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Once you have sized your battery bank and solar panel array, determining which charge controller to use is comparatively straight forward. All we have to do is find the current through the controller by using power = voltage x current. Take the power produced by the solar panels and divide by the voltage of the batteries.