Sharing solar technology with people who do not have electricity is a satisfying venture, and learning how to do it well helps us understand the basics of off-grid solar electric system sizing.
The three key design questions are:
• What is the electrical load?
• What PV array capacity is needed?
• What battery capacity is needed?
These three main parts of the system must be sized so that the load is well supplied, and the battery is regularly fully charged and never overdischarged. (A charge controller, which is not discussed here, would also be needed for this system to prevent the battery from being overcharged.) Finding that balance uses watt-hour math combined with some design experimentation and savvy.
Load. All off-grid PV system design starts with the load, how much energy is needed per day. Energy is measured in kilowatt-hours (kWh). In small developing-world systems, it’s measured in watt-hours (Wh). The typical system we install includes three to five DC LED lights and a USB outlet converter for cell phone charging.
Calculating array capacity is based on:
• Daily energy needed
• Peak sun-hours on the site
• Modules available
Example Small System Loads
In this design, we had several 20-watt modules available, and 4 peak sun-hours in the region. Using a basic formula (PV watts × peak sun-hours × 0.65 “reality factor,” which accounts for energy losses of an off-grid system and an unrealistic module rating system), we end up with 52 Wh per day from our 20 W module. This is almost three times the daily load and may seem excessive—but having this sort of headroom is wise with small systems, and accommodates some load growth.
Battery sizing considers:
• Daily load
• Days without sun (days of autonomy)
• Recharge time with chosen PV module
• Batteries available
When we went battery shopping, we found a 12 Ah, 12 V battery, which means its energy capacity is 144 Wh. We routinely cut this number in half to give us the usable capacity, basing our design on an average 50% depth of discharge (DOD). This leaves us with 72 Wh of usable capacity. An 18 Wh daily load will only cycle the battery down about 12.5%. If we design based on a maximum of four days without sun, we are right at 50% DOD.
Also important is how long it will take to fully recharge the battery after four rainy days. With our “extra” PV charging capacity included in our 52 Wh per day, and assuming we continue to use the load at the full 18 Wh per day, it will take just over two days to catch up and fully recharge the battery (52 – 18 = 34 Wh extra per day; 72 ÷ 34 = 2.1 days). And this all assumes that the load doesn’t grow, which is not always a safe assumption.
Finding the balance between the electrical load, solar generation, and battery capacity is a bit of math and a bit of art based on experience, and must take into consideration the real conditions at the site and product availability. Understanding the basic questions to ask will get you off to a good start for your own solar-electric system.