THE SHORT ANSWER
Use effective daily energy—not maximum watts multiplied by the whole schedule
The conservative ceiling is simple: fixture watts × hours × quantity ÷ 1,000 × electricity rate. But dimmable reef lights normally ramp and run channels below their combined maximum, so multiplying label wattage by the entire photoperiod can overstate real use. Apply an effective duty percentage or measure the fixture with a plug-in energy monitor.
At 26.11p/kWh, a 59W light at full power for ten hours daily is about £56 per year; 95W is about £91; two 95W fixtures are about £181. These are ceilings under the stated assumptions, not promises about a particular programme.
DECISION TABLE
Choose by the constraint that actually changes the answer
Purchase price dominates and duty is modest
Calculate, but do not sacrifice coverage for a few pounds.
Quantity doubles several ownership lines
Model both fixtures and every supplementary bar.
Visual brightness does not reveal electrical draw
Measure representative periods or apply a cautious duty estimate.
Output and field may differ
Compare cost only after confirming both layouts meet the same job.
01 / LIGHTING INTELLIGENCE
The correct formula
Convert watts to kilowatts by dividing by 1,000. Multiply by hours per day, an effective duty fraction, fixture quantity and 365 days. Multiply the resulting kilowatt-hours by the tariff in pounds. If the tariff is written in pence, divide by 100. Standing charges are excluded because the household pays them whether or not the aquarium is present.
For a 95W fixture running ten hours at an estimated 70% effective duty: 0.095 × 10 × 0.70 × 365 equals about 243kWh. At 26.11p/kWh, that is roughly £63.50 per year. The same light treated as full power for all ten hours would be about £90.56.
02 / LIGHTING INTELLIGENCE
Why effective duty matters
A programmed reef light may ramp from zero, hold a peak, then ramp down. Individual channels may be limited, and the fixture may not reach label maximum even when one channel shows 100%. Effective duty compresses the whole daily power curve into one planning percentage.
Use 100% when you want a conservative ceiling. For a better estimate, take power measurements during several programme points and calculate a weighted daily average. The easiest route is an energy monitor that reports accumulated kilowatt-hours over a week. Divide by seven, then annualise at the current tariff.
03 / LIGHTING INTELLIGENCE
Worked maximum-cost comparison
Using ten hours daily at full label power and 26.11p/kWh: a 59W Prime is about £56.22 per year, a 60W ReefLED about £57.18, a 90W A360XE about £85.77 and a 95W XR15 G6 Pro about £90.56. A 115W fixture would be about £109.61. These figures compare electrical ceilings consistently; they do not rank usable light.
Two fixtures double the ceiling before duty is applied. Supplementary bars must be added separately because their schedules may differ. If bars run only six peak hours, do not give them the main fixture’s ten-hour assumption.
04 / LIGHTING INTELLIGENCE
Electricity cannot rescue a poor lighting comparison
A lower-wattage fixture is not economical if two are needed where one broader product would meet the field. Conversely, a high-power premium light is not efficient merely because it can be dimmed. Compare the complete layout at the settings needed to solve the same coral-zone problem.
Coverage, spectrum, coral placement and measurement decide whether the arrangement is suitable. Cost then distinguishes suitable arrangements. This ordering prevents a small annual saving from creating dark edges, premature upgrades or stressed livestock.
05 / LIGHTING INTELLIGENCE
Tariffs and seasonal heat
Use the unit rate from your bill rather than a national average whenever possible. Time-of-use tariffs complicate the calculation because the photoperiod can span different rates. Multiply energy used in each price window separately. Moving the schedule solely to cheap hours may conflict with household viewing or stable routine, so treat tariff optimisation as secondary.
All electrical consumption ultimately becomes heat in the room. Lighting can reduce heater demand in cool periods or increase overheating risk in summer, but the net effect is installation-specific. Measure heater and cooling duty rather than assuming one cancels the other.
06 / LIGHTING INTELLIGENCE
How to reduce cost without destabilising the reef
First confirm the programme is delivering more than the livestock plan requires. Reduce excess peak intensity or unnecessarily long plateaus gradually and monitor the measured field. Clean salt and dust from approved surfaces because fouling can tempt owners to compensate with more output. Do not shorten or dim a schedule abruptly around established coral.
The largest saving may come from avoiding an unnecessary second fixture through a better aquascape and mount, or avoiding a single overstretched fixture that later has to be replaced. Energy efficiency begins with a coherent lighting layout.
07 / LIGHTING INTELLIGENCE
The Reef Bench ownership rule
Publish the rate, wattage, hours, duty and quantity beside every cost. A currency figure without those inputs cannot be audited when tariffs change. Reef Bench uses 26.11p/kWh as a dated example for summer 2026, while every calculator lets the reader replace it.
Review the estimate after one month of normal use and again after major programme changes. Record kilowatt-hours and the date range. A measured ownership history is more valuable than a precise-looking forecast based on label maximum.
WORKED UK EXAMPLE
Two-light 90cm reef with a six-hour peak
Two 95W fixtures ramp for two hours, hold a six-hour peak and ramp down for two hours. The owner initially calculates 190W for ten full hours, producing a ceiling around £181 per year. A plug-in monitor records an average of 1.25kWh per day for the complete lighting system.
At 26.11p/kWh, the measured annual estimate becomes about £119. The difference is not treated as permission to increase intensity; it simply replaces a coarse ceiling with evidence from the actual programme. When two fill bars are later added for four hours, their measured energy is added separately and the ownership record is updated.
THE REEF BENCH CHECK
What this page can—and cannot—prove
Electrical input, physical dimensions, published spread, control method and included accessories are facts that can be checked. They help eliminate unsuitable choices and expose the complete installed cost. They do not prove the PAR at a coral, long-term colour response, fan noise in your room or reliability over several years. Those outcomes depend on the individual unit, installation, programme, water, aquascape and maintenance.
Before changing a working aquarium, record the present fixture, mounting height, schedule, channel intensities and livestock positions. Photograph the tank under the same settings and, where coral demand justifies it, borrow or rent a suitable PAR meter. Change one meaningful variable at a time, begin conservatively and use acclimation controls. A copied online schedule is a starting hypothesis, not evidence that the same percentages will suit another aquarium.
Our recommendations therefore use conditional language. “Best for” means the product or layout solves the stated constraint more convincingly on documented evidence; it does not mean every buyer should choose it. A lower-cost fixture can be the stronger decision for a compact soft-coral island, while a higher-cost system becomes defensible when broader distribution, multiple-light coordination or future expansion is a real requirement.
OWNERSHIP COST
Compare the complete mounted system
Add the fixture, arm or hanging kit, rail, diffuser or shade, controller where required, and any second light needed for the intended footprint. Then calculate electricity from the programmed output and photoperiod rather than assuming the label maximum runs all day. A premium light may justify its cost through distribution or control; it does not justify itself merely by having a higher wattage or a longer feature list.
Also price the parts most likely to interrupt ownership: power supplies, fans, mounting hardware and the route for UK warranty support. App-based control is convenient, but confirm how the fixture behaves after a power cut, router change or service outage. Save the manual and final schedule outside the app. The best installation remains understandable and recoverable when the phone or internet is unavailable.
COMMON QUESTIONS
Frequently asked questions
How much does a reef light cost to run?
It depends on watts, programme, quantity and tariff. A 59W light at full power for ten hours is about £56 yearly at 26.11p/kWh.
Do LED lights use maximum wattage all day?
Usually not when dimmed and ramped. Measure or apply an explicit effective duty estimate.
Should I include standing charge?
No, not when estimating the aquarium’s incremental cost, because the household pays it regardless.
Can I run reef lights at night on a cheaper tariff?
The schedule should serve livestock and household routine first; calculate time bands separately if using a time-of-use tariff.
Does lowering intensity save electricity?
Normally yes, but make gradual livestock-safe changes and confirm the remaining light field is suitable.
PRIMARY SOURCES
Check the specifications yourself
Links below lead to manufacturer information used for the factual comparison. Coverage remains a manufacturer claim unless explicitly described as our measurement.