What Size Landscape Lighting Transformer to Use? – 2021 Guide

Knowing how much power your landscape lighting system uses is important because it allows you to estimate the total cost of operating the fixtures. This is very helpful for determining whether or not a new landscape lighting system will be able to pay for itself over its lifetime, and in some cases may even reduce your property taxes.

It can also help you choose which types of fixtures use less energy than others if you’re looking into shopping around for more efficient options.

To start this article off, we’ll go through a couple of easy steps that will give you an idea of what type of landscape lights are drawing how much wattage from your home’s electrical outlet(s). It’s important to keep in mind though that these numbers aren’t entirely accurate and should only be used as a rough estimate.

Source: hgtv.com

Collect Information About Your Lights

The first thing you’ll need to know is how much wattage your fixtures are pulling from the wall before they start shining their light. This information can usually be found by looking at the back of each fixture or by searching online for information on your specific lights.

If this isn’t possible or if you want to get an idea about what ballpark you’re in, most modern landscape lighting systems draw anywhere between 100 and 500 watts before they begin lighting up.

The main factor that affects how much power the lights use is not actually related to the number of watts used but instead how much voltage it draws. Voltage can change even though the amount of power stays constant because different types of bulbs draw different amounts of power at different voltages.

For example, a typical landscape light that is rated for 100 watts and 120 volts would actually only use 80 watts if it is halogen instead of LED.

Calculate the Average Wattage Used per Fixture

To figure out the average wattage used by your system, take the number of fixtures you have and multiply them by their wattage rating (before shining their light). Then divide this number by 1000.

An easy way to do this with odd numbers is to round up and then divide them in half, which will give you a very accurate estimation. So if you have 24 lights that are each drawing 300 watts before shining their light, you would divide 600 (24 lights x 300 watts) by 1000 to get an average of 0.6 amps per fixture.

Calculate the Total Amperage Used by Lights

Now that you have the number of fixtures and the average wattage used per fixture, you can easily calculate how many amps your landscape lighting system is drawing from the outlet(s). This will help give a rough estimate on why your electricity bill has gone up in the past month or so.

To do this, multiply your total number of fixtures by their average wattage used per fixture and then add together these numbers for every set of outlets in your home. Then multiply this number by 1.1 to compensate for other appliances that might be on the same circuit as your landscape lights.

An example would be if you had 24 fixtures that used 0.6 amps per fixture and were plugged in to two separate outlets, you would multiply 24 x 0.6 + 24 x 0.6 = 55.2 + 55.2 = 111 amps.

Calculate the Total Kilowatts Used by Lights

Kilowatts Used by Lights
Source: inchcalculator.com

To calculate the total amount of energy used by your lights, multiply your average wattage per fixture by the number of hours they are left on each day. For most residential homes this will usually mean leaving them lit for around 6-10 hours each night depending on what time you get home from work. The last thing you need to do is multiply the total kilowatt-hours per day by 365 days in a year. This will give you the total number of kWh used by your landscape lighting system per year.

An example would be if you had 24 lights that each used an average of 0.6 watts per hour and they were on for 10 hours every night for 300 days, you would multiply 24 x 10 x 300 = 72,000 watts x 1000 to get 719,600-watt-hours per year.

Calculate Your Annual Cost per Kilowatt Hour

This step won’t always be possible but if it is available to you then use it because it is a very easy way to calculate how much your lighting system will cost you per year. All you need for this is the nameplate rating on your transformer which can look like one in the photo below.

Malibu 120 Watt Power Pack with Sensor and Weather Shield for Low Voltage Landscape Lighting Spotlight Outdoor Transformer
Source: amazon.com

These numbers are usually something like 115/120, 208/220, or 347/360 and then either “V” or “V-ph-Hz”. This information can be found on the side of most transformers and is generally located near where the power cord meets the box itself.

Once you have this number (which should be in volts), multiply it by the total amps used by lights (found above) and divide this product by 1000. Then multiply that result by your kWh per year number (found above). This will give the estimated amount of money spent by your landscape lighting system per year.

An example would be if you lived in a home that used 347/360 volts and had 24 lights that each used 0.6 amp per hour for 10 hours a night 300 days a year, then your estimated cost per kWh would be 347 x 0.6 + 347 x 0.6 = 307 + 614 = 1011 amps x 1000 to get 1,011,000 watts x 365 to get 365,825,000 watt hours / 1000 to get 365,825 kWh x $0.10 (average) = $36,582 .25 (annual cost)

Conclusion

Remember that this is only an estimation and there are other factors that can alter your annual cost per kWh. If you know the actual nameplate rating on your transformer, for example, you could use a different equation to get even more accurate results. Also, keep in mind that the prices of your utility company.

An important thing to remember when reading this article is that every home, climate, landscape light set up, and electrical system is different so it’s going to be impossible to calculate exactly how much power your lights use unless you have a meter installed or a clamp-type ammeter handy.

There are many things that do not get taken into consideration when using these equations such as how many fixtures are on a given circuit, the actual wattage of your lights, temperature, etc.