Off-Grid Electricity — Part 2

Larger Off-Grid System

In the first part we offered a minimalist system to supply power for lights in an off-grid environment. Here we will do the homework necessary to size a larger system. In this system we will assume your electric needs will be at 120 Volts AC.

There are three things you need to know in order to size the system:

  1. The maximum power you will consume
  2. The total energy you will consume in a 24 hour period
  3. The amount of solar energy you receive

You need to know the maximum (peak) power consumption in order to size the inverter — the unit that converts the DC from your solar cells and battery into 120 Volts AC. I recommend making a table where you can note this information. For example,

DescriptionWattsHoursTotal WH
Phone Charger15460

In this table there are four items listed. In the Watts column you put the power it uses when operating. The total of this column shows the maximum power you will be using, in this case 515 watts. This tells you the power your inverter must be capable of producing.

In the Hours column, put the number of hours the item will be operating per day. Note that I only put 4 for the refrigerator. Refrigerators have a compressor which cycles on and off when needed to maintain the set temperature. How many hours will be a function of how hot the room temperature is, how well insulated the refrigerator is and how often you open the door.

The Total WH column is just the Watts multiplied by Hours. The total of this column is the amount of energy you use in a 24 hour period. You use this number to both determine battery size needed and the size of the PV panels you need to recharge the battery. Note that not everything is 100% efficient so you are better of to err on the high end. In this example the Total WH comes out to 755 so rounding up to 1000 will both make your arithmetic easier and will give you some space to allow for system losses.

If you don’t have a good idea how much power something uses in a 24 hour period (the refrigerator, for example) you can buy an inexpensive meter that measures both Watts and Watt Hours and run a test. For example,

will do the job. They cost $15 or less.

You now need to calculate the amount of solar panels you would need to charge the batteries. This will depend on how sunny your location is. Where I live it is sunny most of the time during maximum sunlight times so I use about six watt hours of energy produced per day for each watt of solar panels. A 200 watt panel would be more than sufficient for me. But, if I was in Seattle, for example, I would probably use two watt hours per watt in my calculation which would mean a 500 watt panel would be needed.

In order to size the system components you will need to decide on a battery voltage. For a system this small, a 12 volt system would probably sufficient. Assuming a 12 volt system, you would use 62 Ampere Hours (755 / 12) of energy per day. A 100 Ampere Hour battery might be sufficient but you would probably be better of with something larger to cover cloudy days.

You also need a charge controller. It is basically a regulator that prevents you battery from being overcharged. There are two types and the type you want is called MPPT, Maximum Power Point Tracking. With this type of controller you don’t need to be concerned about the PV panel voltage vs. battery voltage (as long as the panel voltage is higher than the battery voltage). If you have a 200 watt panel an MPPT controller that can handle 10 amperes is sufficient. If you went with a 500 watt panel you would need a 40 ampere controller.

To complete your system you will need mounting hardware for the solar panel, wire, circuit breakers and other assorted hardware. If this is sounding way too complicated, jump into the forums ( and either explain what you intend to do and ask if it sounds reasonable or just identify what you have for loads and ask for help.

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