Monday, March 17, 2014

Economics of Growing Greens by Artificial Light

We had a thought the other day about growing greens like lettuce, spinach, swiss chard, and kale inside during the winter months.  What would it cost us to do that, in the limiting case that the greens would get no natural light through our windows?  (That's actually not too far from the truth--most of our windows face north or are shaded for all but a few hours a day.)

We went back to take a look at our aquaponics setup, which makes use of four 4-foot-long T8 fluorescent light bulbs, and which we used to grow some swiss chard, thyme, basil and dandelion greens at our old place (there were other plants, too, but those were the four that grew fast enough to give appreciable yields).  We were pretty satisfied with the amount of greens we were able to produce from that little 48" x 24" area, but we never actually crunched the numbers on the economics.  So, today we decided to do a quick calculation to see how much it was costing us to grow all those greens, at least as a ballpark figure.

A picture of the working aquaponics grow bed at a young age.  Later it would look much bushier.

The lighting system uses four 32-Watt bulbs, and our electricity rate is about $0.13/kWh. That is, it costs us thirteen cents to consume 1,000 Watts for one hour.  Watts are actually a rate of energy consumption--Joules per second--but total energy consumed is what gets billed.  So instead of doing the logical thing and using Joules as a standard unit, utility companies use the convoluted unit of kilowatt-hours.  Add that to the list of science-business conventions that don't make sense.  But at least it's based on the metric system!

So, when the four bulbs are all lit up, we're consuming 128 Watts (or 0.128 kilowatts).  If we run the lights for 10 hours a day, we consume 1.28 kWh of energy, which costs us $0.17. (Differences in rates for peak hours, sales tax, and all the other junk the utility company charges for are included in that number.)  Seventeen cents per day doesn't sound too bad.

In the aquaponic system pictured above, we had four swiss chard plants among the other stuff, and were harvesting about 0.1 lbs of chard /week total based on our records, which works out to about 1 lb from those four plants over a 60 day period, or $9.97/lb. (!!)  Not nearly up to expectations, but keep in mind that chard wasn't the only thing in the bed (so the whole cost of the light can't be attributed just to chard), and since our fish were stubbornly refusing to grow, these plants were extremely nutrient limited.  Fortunately, the thyme and basil that we grew at the same time cost a lot more at the store than chard, so it might all even out.  

 But, just for kicks, what would the cost be if we filled the grow-bed with dirt, planted all chard, and it grew a little closer to expectations?

We can assume that ten swiss chard plants will fit in the grow-bed since we had four plants in less than half the space.  A generally-accepted representative time-to-harvest for swiss chard is 60 days, although it's usually possible to start selectively eating the leaves well before that (which we did).  But if we take a 60-day baseline, and equate one plant to one 'bunch' that you'd find at the grocery store, we'd have 10 bunches in 60 days.  So, the calculation then becomes $0.17 per day, times 60 days, divided by 10 plants to give $1.00 per plant (or bunch).  Considering that we currently find it for around $1.99/bunch at the grocery store, that seems like a much more reasonable deal.

Generalized equation to figure out the cost of growing a plant under artificial lights, assuming the artificial light is the only cost.  For "C," use the inverse of the number of plants in the bed (e.g., if you had 10 plants in one bed, use 1/10).  Similarly for "G," if you got, e.g., 8 pounds from 10 plants, use 10/8.

Looks like it might be worth giving it another shot at growing some winter greens indoors!  Next time we'll use real dirt, and won't count on a few anorexic minnows to provide all the fertilizer.  And now that spring is on its way, we probably won't get to this experiment again until the fall, but hang tight!  We won't forget!

Do you grow your own greens indoors during the winter?  How cost effective is it for you?  Let us know in the comments section below!


  1. When you get to doing the growing, also factor in that "your" veggies are fresher than the stores, so besides being organic, they might be more healthy, as nutrients fade with age. Can't beat home grown!

  2. Very true--can't beat home grown!