A friend reading my book wondered if growing vegetables indoors could be cost-effective, at his UK power prices. Chris pays about 22 cents per kWh in daytime, 15 cents at night. (Converted from the pence.) Here in Connecticut, our power prices are some of the highest in the US, at about 15 cents per kWh.
And it matters to me. Not just in cost-effectiveness, but in terms of environmental impact. I choose to keep my footprint light on this planet. I'm sure most gardeners feel the same way.
But for the moment, let's just look at power costs and food prices, and run the numbers. The answer is - it depends on what you're growing, how you're growing it, and what your other options are.
To grow lettuce in my home, it is most cost-effective to grow my own, under electric lights, hydroponically. Provided I don't pay too much for the hydroponic rig. Of course, in theory it would be cheaper to grow it outdoors, and I do get a few heads of lettuce that way each year. But at least 10 months of the year, the options are to grow it indoors, or buy it.
The picture shows my current splasher set-up. Under the right light is 2 splashers growing flowers and toy choi. Under the left is 4 lettuce splashers, growing 8 lettuce plants, under a single 23W CFL lightbulb, running about 12 hours a day. A 23W CFL lightbulb actually draws about 25W of power.
25W x 12 hours/day x 7 days/week ÷ 1000 W/kWh = 2.1 kWh/week
So I spend about 32 cents a week on electricity growing those 8 lettuce plants. They grow for at least 12 weeks, and are harvestable for at least 8 of them. So for about $ 3.84 in electricity, I get 8 weeks of more lettuce than two people want to eat. At the farm market, I can buy a single head of locally grown lettuce for $3, and it stays fresh a couple days. That is not organic lettuce though. Organic would cost me $4, in season. At the supermarket, if I only bought cello-bag lettuce when it was on sale, I might get two bags for $ 2 each, and have fresh lettuce two different weeks. At who knows what cost to the environment in chemicals and transportation costs.
It's no contest. The splasher-grown lettuce is cheaper by a huge margin, provided I already have the equipment. Which is also cheap, in this case.
I could grow much more lettuce and flowers and toy choi with my seedling shelf, under a 64W 2-tube 4-foot T8 fluorescent fixture, which costs about $0.86 per week to operate. But it's slightly cheaper to run two 23W CFLs, and I don't want to manually splash any more splashers.
Now, what if I grew the lettuce in soil? It would take 2-3 weeks longer to reach harvestable size, and continue to grow slower. So, I'd increase my wait and electrical costs by 50-75%, for the same harvest.
Now let's go to the other extreme, and grow tomatoes indoors. My best indoor tomato grow ever took about 5 times the power, on 2 plants. It produced 125 cherry tomatoes in 3.5 weeks, after 12.5 weeks growing before first harvest. The equipment was not cheap - I used a Deluxe Aerogarden and lightscreen. But on power alone, it cost about $ 25.60, or about 20 cents per cherry tomato. That's not a horrible price for clean local cherry tomatoes. But it's not wonderful, either. And as I said, that was my best indoor tomato grow ever. Most are less effective - more weeks, more power per week, way lower yield.
I'm not disappointed one bit by Chris' question about the power. I'm delighted! Because that's the main thing I wanted to accomplish with the book - to empower readers to make these trade-offs. And then grow something - their own way! Chris didn't give up. Thinking this through, he saw the power of reflectors and realized he could extend the season in his garden allotment. He's happily plotting how to add parabolic reflectors and maybe some organically generated heat to his hoop house. Good for Chris!