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Sizing an "Off Grid" System

There are several considerations for sizing “Off Grid” Solar Power Systems. Some will depend on finances and available space, but here we will consider the primary design parameters.

The Solar Array

The solar array is sized so as to provide the required energy in the winter (highest consumption time)

It needs to collect enough energy to supply the following items:

  • Daily usage of all electrical devices
  • Inverter power consumption
  • Battery self-discharge, inefficiency and full and proper charging of the batteries.

As much as people always want to try and get away with the minimum size system and just calculate it that way, it is a poor way to approach sizing a solar array. It usually leads to an inadequately sized system which then leads to frustration and usually battery damage.

Consider the following battery recharge curve taken from the display of my system controller.

This picture shows the charging/discharging current to and from the batteries over about 1 ½ days.

As the sun comes up, it starts to charge the batteries. By about 10:00 or 11:00 AM in this case, the batteries reach the peak current draw and as they continue to charge, require less and less current until by about 12:30 or 1:00 PM the batteries are fully charged. In this example, the transition to Float voltage occurred about 2:00 PM.

The time period from the point where the batteries are no longer accepting all of the available power then is the best time to use power for laundry, cooking etc.

This also shows that if you were to size the system based on just the required loads, you would not have sufficient power to charge the batteries properly as some available energy generally has to be wasted for the finishing charge of the batteries.

In the above example, it was a sunny couple of days, so the peak of the curve is quite early in the day. On other days where it is heavily overcast, you may not even get a full charge, so it is quite important to actually “oversize” the solar array.

If the solar array is not sized large enough, then you will have a lot of generator run time and will likely sulfate the batteries.

The size of the Solar Array relative to the consumption in an “Off Grid” system in South or Central Alberta should be a minimum of ½ the daily usage. For example, if the projected consumption is 8 KWH per day, the minimum Solar Array should be 4 KW (4000 Watts). This will require a generator to make up the difference and may have some significant generator run time. Depending on the usage patterns, the ratio may go as far as about 1 ½ to 2 times the daily usage. For example, the same 8 KWH per day usage, if no generator is used, may require an array size of 12 to 16 KW (12,000 to 16,000 Watts). Most people will choose a size somewhere in between and have some generator run time. A 1 to 1 ratio provides a good economical balance where the generator run time is minimal. My system is sized to not require a generator.

The inverters

The inverters are sized to provide the power required to operate all the loads that may be on at any one time. This will be determined by adding up all the wattages of the various electrical loads.

Another word of caution here. It is generally not a good idea to run electronics at the maximum capacity for very long, even if they are rated for it. The heat generated will tend to shorten the life. Also, motor loads require a much higher current than what they are rated at while starting.

Most systems can also be configured to allow for future expansion if so desired.

The Batteries

The battery bank in an “Off Grid” system is usually sized for about a 3 to 5 day autonomy. What this would mean then is for example.

  • The total loads per day are calculated to be 8500 Watt hours
  • The system voltage will be 48 Volts
  • A 5 day autonomy is desired
  • Maximum discharge of the battery bank will be 50%
  • So, 8500 Watt hours multiplied by 5 days gives us 42,500 Watt hours required.
  • If we want a maximum Depth of Discharge of 50%, then we would require a battery bank rated at 85,000 Watt hours.
  • Batteries are rated in Amp Hours (which does not have the Voltage component). The Voltage component is the system voltage, therefore a 48 Volt system will have 24 cells, since each cell is about 2 Volts.
  • We would then choose a battery bank in Amp Hours (85,000 Watt hours divided by 48 Volts) rated at about 1770 AH at the 100 Hour rate.

Boyd Solar Electrical Power Sheet (PDF)

Boyd Solar Electrical Power Sheet (Excel Spreadsheet)

Putting it all together

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