PAR vs Watts per GAllon

Add some green (or red or pink or purple) to your aquarium!

PAR vs Watts per GAllon

Postby ApistoGrandma » Sun Jan 13, 2013 11:52 am

So I've been trying to figure out why my plants become spindly in my 55. weather its ferts( they are in year old substrate). Lack of co2( no added CO2). or if its a lack of ( 96 watt 4 bulb t5ho,w/reflectors).

I'm pretty sure is a little of all the problems but i figured lighting would be the easiest to calculate....then i ran into the PAR vs W/gal debate. anyone have 2cents they'd like to give?
Last edited by ApistoGrandma on Mon Jan 14, 2013 2:07 pm, edited 1 time in total.
Posts: 74
Joined: Mon Jul 04, 2011 12:17 pm

Re: PAR vs Watts per GAllon

Postby im2smart4u » Sun Jan 13, 2013 4:04 pm

Try going to and reading the info in the lighting area of the forum. PAR is what is actually important, but unless you have a PAR meter you have no idea of what the par in your tank actually is. There is a chart on here : ... p?t=105774 that gives an idea of what lighting you would have based on the type and number of bulbs, but this still isn't real accurate due to the way the fixtures focus the light (some, for instance waste the light from the back side of the bulb, and some have reflectors to reflect that light into the tank... and some reflectors are a lot better than others.)

Watts per gallon is a bad way to calculate, because T8, T5, T5 HO, compact, and LED all put out different amounts of light per watt. So a 50 watt fixture of each would give you VERY different amounts of light.
This link explains it better and in more depth: ... p?t=184368
Posts: 95
Joined: Tue Feb 15, 2011 7:39 pm

Re: PAR vs Watts per GAllon

Postby mewickham » Sun Jan 13, 2013 7:13 pm

I agree that watts-per-gallon is not a good way to calculate lighting. However, it's the only choice available to most people because they don't have expensive PAR meters. Even if you have a PAR meter, it only does you good AFTER you've made your lighting purchase decisions-- and that's too late. You need to know the PAR BEFORE you buy your lights, so that you buy the correct combination. And where the heck would you find those numbers? The wpg rule has worked for years, so I think those that completely poo-poo it in the pursuit of perfection are just being hypercritical.

So I recommend that most people use the watts-per-gallon rule. I would note that the watts-per-gallon rule is not meant to be 100% accurate. It's a rough and variable guideline. That's why they typically say "2 to 5" watts per gallon. If your tank is shallow, 2 WPG may be enough. If it's 30" deep, you probably want the full 5 watts (which will still mean a dimmer substrate than in a 12" deep tank at 2 WPG, but a brighter light near the surface). The WPG rule was originally written for standard fluorescent bulbs, so 2-5 WPG can be reduced for power compact, T5, HO bulbs, and LEDs.

There are just too many variables to ever achieve accuracy with any rule. If you measure the perfect PAR at the substrate level of your aquarium and then add watersprite to the tank, soon the surface will be covered and the substrate will receive virtually no light. No PAR. Tall plants cause shadows below, too. So in addition to all the talk of bulb output, bulb spectrum, bulb distance, bulb diameter, bulb length, shape and reflectivity of the housing, etc., the plants grow and change everything. Growing plants are not a static environment.

Anyway, plenty of people grow great plants without having ever seen or used a PAR meter. I'm not against PAR meters. I just wouldn't bother spending my own money on one.
User avatar
Site Admin
Posts: 2602
Joined: Sun Mar 15, 2009 12:50 am
Location: Rogers, Arkansas

Re: PAR vs Watts per GAllon

Postby ApistoGrandma » Mon Jan 14, 2013 2:25 pm

well thanks fer yer 2 cents y'all
yeah par metters are pricy little gagets aint they!
Posts: 74
Joined: Mon Jul 04, 2011 12:17 pm

Return to Aquatic Plants

Who is online

Users browsing this forum: No registered users and 1 guest