Converting watts to par? - The Planted Tank Forum
LinkBack Thread Tools Display Modes
post #1 of 5 (permalink) Old 06-17-2015, 08:09 PM Thread Starter
Algae Grower
DennyB's Avatar
PTrader: (0/0%)
Join Date: Oct 2014
Location: South Jersey
Posts: 85
Converting watts to par?

Is there a way to convert the light t5/t8 fixtures give off to an approximate par measurement without a par meter?
DennyB is offline  
Sponsored Links
post #2 of 5 (permalink) Old 06-17-2015, 08:12 PM
Children Boogie
mistergreen's Avatar
PTrader: (13/100%)
Join Date: Dec 2006
Location: Cincinnati
Posts: 16,743
nope. Watts measure volts X amps. PAR is micromols per second and square meter (μmol m-2 s-1).

Your best luck is a LUX meter conversion.

Not accurate, but close enough.
mistergreen is offline  
post #3 of 5 (permalink) Old 06-18-2015, 12:37 AM
Planted Tank Guru
PTrader: (0/0%)
Join Date: Jan 2010
Location: Contra Costa CA
Posts: 11,721
You could look at the color charts- If their peaks are in the right range for plants, then you know they at least are better for plants than some other bulbs.

But no, there is no way to convert watts to par.

Have a look here, some research by some of our members about bulbs, and testing with a PAR meter. If your bulbs are listed, this is great.
Diana is offline  
post #4 of 5 (permalink) Old 06-18-2015, 03:36 AM
Planted Tank Guru
PTrader: (84/100%)
Join Date: Dec 2005
Location: Sacramento, CA
Posts: 21,012
One reason you can't convert the bulb wattage to PAR value is that the amount of light a T5 light produces depends heavily on how good the reflector is, and whether or not the ballast is full power or under powered. The PAR from a given bulb can vary by a ratio of more than 2 to one just from the reflector. Also, the PAR from that bulb depends on how far from the bulb you are measuring the PAR. PAR drops with distance from the bulb.

Hoppy is offline  
post #5 of 5 (permalink) Old 06-18-2015, 02:19 PM
Planted Tank Guru
PTrader: (3/100%)
Join Date: Aug 2014
Location: Maryland, USA
Posts: 2,339
+1 on Hoppy...

Put another way:

Par measures surface brightness (ie: how many photons are hitting a given area in a second)... How much brightness a given light puts on a surface depends on how might light is produced, how far away the light is, and how focused it is.

Watts measures energy usage... This has no direct relationship to the amount of light produced, and even less to how bright the light shines on a given surface.

To convert watts to par you'd need:
- some kind of light-production efficiency factor, spectrally weighted correctly to the PAR band. (or at least close, since most PAR meters aren't perfect at this).
- a measure of how focused the light is
- a measure of the distance from the light to whatever it is shining on.
- a correction for re-focusing of the light by the air-water boundary, based on how far the light is from the water surface, if we're talking about underwater plants.

That's a lot of messy calculations.

New to planted tanks, avid gardener/tinkerer.

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Fluval C4 + Quietflow 30, Finnex Planted+ with
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
, low-tech.
10 gallon secondary tank with Finnex stingray with
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
, AC20, low tech
mattinmd is offline  


Quick Reply

Register Now

In order to be able to post messages on the The Planted Tank Forum forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Please enter a password for your user account. Note that passwords are case-sensitive.


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:


Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode

Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

For the best viewing experience please update your browser to Google Chrome