4 Tips for Sizing a Solar Battery System

solar tubes
  • 1-5 hours
  • Intermediate
  • 500-5,000

Sizing a solar battery system is a lengthy process with complex calculations and trade-offs to consider. To make it easy on yourself, do the calculations in the correct order. Be realistic about your budget constraints and the system's maximum output. Design a system that can create the greatest reduction in your monthly electric bill. Begin by analyzing your building’s electrical load. Size your solar array and inverter based on the load. Choose a battery size that balances your capacity requirements with the maximum charge and discharge rates dictated by your other equipment.

1. Calculate the Electrical Load

Check the electrical draw of all your appliances, light fixtures, computers, entertainment devices, and portable electronics. Multiply the amperage and voltage ratings to calculate the wattage. A device that draws one kilowatt and runs for an hour consumes one kilowatt-hour. This is the common denomination for residential electricity sales. If you are installing a grid-connected system, you can review previous months’ electric bills to determine your total load. Electrical motors, such as those found in the compressors of air conditioners and refrigerators, draw large amounts of energy to start turning. You might not have the space or funds to install a net-zero system. However, you can still install panels to supplement your energy use and reduce your bill. On the other hand, you may wish to store energy collected during the height of the day to displace metered electricity used for cooking and water heating in the evening.

2. Evaluate the Site

solar panels on a tile roof

The total potential output of the photovoltaic array depends on several factors, including latitude, weather, shading, installation area, installation angle, and distance from the electrical load. Professional installers use worksheets and solar pathfinders to make highly accurate estimates of the panel’s output. You may choose to install the array on your roof or on mounting brackets in the nearest clearing. The distance between the panels to the battery will affect the voltage of your system. Wire the components together in 12 volt modules when the panels are closer than approximately 100 feet, use 24 volt wiring for longer distances. Wiring batteries in a series will increase their voltage, whereas wiring them in parallel increases the amperage.

3. Choose Inverter

Inverters are used to convert low-voltage solar power to 120 volt household electricity. If you intend to power 240 volt circuits, you will need to install a step-up transformer as well. Inverters are rated for sustained and surge wattages; choose an inverter based on your previous load calculations.

4. Choose Battery Size

Battery capacity is rated in amp-hours. Divide the electrical load in kilowatt-hours by the battery voltage to find thousands of amp-hours. Deeply discharging lead acid batteries can shorten their life. Lead-acid batteries also lose up to 50 percent of their capacity in cold weather. To find your capacity requirement, multiply the electrical load by the time it will depend on the battery. Divide by 0.8 to factor in a 20 percent discharge reserve. The solar array must be large enough to completely recharge the battery during the next day. Most solar harvesting occurs between nine AM and three PM, so multiply the array’s output in amp-hours by six.