This is continuing from a discussion from an astronomy forum where the "massive" energy savings of 80% were claimed by San Jose for changing from astronomer friendly low pressure sodium SOX lights to white LEDs. Citing the following article in the Mr Roadshow column of the Mercury News. But the numbers just do not add up. Anyone care to comment?
I can't see any obvious reason why European experience of these lights does not apply in the USA.
Low pressure sodium gives something like 160 lumens/watt and almost 200 l/W with optimised solid state ballast. They are mandated near world class observatories (in this case Lick) because they are almost monochromatic narrowband emitters and can be easily filtered out.
Best power LEDs are close to 100 lumens/watt now.
I looked up the PE&G report advocating this change and found that they reckon to power a 55W SOX tube requires 92.5W so I guess they used a half dead antidiluvian magnetic choke ballast as their baseline. I reckon a typical modern ballast would be more like 10W at most and could be easily half that.
The legal disclaimer on page 1 rings alarm bells for me.
They go on to compare this bogus highly inefficient baseline SOX setup with the luminous efficiency of the bare LED units without DC control gear in the first table I. And so claim spuriously large energy savings from making the conversion. The estimates further into the document are slightly more realistic and by table XVII they do include control gear for the LED systems (with some optimistic assumptions).
Nowhere do the numbers approach the 80% in the news article though.
Their numbers in W are SOX 92.5 (extremely high for a 55W SOX tube) LED 100% 75 LED 75% 52.4 LED 50% 34.9
My estimate is SOX* 65 (Philips electromagnetic ballast)
And so by the time you add in the overhead of 5-10% for control gear I reckon the 100% LEDs will use almost as much energy as the original installation is allegedly using at present. I reckon they are in for a nasty surprise after they finish spending $50M on these new streetlights. Retrofitting decent ballasts would be a far better choice with a saving of 30% power for a $20 new ballast component retrofit.
Is there some obvious reason why modern SOX ballasts do not work well on US mains or is this an attempt to make the old lamps look like they are very inefficient so as to create momentum for a change to LEDs?
I am aware that some residents of San Jose hate the low pressure sodium lights on largely cosmetic grounds of poor colour rendering.
There is undoubtedly a maintenance saving since in theory at least since LED based luminaires should last 3-4x longer than SOX tubes.
Regards, Martin Brown