Designing an Off Grid Solar Electric System, Step 3a, Determine the Amount of Solar Needed Using an MPPT Controller

The basic calculation for this step is:

(Load) x (Recharge Factor)  x (MPPT Controller Loss)  /  (Sun Hours) = (Array Size Needed)

We know the load from Step 2.

The recharge factor is a way to account for the probability of a clear day. Without this factor if it is cloudy today you will not be able to fully recharge the batteries tomorrow.  Without getting into too much detail on determining this number here are some basic rules to help.

Recharge Factor

1.15 < Use this factor if this is not a critical load and if it is ok if the system fails to operate.  This could be an area light in a parking lot and if it doesn’t turn on for a few nights a year that is ok.
1.2 <  I use this number as my default. In most location this factor will provide a well design system that will operate all year long without failing.
1.3 < If the system is critical and can not fail then increase the recharge factor. This could be peice of industrial equipment and if it fails to operate might cause a disruption in service.

MPPT Controller Losses

Nothing operates at 100% efficiency so every time power is changed or converted there are losses to account for in the calculation. Because these controllers use slower computer processors to keep costs low they don’t always operate at the exact correct max output of the solar array. They are also converting DC voltages twice so while they offer a huge increase in power output you need to account for their losses. Unfortunately all of the data sheets and tech bulletins from MPPT controller manufacturers that I have reviewed didn’t list their efficiency or losses. Hopefully this will change over time but for now with limited information I would use a multiplier of 1.10 to account for controller losses.

Location, Sun Hours

(irradiance map links coming soon)

Find the location of the site. If you are in the USA use the above link for PV Watts as the best source to determine the suns strength. Please note that PV Watts is not for battery based system designs, we are only using it to find the sun hours for a location.  Above is also a link to some generic irradiance maps that show the minimum sun hours.

With PV Watts, enter your zip code to find the location. On the system info page enter 1 kW for the DC system size and if your system will not face south change the 180 to match the system site. For tilt angle, stepper is better for off grid systems. In the USA, the winter has the shortest days so we need to design and optimize the system for this time of the year. Start at 50 Degrees and run the program a couple of times to see if this is the right tilt for your location, vary the tilt up and down until you find the highest worst case sun hour.  The number to focus at on the result page is the minimum number under the column labeled Solar Radiation ( kWh / m2 / day). The minimum number will typically fall on either in December, January or February. This is the worst case sun for the location and the design parameter needed. This min number can be referred to as the sun hours and is essentially the number of hours. If you graph the output of the sun throughout the day and total the area under the graph is the sun hours.

For round numbers let’s use 3 hours.

Putting it all together:

(Load) x (Recharge Factor)  x (MPPT Controller Loss)  /  (Sun Hours) = (Array Size Needed)

(985 Wh/day) x (1.2 Recharge Factor) x  (1.1 MPPT Loss) = (1300.2 Wh/day)

(1300.2 Wh/day)  represent the amount of power we want the solar array to produce

(1300.2 Wh/day)  /  (3 sun hours) = (433.4 watts of solar modules is needed)

If using a 280 watt module this would round to 2 solar modules.