THE SHORT ANSWER
Work from the energy your lights actually use, not the maximum wattage
The worst case is simple to work out: watts × hours × number of lights ÷ 1,000 × your electricity rate. But dimmable reef lights ramp up and down and usually run below their full power, so multiplying the label wattage by your whole schedule can overstate what you really use. Apply an average power percentage, or measure your lights with a plug-in energy monitor.
At 26.11p/kWh, a 59W light at full power for ten hours a day costs about £56 a year; a 95W light about £91; and two 95W lights about £181. These are worst cases based on those assumptions, not what any particular schedule will cost.
DECISION TABLE
Which one is right for your tank?
The purchase price matters most and power use is low
Work it out, but don’t sacrifice coverage to save a few pounds.
Doubling up doubles several running costs
Include both lights and every extra bar.
How bright it looks tells you nothing about power use
Measure at typical times, or use a cautious average.
They may put out different amounts of light, over different areas
Only compare costs once you know both setups do the same job.
01 / IN DETAIL
The formula
Turn watts into kilowatts by dividing by 1,000. Multiply by hours per day, your average power as a fraction, the number of lights and 365 days. Then multiply those kilowatt-hours by your rate in pounds (if it’s in pence, divide by 100). Leave out the standing charge, because you pay it whether or not you have an aquarium.
For a 95W light running ten hours a day at an estimated 70% average: 0.095 × 10 × 0.70 × 365 comes to about 243kWh. At 26.11p/kWh, that’s roughly £63.50 a year. The same light counted at full power for all ten hours would cost about £90.56.
02 / IN DETAIL
Why average power matters
A programmed reef light may ramp up from zero, hold at its peak, then ramp back down. Some channels may be capped, and the light may not reach its label maximum even when one channel shows 100%. Average power squeezes that whole daily curve into one percentage you can plan with.
Use 100% when you want a cautious worst case. For a better estimate, measure the power at several points in your schedule and work out a weighted daily average. The easiest way is an energy monitor that adds up kilowatt-hours over a week. Divide by seven, then multiply up to a year at your current rate.
03 / IN DETAIL
Worst-case costs compared
At full label power for ten hours a day and 26.11p/kWh, a 59W Prime costs about £56.22 a year, a 60W ReefLED about £57.18, a 90W A360XE about £85.77 and a 95W XR15 G6 Pro about £90.56. A 115W light would cost about £109.61. These figures compare worst-case electricity costs fairly; they don’t rank how much usable light you get.
Two lights double the worst case before you allow for average power. Add any extra bars separately, as their schedules may be different. If bars only run for a six-hour peak, don’t give them the main light’s ten hours.
04 / IN DETAIL
Electricity costs shouldn’t decide your lighting
A lower-wattage light isn’t cheaper if you need two of them where one wider light would do. Equally, a powerful premium light isn’t efficient just because it can be dimmed. Compare the whole setup, at the settings needed to light the same coral area.
Coverage, spectrum, where your corals sit and your measurements decide whether a setup is suitable. Cost then helps you choose between suitable setups. In that order, a small yearly saving can’t lead you into dark edges, an early upgrade or stressed livestock.
05 / IN DETAIL
Tariffs and heat through the year
Use the unit rate on your own bill rather than a national average where you can. Time-of-use tariffs make it more complicated, because your lighting schedule may span different rates. Work out the energy used in each price band separately. Moving your schedule to cheap hours only can clash with when you want to watch the tank, or with a steady routine, so treat it as a secondary concern.
All the electricity your lights use ends up as heat in the room. That can mean your heater works less in cooler months, or add to overheating in summer, but the overall effect depends on your setup. Measure how hard your heater and any cooling work rather than assuming one cancels out the other.
06 / IN DETAIL
Cutting the cost without upsetting your reef
First, check whether your schedule gives your livestock more light than it needs. Bring down excessive peak intensity or unnecessarily long peaks gradually, and keep measuring. Clean salt and dust off approved surfaces, because a dirty lens can tempt you to turn the output up to compensate. Never shorten or dim the schedule suddenly in a tank with established corals.
The biggest saving may come from avoiding an unnecessary second light through a better aquascape and mount, or from avoiding one overstretched light that you’ll end up replacing. Efficient lighting starts with a layout that makes sense.
07 / IN DETAIL
How we show running costs
We show the rate, wattage, hours, average power and number of lights next to every cost. A pound figure without those can’t be checked when prices change. We use 26.11p/kWh as a dated example rate for summer 2026, and every calculator lets you enter your own.
Check your estimate after a month of normal use, and again after any big change to your schedule. Note the kilowatt-hours and the dates. A record of what you’ve actually used is worth more than a precise-looking forecast based on the label maximum.
WORKED UK EXAMPLE
Two lights on a 90cm reef with a six-hour peak
Two 95W lights ramp up for two hours, hold a six-hour peak and ramp down for two hours. The owner first works out 190W for ten full hours, a worst case of around £181 a year. A plug-in monitor then shows the whole lighting setup uses 1.25kWh a day on average.
At 26.11p/kWh, that measured figure comes to about £119 a year. The difference isn’t a reason to turn the lights up; it just swaps a rough worst case for evidence from the real schedule. When two fill bars are later added for four hours a day, their energy is measured and added separately, and the running cost record is updated.
WHAT WE CAN AND CAN’T TELL YOU
What this comparison can prove, and what it can’t
Power draw, dimensions, published spread, controls and what’s in the box are facts we can check. They help rule out the wrong lights and show the true cost once installed. They can’t prove the PAR at a particular coral, how colours develop over time, how loud the fan will be in your room, or how reliable it will be over several years. Those depend on the individual light, how it’s installed and programmed, your water, aquascape and maintenance.
Before changing a light on a tank that’s doing well, write down your current light, its mounting height, schedule, channel settings and where each coral sits. Take photos under the same settings and, if your corals are demanding, borrow or rent a PAR meter. Change one thing at a time, start cautiously and use the acclimation mode. A schedule copied from the internet is a starting point, not proof the same percentages will suit your tank.
That’s why our recommendations are conditional. “Best for” means the light handles that situation best on the evidence available, not that everyone should buy it. A cheaper light can be the smarter choice for a small soft-coral island, while a pricier system earns its place when you genuinely need wider coverage, several lights working together or room to expand.
RUNNING COSTS
Compare the full cost once it’s mounted
Add up the light, the arm or hanging kit, any rail, diffuser or shade, a controller if it needs one, and a second light if your tank needs it. Then work out the electricity from the output and hours you’ll actually programme, rather than assuming it runs at full power all day. A premium light can be worth it for better spread or control, but not just for higher wattage or a longer feature list.
Also price the parts most likely to fail: power supplies, fans and mounts, and check how UK warranty claims work. App control is convenient, but find out what the light does after a power cut, a new router or an app outage. Keep the manual and your final schedule somewhere outside the app, so you can recover your setup without your phone or the internet.
COMMON QUESTIONS
Frequently asked questions
How much does a reef light cost to run?
It depends on the wattage, your schedule, how many lights you have and your tariff. A 59W light at full power for ten hours a day costs about £56 a year at 26.11p/kWh.
Do LED lights use their maximum wattage all day?
Usually not, if they’re dimmed and ramped. Measure them, or use a clear average power estimate.
Should I include the standing charge?
No, not when working out what the aquarium adds to your bill, because you pay it either way.
Can I run my reef lights at night on a cheaper tariff?
Your schedule should suit your livestock and your household first. If you’re on a time-of-use tariff, work out each price band separately.
Does turning the lights down save electricity?
Usually yes, but make gradual changes that are safe for your livestock, and check the remaining light still suits your tank.
SOURCES
Check the specifications yourself
These links go to the manufacturer information we used. Coverage figures are the maker’s claims unless we say we measured them.