# Watts per Gallon



## Diana (Jan 14, 2010)

Depends on what you want to measure.

PAR, no matter what the source of light, is really the best way to determine if a light is good for the plants. This measures the actual amount of the wavelengths that plants use the most of. 

WPG are usually pretty easy to figure out, because fluorescent and incandescent bulbs are usually labeled in watts. But how many of those watts are going to 'feed' the plants? Often there is a color chart that shows which colors the light produces. This can help. A moderate wpg with the right color chart is better than a higher wpg that does not really have the right wavelengths for the plants. 

Degrees Kelvin (K) value can help a bit, too. This is a generalized value that shows what the blended wavelengths look like to our eyes. Our eyes are not plants, though. We see the yellow to green wavelengths as the brightest, but plants use certain reds and blues.

Names like 'Cool', 'Warm', 'Daylight' and similar are also rather vague, and aimed at our vision, not plant needs. 

Here is how I did it using wpg:

Get a fixture that will hold several bulbs, or use more than one fixture over larger tanks. 
Mix-and-match the bulbs including one 'plant' bulb and one 'cool' or 'daylight' bulb. 
Aim for a minimum of 2 wpg, and keep the tank near a window. 
The bulb labeled for plants looks rather pink-purple, and does not bring out the colors of the fish very well. The daylight bulb has more of the colors we see, and does help the fish and hardscape look good. The combination helps the plants grow better because at least some of the light is in the right range.


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## Steve001 (Feb 26, 2011)

BDpups said:


> Is this now the inch per gallon rule of fish? I'm hearing it is "outdated and inaccurate" It's because of LED's and PAR rating. I get that when it comes to LED's. But if you are using florescent bulbs, it would still seem to be a valid tool to use to get started. Am I wrong about this?


The only correlation is a higher wattage bulb will appear brighter generally. Any light can have a PAR or PUR rating. It is an antiquated way. Think about it.


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## BDpups (Aug 9, 2015)

I understand all lights have a par rating. But a higher wattage bulb will not only appear brighter, it will also grow more harder to grow plants in my experience. I would not try to grow HC with a single T8. Mainly because I have tried and failed miserably. 

Diana, thanks for the detailed response. I get all of that. I am not new to keeping planted tanks. I just guess I am old school because I still use HO T5's. None of mine have stopped working so I don't feel the need to replace them with LED's. But replacing the bulbs annually can get a bit pricey so I may change before too long. 

I have just seen on another forum how people are saying that the WPG is not a good "rule" and outdated. I just don't see how it can be true when in my experience if using florescent bulbs, WPG it is a good starting point to know what kind of plants you can grow.

I do use LED's on non planted tanks because I feel they just give off a more appealing light. You can see the water shimmer. Plus they are not that expensive and changing out the florescent for the LED's was not a huge hit to my bank account.


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## ac0xr (Aug 13, 2014)

I found this thread very informative: http://www.plantedtank.net/forums/showthread.php?t=184368


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## mistergreen (Dec 9, 2006)

+1 yes read Hoppy's article. Basically, fluorescence are so efficient these days, where's no telling how much light (too much) output given the wattage.

There's no need to replace the T5's annually btw. I have a PAR meter and measure my t5 HO from time to time. There's no major drop in output. I've had them for 4 years.

I do plan to swap them out for LEDS though someday only because of size.


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## BDpups (Aug 9, 2015)

ac0xr said:


> I found this thread very informative: http://www.plantedtank.net/forums/showthread.php?t=184368


Thanks. 


mistergreen said:


> +1 yes read Hoppy's article. Basically, fluorescence are so efficient these days, where's no telling how much light (too much) output given the wattage.
> 
> There's no need to replace the T5's annually btw. I have a PAR meter and measure my t5 HO from time to time. There's no major drop in output. I've had them for 4 years.
> 
> I do plan to swap them out for LEDS though someday only because of size.


I was always suspicious of the replacing of fluorescent bulbs. That seemed like more of a myth than the WPG. Thanks.


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## keymastr (May 25, 2015)

T8 and T12 bulbs did wear out much faster than T5 bulbs, and when you start to see the dark band near the end of the bulb they should be replaced. That also is harder on the ballast and can cause premature failure of the ballast and starter capacitor.

The watts per gallon rule was started back in the T8 days and once T5 came along it did not really apply because 50 watts of T8 lighting was about the same as 24 watts of T5. Then par meters started becoming more accessible and suddenly you could compare light levels at different depths and that is much more useful information.


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## Steve001 (Feb 26, 2011)

BDpups said:


> I understand all lights have a par rating. But a higher wattage bulb will not only appear brighter, it will also grow more harder to grow plants in my experience. I would not try to grow HC with a single T8. Mainly because I have tried and failed miserably.
> 
> Diana, thanks for the detailed response. I get all of that. I am not new to keeping planted tanks. I just guess I am old school because I still use HO T5's. None of mine have stopped working so I don't feel the need to replace them with LED's. But replacing the bulbs annually can get a bit pricey so I may change before too long.
> 
> ...


 I'm going to make an assumption that the lights you've been using fit somewhere in the daylight range and are triphosphore that's why they work. Higher wattage would correspond too higher light, PAR, PUR.


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## BDpups (Aug 9, 2015)

keymastr said:


> T8 and T12 bulbs did wear out much faster than T5 bulbs, and when you start to see the dark band near the end of the bulb they should be replaced. That also is harder on the ballast and can cause premature failure of the ballast and starter capacitor.
> 
> The watts per gallon rule was started back in the T8 days and once T5 came along it did not really apply because 50 watts of T8 lighting was about the same as 24 watts of T5. Then par meters started becoming more accessible and suddenly you could compare light levels at different depths and that is much more useful information.


Good to know. Thanks. 


Steve001 said:


> I'm going to make an assumption that the lights you've been using fit somewhere in the daylight range and are triphosphore that's why they work. Higher wattage would correspond too higher light, PAR, PUR.


And yes. I use 6700k and 10,000k together in my planted tanks.


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## Christophe (Oct 23, 2013)

Watts per gallon is kinda like trying to figure out how fast you're driving by your gas mileage.


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## BBradbury (Nov 8, 2010)

*Using Watts Per Gallon*



BDpups said:


> Is this now the inch per gallon rule of fish? I'm hearing it is "outdated and inaccurate" It's because of LED's and PAR rating. I get that when it comes to LED's. But if you are using florescent bulbs, it would still seem to be a valid tool to use to get started. Am I wrong about this?


Hello BD...

Using "watts per gallon" is fine when it comes to florescent lighting. I try to get to 2 watts per gallon for my planted tanks. By using lights from the hardware store, either T8s at 32 watts or T12s at 40 watts per bulb. Bulbs in the 6500K range will do nicely. You can grow most of the plants available at the local pet stores with a fixture that holds two bulbs or four depending on the requirement of the plants you want. This type of lighting is inexpensive and a bulb will last well over a year.

B


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## jboone82590 (Jul 4, 2015)

Damn I just got the most info about lights whit this then I ever wanted thanks for all the info guys now I know what to get for my 29 I'm about to get bulbs for and put some little plants in 

Sent from my SAMSUNG-SM-G870A using Tapatalk


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## exv152 (Jun 8, 2009)

To take it a step further, for me, if a manufacturer of a fixture hasn't released, or has no plans of releasing, PAR data for its products, then that's an automatic disqualifier for me. And there are several out there that still haven't done this. Likewise, if I go to a fish shop where the staff speak in WPG, that gives me a good indicator of what I can expect. But for the day to day hobbyist who just wants to enjoy some healthy plants in a glass box, the WPG rule is just fine.


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## Hoppy (Dec 24, 2005)

exv152 said:


> To take it a step further, for me, if a manufacturer of a fixture hasn't released, or has no plans of releasing, PAR data for its products, then that's an automatic disqualifier for me. And there are several out there that still haven't done this. Likewise, if I go to a fish shop where the staff speak in WPG, that gives me a good indicator of what I can expect. But for the day to day hobbyist who just wants to enjoy some healthy plants in a glass box, the WPG rule is just fine.


T8 light fixtures are not made for aquariums, except for the very cheap light hoods sold with aquarium kits. So, there will never be a manufacturers listing of PAR data for those. The PAR you get from any light varies with distance from the light, and depends greatly on how good the reflectors are. One light fixture with a single T8 bulb may give less than half the PAR another light fixture with a single T8 bulb, and a good reflector, will give. PAR data has to be PAR vs distance, or it is useless.


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## exv152 (Jun 8, 2009)

When I say PAR data, distance was implied.


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## BDpups (Aug 9, 2015)

Christophe said:


> Watts per gallon is kinda like trying to figure out how fast you're driving by your gas mileage.


That's a bit of an odd and inaccurate comment. 


BBradbury said:


> Hello BD...
> 
> Using "watts per gallon" is fine when it comes to florescent lighting. I try to get to 2 watts per gallon for my planted tanks. By using lights from the hardware store, either T8s at 32 watts or T12s at 40 watts per bulb. Bulbs in the 6500K range will do nicely. You can grow most of the plants available at the local pet stores with a fixture that holds two bulbs or four depending on the requirement of the plants you want. This type of lighting is inexpensive and a bulb will last well over a year.
> 
> B


This has been my thinking as well. Thanks.


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## burr740 (Feb 19, 2014)

Like Hoppy said, it all depends on reflectors and distance. That's why WPG cant be relied on.

It often helps me understand something to look at it from extremes. Take for example a nice 48" 2 bulb T5HO unit, and let's say it's mounted 2" above the surface of a 55 gallon tank. We'll call that 2 wpg (54W x 2). And the PAR level is whatever it is. 

But if you raise the fixture 4 feet above the tank, the PAR level would change drastically....still be 2 WPG though.

Same thing would happen if you removed the reflector entirely. PAR takes a nose dive, WPG remains the same.

That's why WPG can literally mean anything. While a having certain PAR means exactly one thing, always.


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## Christophe (Oct 23, 2013)

BDpups said:


> That's a bit of an odd and inaccurate comment.


No, it's pretty much exactly what you're saying in the phrase 'Watts per Gallon'.

Watts is a measure of the power you are putting into the lights. How much light you get out of them depends on their efficiency. The efficiency of fluorescents tends to change over time, so how much light you are getting is a guess (unless you are using a PAR meter). Now consider the fixtures reflectors, if the fixture has them. They drastically effect the amount of light you get also. Two setups of the same wattage can perform very differently. Yes, more watts implies more light, but it ain't necessarily so.

Now consider Gallons. Is this a short, squat tank? A tall, slim tank? A twenty gallon long is a completely different lighting scenario from a twenty gallon high.


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## BDpups (Aug 9, 2015)

Christophe said:


> Watts per gallon is kinda like trying to figure out how fast you're driving by your gas mileage.





Christophe said:


> No, it's pretty much exactly what you're saying in the phrase 'Watts per Gallon'.


You keep on thinking this is an accurate comparison.:icon_roll

BTW. I'm not the one who came up with the phrase, "watts per gallon"


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## Hoppy (Dec 24, 2005)

When everyone used T8 lights for their tanks, and all tanks were of a similar geometry, watts per gallon was a usable way to decide how many bulbs you needed to adequately light the tank. None of the light fixtures in use then had effective reflectors, so that variable was out of the picture, too. Since then the options for lighting have increased substantially, reflectors are widely used, and vary widely in how effective they are, some manufacturers use underpowered ballasts for their fluorescent fixtures, open topped tanks with light fixtures suspended above the tank are now common, and glass/plastic tanks are now available in many different geometries. That leaves only T8 lights sitting on conventionally shaped tanks that can use watts per gallon as a meaningful parameter.


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## Maryland Guppy (Dec 6, 2014)

I can remember only T12 lamps or incandescent.
HO and VHO also available, 800 and 1600mA respectively.

Watts per gallon is not at all accurate.

I have suggested before an app for one's phone for lumens or lux.
It will prove to many the further away the less intensity.
Distance from substrate = less PAR/PUR.
Lumens measurement still does not equal PAR measurements but opens one's eyes to distance related PAR/PUR measurements.

As distance is increased PAR is reduced and so forth.

Some fixtures with available PAR ratings could be measured in lumens.
Math then applied to establish a possible PAR rating for greater distance.


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## Christophe (Oct 23, 2013)

BDpups said:


> You keep on thinking this is an accurate comparison.:icon_roll


My ridiculous phrase was just another example of deriving an output value from the one of the inputs when there are many other variables in between.


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