How Much Battery and Solar Panel for a 36w 120V LED Security Light?

The size of battery and solar panel needed to power a 36-watt, 120-volt LED security light for 12 hours a day depends greatly on your location. Factors such as the duration and intensity of sunlight, as well as cold weather and potential snowfall, play significant roles in determining the necessary components. Understanding these factors helps in ensuring that the system functions effectively throughout the year.

Factors Determining the Battery and Solar Panel Size

For a 36-watt, 120-volt LED security light operating 12 hours a day, the power consumption can be calculated as follows:

36 watts x 12 hours 432 watt-hours per day.

With inverter losses, you need approximately 500 watt-hours per day.

A key factor is how long the battery can run without solar power. Ideally, the battery should provide power for three consecutive days. Therefore, the battery capacity should be approximately 1500 watt-hours (125 ampere-hours).

Given that these are 12-volt batteries, a 125 ampere-hours (Ah) battery size would be roughly equivalent to two standard deep-cycle 12-volt batteries.

Note: This calculation assumes you have sufficient sun or battery backup. For extreme weather conditions, additional factors must be considered.

Factors Specific to Your Location

The solar panel size needed depends on your location. Generally, you can expect about half the rated output at noon on a sunny day. Over the course of 12 hours with 6 peak hours, the output can be estimated as follows:

At 100 watts per panel, with 6 peak hours, a 100-watt panel would produce 600 watt-hours per day.

This amount should be sufficient to charge the battery and provide power for the night.

Solar Charge Controller and Inverter: To maximize efficiency, a solar charge controller and an efficient 12V to 120V inverter are essential components of the system.

Note: If you are located in a northern region, solar power will not help during long winters when the sun does not rise. In such cases, additional battery storage or alternative power sources may be necessary.

Practical Example: My Personal Experience

I have a similar setup with a small solar panel and a small deep-discharge battery powering an electric fence to keep deer out of our vegetable garden. However, due to the long winter months with heavy snowfall, I have to take down the fence and bring the battery indoors by late October and cannot put it back up until the snow is mostly gone, usually in early April.

This highlights the challenges of relying solely on solar power in regions with harsh winters. Adequate planning and sometimes seasonal adjustments are necessary to ensure the system remains functional.

Conclusion

For a 36-watt, 120-volt LED security light, you need a combination of a battery and solar panel that can provide sufficient power for its operation. The exact size depends on the specific conditions of your location. With proper planning and occasional adjustments, you can ensure the system runs smoothly year-round.