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    Reef light schedule and PAR calculator

    Plan how long your light runs, how it ramps up and down, and whether the PAR your corals sit in matches what the sources suggest. It also shows what the light costs to run.

    Formulas and sources last checked . How we check

    Calculator

    Corals

    From a PAR meter, or the maker’s chart for your light and height.

    in PAR
    in hours
    in hours
    in hours
    in watts
    in per cent

    UK price cap, 1 October to 31 December 2026.

    in p per kWh

    Result

    Daily light3.9 mol per square metre a day

    Light on
    10.0 hours
    Soft corals range (BRS 2025)
    75 to 150 PAR
    Light energy
    0.68 kWh a day
    Light running cost
    £64.85 a year

    OK: 120 PAR is within the published range for soft corals.

    How long should reef lights be on?

    PAR is the light corals can use, measured in micromoles of light per square metre per second. Multiply it by the seconds the light is on and you get the daily light integral (DLI): the total a coral receives in a day. A straight ramp up or down counts as half its length at full power. Energy is the light’s watts, times the share of full power, times the hours.

    DLI (mol per square metre a day) = peak PAR x (full-power hours + ramp hours / 2) x 3600 / 1,000,000kWh a day = watts x power share x (full-power hours + ramp hours / 2) / 1000

    Bulk Reef Supply (2024) suggests 10 hours: a one-hour ramp up, 8 hours at full power and a one-hour ramp down, and says most keepers run 8 to 12 hours. Practical Fishkeeping also gives 8 to 12 hours, with most corals in 100 to 300 PAR and many lights run at about 75 per cent.

    Worked example: soft corals at 120 PAR

    1. A one-hour ramp, 8 hours at full power, a one-hour ramp down: 10 hours on, counted as 9 hours at full power.
    2. 120 PAR x 9 x 3600 / 1,000,000 = 3.9 mol per square metre a day.
    3. A 100 W light at 75 per cent for those 9 effective hours uses 0.675 kWh a day: about £65 a year at 26.32p per kWh.

    How much PAR do soft, LPS and SPS corals need?

    PAR ranges and where sources disagree
    TopicWhat sources sayOur reading
    Soft corals
    • Bulk Reef Supply (2025): 75 to 150.
    • Bulk Reef Supply (2019): start at 150 to 250; 100 to 200 is a typical captive average.
    • Several retailers say 50 to 150 (typical, check).
    		The newer BRS figure is lower than its 2019 advice. Start low; most soft corals and LPS do well around 100 to 150.
    LPS corals
    • Bulk Reef Supply (2025): 75 to 150.
    • Bulk Reef Supply (2019): start at 150 to 250; 100 to 200 is a typical captive average.
    • A BRS coral pack page suggests starting at 100 to 150; other vendors give 150 to 250 (typical, check).
    		The newer BRS figure is lower than its 2019 advice. Start low; most soft corals and LPS do well around 100 to 150.
    SPS corals
    • Bulk Reef Supply (2025): 200 to 350.
    • Some keepers run higher (typical, check). Natural reefs measure about 150 to over 450.
    		Start at the low end and raise slowly over weeks while you watch colour and polyp extension.

    What are the limits?

    Other calculators

    Sources

    1. Bulk Reef Supply, Dial in your reef lighting with a PAR meterPublished 7 April 2025. Read 21 October 2026, read in full. Used for: PAR ranges by coral group.
    2. Bulk Reef Supply, Reef aquarium lighting guide: what is the right PARPublished 17 May 2019. Read 21 October 2026, read in full. Used for: PAR on natural reefs, acclimatising to new light.
    3. Bulk Reef Supply, Best reef tank lighting schedulePublished 25 March 2024. Read 21 October 2026, read in full. Used for: 10 hours with one-hour ramps.
    4. Practical Fishkeeping, Beginner’s guide to reef tanks: lightingRead 21 October 2026, read in full. Used for: 8 to 12 hours, most corals 100 to 300 PAR.
    5. Ofgem, Changes to energy price cap between 1 October and 31 December 2026Read 21 October 2026, search summary only. Used for: 26.32p per kWh unit rate.