Photo courtesy Wikimedia CommonsPhoto courtesy Wikimedia Commons

A commenter recently asked how much it costs to run the lights we have on our seedlings. I’ve never calculated this up and really had never thought about it, so I figured it was time to start.

Time for a little math.

Each shop light houses two 40 watt bulbs for a total of 80 watts. Running the light for an hour uses 80 watt hours of energy, or 0.080 kilowatt hours (kWh).

In Columbia our electricity rate is 9.44 – 10.88 cents per kWh depending on your total energy usage. This seems to be a fairly average energy cost. I’ll use the lower number for the calculation.

Generally it’s recommended that lights be left on seedlings for 14-16 hours. Let’s assume we leave them on for 16 hours. This gives us all the information we need for our energy calculation.

For one shop light with two 40 watt bulbs:

0.080 kWh/hr * $0.0944/kWh * 16 hr = $0.12

This means that for each day I run one shop light, the energy costs me $0.12. If I run the light for a month it costs $3.62.

Of course I don’t just have one light going; I have six. So running all six lights for a month costs $21.75.

I will probably run these lights for 2-3 months, depending on the plants I have growing. I also am not likely to have all six lights on from the very beginning, since I don’t start all my seeds at the same time. But let’s just assume for simplicity that all the lights stay on for a solid three months. That’s a grand total of $65.25 for running the lights for three months.

This isn’t a huge expense, but it’s not negligible, either. I’ve never seen numbers like this before, so it’s helpful to know that every time we install another light, we’re going to spend roughly an additional $3-4/month on energy.

(If the math is too much for you, you can also use an online energy calculator like this one.)

But let’s not forget the heat mats! They require energy, too.

I’ve got a 48×20″ mat which pulls 107 watts, and a 9×19.5″ mat which pulls 17 watts. Based on the same calculations above, running these two mats for 16 hours a day costs $0.19/day, or about $5.62/month. Over the course of 3 months, running these mats will cost around $16.85 total.

The whole getup together will cost around $82.10 to run for three solid months.

So. Starting seeds at home is way cheaper than buying seedlings when you’re growing on a large scale, and gives you the flexibility to grow the varieties you want with methods that you agree with. But just like buying seeds, there are some definite costs involved.

Who wants to build me a passive solar greenhouse?

5 Responses to “The cost of running seedling grow lights”

  1. Scott
    20 February 2012 at 1:57 pm #

    That guy over there wants to build it! I think his name is Charlie or something.

  2. pattyskypants
    20 February 2012 at 7:25 pm #

    Love the math and thank you very much for the energy calculator link! Considering all that you are able to grow, it seems pretty reasonable!

  3. Eric Reuter
    21 February 2012 at 7:45 am #

    We’re building one in March, once I get the lumber milled. You can learn from my mistakes once it’s done…

  4. CoMo Homestead
    21 February 2012 at 9:34 am #

    Eric – OOOH!!! This is excellent! Let me know how it goes!

  5. CoMo Homestead
    21 February 2012 at 9:37 am #

    Scott – He actually does want to build it! He built solar cars in college, so it’d be a fun project. We just need the space and the cash.

    pattskypants – Thank you for the suggestion! It was fun working the math all out.

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