Sizing up a solar lighting or power system can be a difficult task. There are two major ways for getting all the calculations correct for two completely different types of systems; grid tied and off grid. Understanding the way your system will operate will make sure you get the correct information to size the system properly.
Grid Tied System Sizing
Grid tied solar lighting and power systems use averages to size the system. The average sun hours are used to calculate the available sun for your area and the systems are sized according to that. They also use the average length of night and the average power consumption.
For example, if you have a 100 Watts operating 24/7/365, and you have an average of 4.5 sun hours, you would need around 750 Watts of solar power. During times when the sun is stronger and you have more available sun, this would feed the grid more than what is being used. During times when the sun isn’t as strong, the system would pull energy from the grid to offset what the panels couldn’t do.
This is a NET Zero application for grid tied systems and is how most homes and buildings that are still grid tied are designed. If less solar is installed, the system offsets only a percentage of the total electrical usage; however, in either situation, the systems shouldn’t fail since grid power still exists.
Off Grid System Sizing
Off grid system sizing is completely different than grid tied. Worst case scenarios are used to determine the sizing of the system, which typically includes the longest night of the year as the operation time and the lowest amount of sun available, usually December sun hours (insolation). Other factors such as low temperatures and humidity conditions are also taken into account with this size system.
Worst case sun hours vary from region to region. Here in most of Florida, we have over 4 hours of sun in December. Areas like Arizona have upwards of 5.25 sun hours. Maine has 2.5-3 sun hours and most of Washington State has less than 2 sun hours. Overcast weather affects this greatly and should also be remember during the sizing process.
The longest night is also a factor for dusk to dawn operation. Florida has 13 hour nights, but Maine has 14.75 hour nights. If the systems can have different control option, for example using a split time operation turning off in the middle of the night, this can greatly impact the system sizing and still allow for good system design.
Since off grid systems are just that, off grid, battery storage systems are used as a backup. Autonomy, or backup power, is provided for a minimum of five days in properly sized systems, longer in colder regions since the depth of discharge on the batteries is greater in these climates. This means, if the area of installation drops to zero degrees or below Celsius, or 32 degrees or below Fahrenheit, you can lose up to 20% of the battery’s capacity. Once you hit -20°C or -4°F, you are down to 50% capacity. Having only a three day backup built into the system will cause the system to fail quickly.
Taking into account all these different factors when sizing an off grid system and looking at the worst case scenario for conditions, will ensure that the system is properly manufactured to operate during the worst time, and therefore, flawlessly the rest of the year. This is essential when there is no backup grid power available to a system.