I like the concept of solar lighting but don’t like the choices. Mass marketed solar landscape lighting is pretty wimpy and in its current form will have difficulty competing with traditional low voltage landscape lighting. Being the Techno-nerd I am, this seemed like a great opportunity to create a centrally powered solar landscape lighting system. By appearance, it looks a lot like a low voltage light system you would buy from a home center, but with notable differences:
Removed: AC Transformer, Incandescent Light Bulbs
Added: 15 Watt solar panel as a power source, 35AH 12 volt solar battery for power storage, and 6- 21 pin LED light bulbs with a MR-16 base (MR-16 base is used in most landscape light fixtures)
The most important addition was the use of LED light bulbs in lieu of traditional incandescent bulbs. Granted, the LED bulbs are not cheap but may last over 25 years running them 4 hours a night. When I started this project I paid close to $20 a bulb. Today, that same bulb sells for about $8-$10. The light bulbs alone change the power usage from 500 Watts per hour to less than 15 Watts.
Standard 12 Volt AC Landscape Lighting Systems: This drawing is a general depiction of a landscape lighting system including 8 light fixtures with a 500 Watt transformer. In this scenario, using the 80% rule you have 420 Watts of usable power. If you used 50 Watt bulbs you have a budget of a little over 8 fixtures.
Solar Powered 12 Volt DC Landscape Lighting System: For me, it’s cost prohibitive to build a 500 Watt solar lighting system. By converting the bulbs to LED and reducing the fixture count to 6, I created a similar system using solar power. Granted, the brightness will be good but not near as bright as the 50 Watt incandescent bulbs, but 6 to 10 times greater than the current breed of solar light fixtures on the market today. If you look at the second drawing, I have removed the transformer, reduced the fixture count to 6, removed the connection to the utility AC and added the solar equipment and battery.
The Design: The drawing of the 12 VDC system is very similar to the 12 VAC system. Since I built the system from scratch, I allowed for growth considerations by enlarging the wire and battery sizes. If you are considering converting an existing system, validating your voltage drop numbers may alleviate future problems. Changing the bulb size or the number of bulbs can also alter the calculation, so adjust the calculation and components as necessary. Here is the design criteria:
- System controller has a non-adjustable LVD (low voltage disconnect) at 11.7 VDC (per Morningstar)
- System controller will re-connect at 12.8 VDC (per Morningstar)
- The CSB 12340 battery can provide 5.88 Amps constant current to down to 11.7 VDC (per their documentation) for 4 hours
- Each circuit will not exceed 1 Amp of drain (= I in calculation)
- Use a voltage drop of .30 looped voltage drop (=V in calculation). Using a .30 will allow the bulbs to operate to 11.4 VDC before the circuit disconnects at the LVD of 11.7
- The circuits will use 14 ga. (4070 CM) copper wire for each circuit
- Using the voltage drop calculator to calculate cable length, each circuit needs to be less than 55 feet in total length. If you need longer cable runs, you can increase the cable size from 14 to 12 or 10 gauge.
You can also use this on-line calculator from Southwire. The calculation is similar but is geared toward an AC circuit. But you can play around with the numbers easier with the calculator. Because they use % voltage drop in lieu of true voltage drop, use 2.5% to achieve a .30 drop.
Voltage drop Calculation (CM/11.1)/I*V=L
- CM= Circular Mil area of the cable
- 11.1= Conductivity factor for copper cable
- I= Peak Current
- L= One way cable length
- V=Allowable voltage drop
Product Solutions based on Design:
- Battery: Using the CSB 12340 , I have a total Ampacity budget of 5.88 Amps for 4 hours of run time. NOTE: In an attempt to find a battery locally, the closest match was the CSB 12340 battery. Since it exceeded my requirements, would support future growth … I took it. With the 12340 I have enough battery capacity to support a larger demand based on my 4 hour requirement
- Solar Panel: This panel can produce 15 Watts or 1 Amp of 12 VDC electricity. Based on the battery, I can enlarge my system by 5 more panels, but with only one panel installed, this element remains as the limiting factor in the system
- Wire: One 14 ga cable will allow up to 1 Amp of current for a total of 55 feet. TIP: To create greater flexibility, I ran multiple runs of 14 ga. cable to different parts of my landscape, in lieu of one (or two) long cable(s). This way, I spread the lighting budget over more than one conductor with separate circuits. Again, this allows me flexibility for growth
- Bulbs: Using the Voltage drop calculator; at .16A per (21pin) LED, I can support 6 lamps over a 55 foot circuit. The LED bulb was a direct replacement for the 20 Watt bulb supplied by Malibu
- 200 feet of #14 ga copper landscape wire. Even though the wire looks very similar to lamp cord, the rubberized sheath is designed for outdoor and underground usage. It’s important to use the wire specifically designed for this purpose. I bought this from the Orange Box Store but you can buy this in bulk over the Internet a bit cheaper. As mentioned I created multiple circuits to build the system
- 15 Watt Solar Panel. Purchased from Northern Tool Company. The one I chose will allows up to seven additional panels. Based on my current design, I am at the limit of one 15 Watt panel, so I will be adding at least one more panel to increase my fixture count
- 6 Malibu Landscape light fixture. I used the basic $15 fixture from Malibu Lights. Most normal light fixtures are rated by wattage and since I am using LED bulbs the Wattage ratings are insignificant to the project.
- Battery Charge Controller & Light Timer.The Sunlight solar light controller from Morningstar is a perfect controller for the application, it includes a charge controller, adjustable timer, low voltage disconnect, and it uses the solar panel to determine when to start the lighting cycle. See the MorningStar Sunlight Controller webpage
- LED Light Bulbs. LED’s light bulbs are on the cutting edge of new lighting designs. For that reason they are still expensive with limited standards defining their make up and performance. The first round of bulbs cost over $20 each and lasted about 6 weeks. The second set I bought for $10 each have been working for over 6 months. However, the light output has varied with each shipment from a yellow tint to blue even though they are all supposed to be cool white. To date, LED bulbs increase their light output by increasing the number of individual LEDs in the bulb, I chose a 21 pin bulb with a wattage demand of 2 Watts per bulb (new styles are starting to show up on the market using fewer LED’s requiring more power and greater output)
Innovation Comes With a Price:
Here is a basic breakdown on the cost of the system. Both the copper wire and LED light bulbs were reduced to their current cost. I included a cost analysis on the electricity used for a standard AC derived system based on a 1000 Watt system. So, with a 500 Watt system the savings would be about $86.40 per year with a payback at almost 7 years. I believe the system could be cost-reduced a bit more over time, but it will still not compete in price with an off the shelf system AC powered system.
The Cost of Operating a 1000 Watt Landscape lighting System:
- 1000 Watt used per hour of run time
- 4 hours of run time per night, 7 days a week
- 12 cents a Kilowatt, per hour charged by local electric utility
- 1000 Watts = 1 kW
- 1000w x4 hours = 4000 W. 4000W x 30 days =120,000 watts per month
- 120,000w /1kWH= 120kWH per month
- 120 kWH x .12 cents = $14.4
- $14.40 per month to run landscape lighting or $172.80 per year
The Completed Solution and Conclusion: I started this project about 2 years ago and just worked on it when time and money permitted. For the most part it was fun to put the project together. Overall, I am happy with the results. I used the lamps with a bluish tint (cool white) and it gives the house an interesting look over the warm light found with incandescent bulbs. Based on the calculations, I will be adding another panel soon. By adding the second panel I can add up to 6 more light fixtures.
In comparison, the centralized solar system is superior to the stand alone solar fixtures hands down. It allows the use of all the different fixtures available on the market today as you are not restricted to fixtures with a solar panel attached to the top. Additionally, the battery life expectancy is a bit better than the small AA batteries found with the stand alone units. And even though the centrally powered solar system does not equal the AC powered version 1 for 1, it’s a lot closer in comparison.
If you are thinking of building your own system and have questions, drop me a note at HomeownerBOB@gmail.com
Make sure and take a look at my update on this project. The prices of solar panels have continued to fall allowing me to increase the size of the system.