Sizing an off-grid solar system for air conditioning — best practices?

Calculating Total AC Wattage

To determine the size of the off-grid solar system needed to support air conditioning, first calculate the total wattage required by the AC unit. This typically involves multiplying the AC unit’s wattage by the number of hours it will be running per day. For example, a 9,000 BTU mini-split AC unit with an energy consumption of 0.75 kWh per hour would require 4.5 kWh of energy per day (9,000 BTU / 3.41 Btu/kWh = 2.64 kW, then 2.64 kW * 1.7 hours = 4.488 kWh * 1.05). This calculation should be based on a peak load assumption to ensure the system can handle the highest energy demand.

Battery Bank Sizing

Battery bank sizing is critical for an off-grid solar system to store excess energy generated during the day for use during the night or on cloudy days. A general rule of thumb is to size the battery bank to provide at least 14 days of energy storage for a single air conditioner, assuming 75% depth of discharge (DOD). In this example, the required battery bank capacity would be approximately 63 kWh (4.5 kWh/day * 14 days). This calculation should take into account the AC unit’s energy consumption, the number of days the system needs to run on stored energy, and the battery’s capacity and DOD.

System Design Considerations

When designing an off-grid solar system for air conditioning, it’s essential to consider system losses due to inefficiencies in the solar array, charge controller, and inverter. A good rule of thumb is to oversize the solar array by 10-20% to account for these losses. Additionally, the system should be designed to account for the AC unit’s startup surge, which can be up to 2-3 times the unit’s normal operating wattage. This can be achieved by sizing the solar array and battery bank accordingly and using a high-performance inverter to handle the surge.