off-grid

Planning / Design Off-Grid Solar System

by Kristie H. on June 20, 2012

Modern battery-based off-grid solar systems offer more than just a DC-to-AC power source, they can charge batteries, select from multiple power sources, and control external functions. Here are a handful of steps you will need to take to plan for your off-grid purchase.

Appliance & Equipment Load List: List and add up your daily electrical equipment load demand in watt hours (wh):

Watt hours (wh) and amp-hours (aH); Watts = Amps X Volts. Watt hours are the most common measure of electricity usage and are the easiest to understand. Amp Hours = Watt hours / System Voltage. Many professional system designers will use amp hours to size a system because amp hours takes into account real world behavior of solar panels and battery banks. Either method will arrive at the same conclusion if done properly. For our purposes here, we will primarily use watt hours (wh) when sizing the number of solar panels (and inverter) and amp hours (aH) for our battery selection.

Energy Load Worksheet

TIP; What’s the difference between watts and watt hours? A watt = one joule “per second” and is a measurement of the rate of power flowing. 1 watt is a unit of energy equal to the power of one watt operating for one hour (3,600 joule’s). Watt and watt hours are often interchanged and misused. 1 watt hour (wh) = 1 watt of power supplied for 1 hour. Think of watts (w) as the speed you’re running and watt-hours (wh) as the distance you ran. A light bulb rated at 20 watts, in 1 hour it will consume 20 wh, and in 5 hours it will consume 100 wh.
System Voltage:

Higher voltage means less resistance which allows equipment to run cooler. Cooler electrical equipment = longer life.

Your system voltage means the nominal voltage you select for your battery bank, charge controller and inverter (if you are planning to use one). Here are some things to consider when choosing your systems voltage:

The DC system voltage is established by the battery bank in off-grid systems. A major factor in making this decision is how much power will be required from the batteries. As power demands increase it is advisable to raise the battery voltage. This voltage is important because establishes the type of charge controller and inverter that will be selected. The selection of the battery bank voltage affects the currents. A 1200 watt off-grid system operating at 12 volts draws 100 amps. (1200w / 12v = 100A). The same system draws only 25A at 48 volts. Lower amps reduces the size of conductors, over current protection devices, disconnects and charge controllers. Additionally, since voltage drop and power losses are smaller at lower amps, higher voltage off-grid systems are more efficient. As a rule of thumb, off-grid systems up to 1000 watts use a minimum 12 volt battery bank which limits DC currents to less than 84 amps. For 2000 watt systems, 24 volt battery banks are used. For 5000 watt system a 48 volt battery bank should be selected.

TIP; How long each day does a refrigerator run? 120VAC Refrigerators can be a large load for off-grid systems or they can be no draw. How often a refrigerator will run during a 24 hour day depends on many factors and will vary from home to home for the same refrigerator. Some of the variables include; the room temperature throughout the day, how full your refrigerator is (Full refrigerators run less often once the contents are cooled), and how often you open the door throughout the day. You can lower the run time of your refrigerator by turning off the automatic defrost function and keeping your refrigerator full. One off the grid living trick is to keep full jugs of water in a 1/2 full refrigerator which can help. Once the contents of the refrigerator are cooled, an energy efficient model in your home that is full might be expected to run 24% – 35% of the time with a room temperature of 70 degrees or about 6 hours out of 24.

Daily Off-grid System Charge Requirement in Amp Hours:

Since the energy output to the loads must be balanced by the energy input from your solar panels and or wind turbine, we need to calculate your daily charge requirement in amp hours as that number will come in handy later. Take your total daily what hours x 20% (rule of thumb) to account for losses in inverter, circuits and wire transfer. Now divide by the system voltage you chose based on the previous section and write this number down. This is the charge in amp hours your solar panels will have to provide each day to meet your load requirements you have set. Example 5,000 watts daily load total X 20% = 6,000 watts / 48 volt system = 125 amp hours that will need to be generated. If you follow those steps, you are on your way to putting some bling in your off-grid system and enjoying the zen it can bring.

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