Early versions of this manuscript included a ten year time horizon, but based on reviewer comments (including economists), the ten-year analysis was not included in this version for two reasons:

1) The efficiency of the best fixtures from both technologies is equal (1.7 umol per Joule) so there are no long-term savings in electric costs among the most efficient fixtures.
2) As discussed in the manuscript, long-term maintenance costs are uncertain and do not necessarily favor LED technology.
For these reasons there is not an economic cross-over point between technologies and a five-year time horizon (commonly used in business decisions) was sufficient to compare options.

Here we compare the costs over the lifetime of an LED fixture.

Operating life for LED fixtures is generally not specified by manufacturers, but a 50,000 hour lifetime is reported for individual LEDs. Extending this lifetime to the entire fixture makes the assumption that no other component will fail (such as power supplies or fans). If we make the assumption of no premature failures and no maintenance costs over 50,000 hours, the initial capital cost of the most cost effective LED fixtures is $3,200 for 1750 µmol/s of output (the equivalent photon output of one HPS fixture; based on photon output and cost of fixtures in Table 3). After 50,000 hours the photon output of the LEDs is predicted to be 70% of initial output.
If double –ended HPS lamps (bulbs) are replaced every 10,000 hours (at 95% of initial output), and the entire fixture is replaced at 30,000 hours, it would cost $420 (3 x $140) for three replacement lamps and two fixtures (2 x $500 = $1000), for a total of $1,420 capitol and maintenance cost for the same 50,000 operating hours.

LED fixtures do not necessarily have a long-term economic advantage when capital costs are included.

As the title indicates, this article focuses on greenhouse applications, but the data and principles can be applied to indoor plant growth. Indoor lighting can take advantage of unique spectral effects, which we are currently investigating (Cope and Bugbee, 2013; Cope et al, 2014). LED fixtures have many unique properties and have the potential to revolutionize the industry.

We hope the data in this manuscript will help analysts giving energy rebates and incentives to make informed decisions. These incentives should be allocated based on independently verified efficiency ratings (in µmol/J). Both HPS and LED manufacturers tend to inflate claims of fixture efficiency. Bulk discounts and rebates are available for both technologies. The associated on line calculator (http://cpl.usu.edu/htm/pu...) allows users to incorporate individually quoted costs, maintenance estimates, electricity rates, and differences in photon capture efficiency.