One of the frequent questions I get from clients interested in new or expanded landscape lighting systems is some version of ‘How much power does a system like this use?’ Sometimes they will ask about the impact on a utility bill, sometimes it is out of carbon/greenhouse concerns or maybe even concerns in older homes with a history of electrical problems. Sometimes the question is about expense versus vanity. For example, are pool lights worth the extra power expense? With the transition to LED-based fixtures, and my own training and experience I’ve usually been able to answer these questions and remove any concerns the clients have had – but thinking it through raised some questions in my own mind on some specifics I couldn’t easily answer.
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For instance – is it wasteful to use a larger transformer so there is ‘room to expand the system’? How much does the transformer draw as more lights are added? And the big one – does a landscape lighting system really consume less power than a TV? I ran some repeated and repeatable tests with my favorite products to answer just these questions – read on to see the findings!
What size landscape lighting transformer do I need?
We will go over in more detail how do do things like test your transformer and costs associated with landscape lighting transformers. But on average transformers usually range between 150- to 900-watt capacity. It is important to make sure your exterior lighting and transformer capacities match up. a simple way to figure this out is to
- Add up the wattage of each light in your landscape lighting design
- Multiply the total wattage by 1.25 to adjust for loss due to cable distance. For the example, 240 x 1.25 = 300.
- Choose a transformer that supplies no less than the required wattage.
We will go over understanding your transformer in greater detail below!
I have a more detailed write up below, but basically I set up three clusters of 4 lights (12 lights total) and measured the power usage in different combinations on different transformers. The good news is, larger transformers are just as efficient as small ones – so when I recommend sizing up for future growth there is no energy penalty.
I was happy to see some very clear and linear results that matched what I hoped and expected to see. Per the table below – all the transformers only drew a small fraction more power than the total load that was placed on them. For instance – with four 7-Watt lights attached, you have a load of 28 Watts (4 × 7W) and all the transformers showed between 35 and 49 watts draw at the outlet via the Kill A Watt measuring device. With 12 lights and a load of 84 W, the transformers drew between 105 and 122 W.
This answered a few questions for me.
Does a powered-on transformer use much electricity without load?
No. Some people like to leave a transformer on all the time and switch on fixtures with a wall switch or an app, these tests tells me there is small cost to doing that.
Is a large transformer (300W or 600W) inefficient with a small load, such as when people want to grow a lighting system over time?
Not really, the larger transformer from Dauer drew just a few more watts than the smaller one on the same load. It drew fewer watts than the smaller Malibu at all loads, which I take as evidence of superior quality in manufacturing and materials.
Does a landscape lighting system really use about the same as a TV?
I would say a typical system uses less than half of what a TV uses, and only a fraction of what a large home-theater uses. I measured a 42” modern TV at about 250W, or twice what a 12-light system consumes. A 60-70” screen with a supporting stereo would consume several times more power than almost any residential lighting system.
How much does an average system cost to run per year?
10 cents a night or less than $5 a year. Of course electricity costs vary widely across the country and no two systems are the same… but call an average system 12 lights and the national average for power per kilowatt hour (kWh) is about $.13. If a typical system is lit for 4 hours at ~ 125W (see chart below) – then it will use ½ kWh per night (125 W X 4 h = 500Wh or ½ kWh ) at an electrical cost of less than 10 cents.
Power Draw Experiment with LED Lights
For those interested in how I got these results – here are some details. In order to test these systems, I would need to set up some test scenarios and be able to measure them. Since I have a lighting company, some of this was easy – I simply took a selection of transformers and lights off the shelf! But measuring power usage required some test equipment – for this I used the Kill A Watt power meter which is an ingenious device that plugs into any wall outlet and offers a monitored plug to any electrical device (such as a transformer or TV). You can read more info on the Kill A Watt device here – I recommend every homeowner get one. It is a good way to tell how much that old fridge in the garage is costing you, or what it costs to run that fan all day…
For the load, I used a dozen identical Redwoods Aspen fixtures which draw 7 Watts each. These fixtures are some of my favorites with adjustable output, quality construction and unique design. We assembled three pods of 4 lights to have a load of 4, 8 or 12 lights and then tested these against three different transformers, a Dauer 150W, a Dauer 300W and an old Malibu 200W we recovered from a client who upgraded ( Malibu has since gone out of business so any Malibu lights and components are beyond support). I was curious to see what the differences were with a consumer grade Malibu vs the professional grade Dauer.
The Dauer units have a multiple voltage taps, meaning that you can get output of 11, 12, 13, 14 or 15 volts depending on where you connect the circuit to. This is useful to adjust the performance on large systems. I tested the lights at 12 and 15 v outputs to see if there was a difference in power usage – but the results were almost identical, so I only have the one test voltage reported here to keep the results clear.
A Note on Transformers
Transformers are at once very simple, but kind of mysterious. According to Electrical4U, the definition is: A transformer is a static device which transfers electrical energy from one circuit to another through the process of electromagnetic induction. It is most commonly used to increase (‘step up’) or decrease (‘step down’) voltage levels between circuits. But how does this happen? There are no moving parts and a faint hum is usually the only clue that something is happening. Essentially, a transformer two coils of wire wrapped around a core – through the scientific magic of electromagnetism, our transformers step-down dangerous 120V Alternating Current (AC) to nice safe 12V Direct Current (DC) that we can run next to sprinklers and across playgrounds without any risk. The core and windings add up to a lot of metal which explains why transformers are always so dense. If you really want to read up, click here to learn more.
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About the Author: John Moore, Outdoor Lighting Perspectives of South San Diego
After serving in the Marines, John went back to school and stared working in IT sales and services. In Silicon Valley he learned a lot about cabling for large corporations, such as Google and Facebook. John uses these years of experience to bring beautiful outdoor lighting systems to homes in San Diego and surrounding communities. John and his team specialize in everything for extierior lighting, from design, LEDs, and more!