Quick answer: For DIY LED aquarium lights, plan the light reaching your plants separately from the electrical power needed by the fixture. Use the calculations below to size a proposed strip load and estimate running cost; neither calculation establishes whether the tank has enough plant light.
This guide covers planning an above-water freshwater planted-tank light, with a separate note on reefs. It replaces a fixed parts list with decisions about light measurement, power, controls and water exposure. The LED lighting calculator hub brings together the electrical planning tools used for this kind of project.
What should you measure for a planted aquarium?
Use plant-light information to assess the growing conditions; use watts to calculate electrical demand. According to Wikipedia's PAR overview, photosynthetically active radiation covers 400–700 nm, and plant biologists often quantify it as photosynthetic photon flux density, or PPFD, in μmol/m²/s. That describes photons arriving per area and time, rather than electricity consumed.
A fixture's wattage alone therefore cannot supply a PPFD reading at the plants. For a proposed DIY layout, distinguish the electrical calculation you can make from the light measurement you still need. This guide does not assign a universal PPFD target or a preferred colour-temperature number.
LED efficiency is a reason to consider the technology. ENERGY STAR says LED lighting produces light up to 90% more efficiently than incandescent lighting. That is a general lighting comparison, not a promise that a particular aquarium strip will deliver suitable plant light or reduce your tank's bill by that percentage.
How many watts per gallon should you use?
Treat watts per gallon as a rule of thumb, not an LED sizing specification. According to Wikipedia's planted-aquarium article, a long-quoted rule is 2–4 W per gallon. It describes electrical power per tank volume; it does not measure the light reaching a leaf.
Worked example: the old rule applied to an assumed tank
Assumed input: a 20-gallon tank. Apply the source's rule only to see what it implies:
- Formula: reference power = tank volume × watts per gallon.
- Lower end = 20 gallons × 2 W/gallon = 40 W.
- Upper end = 20 gallons × 4 W/gallon = 80 W.
The resulting range is not a recommendation to install that much LED power. Keep the following quantities separate when comparing a strip, a finished fixture and an older lighting recommendation.
| Quantity | What it describes | What to do with it |
|---|---|---|
| Watts | Electrical power | Size the proposed load and estimate energy use |
| Watts per gallon | Electrical power divided by tank volume | Interpret older recommendations without treating them as measured plant light |
| PPFD | Photosynthetic photons arriving per area and time | Assess light at the plants; according to Wikipedia's PAR overview, this is a photon-based measure |
| Strip length and density | Physical length and LED count per length | Calculate the proposed strip's load and emitter count |
How do you calculate a DIY strip's power?
Multiply strip length by watts per metre, then divide power by voltage to calculate current. The LED strip calculator is the closest tool for this part of a DIY build. Its SMD 2835, 60 LEDs/m option uses 4.8 W/m as the calculator's planning figure. This is a site estimate, not a verified rating for every strip sold under that designation.
Worked example: an assumed strip layout
Assumed inputs: 1 m of strip, the calculator's SMD 2835 option at 60 LEDs/m, and a 12 V supply compatible with the assumed strip.
- Formula: strip power = length × power per metre.
- Strip power = 1 m × 4.8 W/m = 4.8 W.
- Formula: current = power ÷ voltage.
- Current = 4.8 W ÷ 12 V = 0.4 A.
- Formula: LED count = length × density.
- LED count = 1 m × 60 LEDs/m = 60 LEDs.
These results describe the strip load. They do not specify an aquarium-safe enclosure, prove plant suitability or represent the complete system's wall-plug consumption. Replace the planning figure with the actual strip's power rating when evaluating a real purchase.
What will the aquarium light cost to run?
Calculate running cost from total input power, operating time and your electricity tariff. The electricity cost calculator uses daily energy = watts × quantity × hours ÷ 1,000, then multiplies by days and price per kWh.
Worked example: an assumed complete fixture
Assumed inputs: one 20 W complete fixture, 8 hours/day, 30 days/month, and $0.17/kWh. The operating time is an arithmetic assumption, not a recommended aquarium photoperiod. This fixture is a separate example from the strip above.
- Total power = 20 W × 1 = 20 W.
- Daily energy = 20 × 8 ÷ 1,000 = 160 ÷ 1,000 = 0.16 kWh.
- Monthly energy = 0.16 × 30 = 4.8 kWh.
- Monthly cost = 4.8 × $0.17 = $0.816, rounded to $0.82.
Use complete-system input power for a complete-system bill estimate. A calculation based only on the LED strip's load leaves other system consumption outside the estimate.
What should the build plan include?
Plan a low-voltage DC light supplied from a GFCI-protected outlet, with mains work handled separately. According to Wikipedia's GFCI article, 5 mA is a typical GFCI outlet sensitivity. That protection is not evidence that a homemade light or connection is suitable for immersion.
Use this checklist before assembling the low-voltage portion:
- Light requirement: decide how you will assess illumination at the plants, independently of the power calculation.
- Power compatibility: match the selected light, driver and controller using their documented requirements.
- Water exposure: choose components documented for the intended location; do not treat heat-shrink or glue alone as proof of underwater suitability.
- Controls: include a timer if you want a repeatable schedule. According to Wikipedia's planted-aquarium article, timers are used to give plants a set lighting cycle.
Use a qualified electrician for mains wiring or outlet changes. This checklist does not provide mains connection instructions.
FAQ
Are LED lights a good choice for planted tanks?
According to Wikipedia's aquarium-lighting article, LED fixtures are generally the most efficient option for planted aquariums. That general comparison does not establish the suitability of a particular DIY strip.
Can I size the light by LED count alone?
No: count tells you how many emitters the layout contains, not the PPFD reaching plants. In the assumed strip example, 1 m × 60 LEDs/m = 60 LEDs, while power is calculated separately as 1 m × 4.8 W/m = 4.8 W.
Can the same DIY light serve a freshwater tank and a reef?
Do not assume that it can. According to Wikipedia's aquarium-lighting article, reef LED arrays combine different colours to provide spectral coverage suitable for coral growth; a freshwater power calculation does not establish that coverage.
How long should I leave the lights on?
The 8-hour assumption in the cost example is not a care recommendation. According to Wikipedia's aquarium article, lighting duration and quality must be balanced with plant mass, carbon dioxide and nutrients, so operating time needs its own husbandry decision.
Why might stronger lighting accompany algae growth?
According to Wikipedia's aquarium article, too few plants or insufficient carbon dioxide can leave nutrients unused for a given amount of light, resulting in algae growth. Increasing fixture power alone does not resolve that imbalance.
Does low-voltage power mean I can submerge the strip?
No: the voltage calculation supplies no evidence of immersion suitability. Keep this proposed build above water and select components for their intended exposure; a calculated current is not a waterproofing assessment.
Jack Shi
Founder & editor, LEDaskJack Shi builds and writes LEDask, an independent LED-lighting tools project operated by clooms. He designs the calculators, checks their formulas and reference values against published engineering data, and writes the guides across the site.
