Filed under: Projects. Tagged as: 12v fluorescent light, barn light fixtures, barn lights, barnlight, barnyard lighting, batteries for solar lights, indoor solar lighting, solar barn, solar lighting system.
One of my readers left a note in the comments asking me for advice on how to build a solar lighting system to run as barn lights in the evening. This article shows how I would go about building one, and includes the math on how to adjust this idea for your own setup.
Before I go into too much detail, here is what the original reader sent me:
Chris, I would like to light my barn for feeding, or working after dark. I would like 2-4′ fluorescent lights. I was thinking of solar panels, batteries, & inverter. What’s your recommendation for a set up like this?
Well Jeff, here is how I would go about creating solar barn light fixtures. I went ahead and assumed 2 foot lights. If you need more barnlight, just duplicate the 2 foot setup, or rework the math below to compensate for the power (watts) used by a 4 foot bulb. These kinds of indoor solar lighting applications can get expensive quickly. That’s another reason for assuming 2 foot lights: I want want to keep costs down.
Choosing Bulbs for Solar Barnyard Lighting
The first place to start is to pick the bulbs you want to use. Once you pick those, then you can size the battery properly. With the lights and batteries nailed down, you can figure out how big of a solar panel you’ll need.
Doing a quick look around Amazon, I found these 2 foot, 14 watt bulbs that should work nicely as a 12v fluorescent light. Most lighting systems will take either two or four bulbs. Since I can’t know which kind you’ll have, I’ll just do the math per bulb. Just add these power/amperage numbers for each bulb you want to operate.
The first step is to figure out how much current this bulb is going draw from your battery. The light is rated for 14 watts, but for the sake of simpler math, we’ll add 4 watts. This compensates for losses through wiring, the inverter, etc. This will make a pessimistic estimate, which is good. At 18 watts of power, you’ll be drawing 1.5 amps from a 12 volt battery:
Choosing Batteries for Solar Lights
Now, you need to estimate what the longest amount of time is that you’ll want to run the barnlight for. Since you mentioned working after dark, I’ll assume 4 hours is the maximum amount of time you’d run them. A 1.5 amp draw for 4 hours works out to be 6 amp-hours.
Knowing the amp-hour draw, you can properly size your battery. Remember: discharging a battery below 80% of its rating will cause deep-discharge damage. Be sure to multiply a batteries amp-hour rating by 0.8 to compensate for this.
For example, this 12 volt, Group 24 Deep-Cycle battery is rated for 75 amp-hours of capacity. Multiplying it by 80% tells us that we have 60 amp-hours of effective capacity:
Diving the 6 amp-hours (per bulb for 4 hours of operation) into the 60 amp-hour of effective capacity of the battery, we know that this battery can power 10 bulbs (for 4 hours). It could also power fewer bulbs for longer, but the take away from all this math is that a deep-cycle Group 24 battery should be able to power your barn lights.
Choosing an Inverter For Your Solar Lighting System
It’s also important to pay attention to the power rating of your inverter. Assuming you wanted to take this setup to the max (10 bulbs), and each bulb is consuming 18 watts of power, then you need an inverter that can put out 180 watts of power. You could get away with a 200 watt inverter, but this 400 watt inverter is inexpensive and has gotten some great reviews on Amazon:
Choosing Panels For Your Solar Barn
Whew! So we’ve got the lights, the battery, and the inverter nailed down. You can finally choose a properly sized solar panel. Since the size of the panel will determine how quickly the battery will charge, I have to make some some more assumptions as to how you want to use you solar lighting system. Typically, I would assume that you want to use your lights on a day-to-day basis in the winter. This means the batteries have to be fully charged in a day and that we may only have the equivalent of 6 hours of light on the solar panels. This may still not be enough in the dead of winter, but it’s a good design target.
The number of lights you run will also play a role here. If you power fewer lights than 10, you won’t need as large of a panel. In fact, for this part of the design, I’ll assume you only need enough power from your solar panel to replace the power consumed by 4 bulbs. This means that the panel only needs to recharge the battery over the course of 2 days, which means you can use a significantly cheaper solar panel. A bulb at 18 watts for 4 hours means you’ll consume 72 watt-hours per bulb:
Assuming you run 4 bulbs, you’ll have to replace 288 watt-hours of power each day:
Assuming the sun only shines on the solar panel for 6 hours in the winter, this means you’ll need a 48 watt panel or larger:
This 60 watt solar panel is a great deal and it’s gotten a couple good reviews:
You’ll also need a charge controller. You’ll fry your battery if you connect this panel up to it directly. I’m a fan of the Sunforce charge controller, but the Kintrex controller has a low-voltage cut-off that will protect your battery from deep discharge damage. This feature will allow you to get significantly longer life from your battery.
Assembling the Barn Light Fixtures