Why include equipment wattage — doesn't the chiller only fight the room?

Lights dump 80–90 % of their input power as heat into the water column. A 200 W reef-bar over 50 gal of water can hold the tank 4 °F above ambient even in a cool room. Always count equipment.

What's the deal with HP vs BTU/hr?

BTU/hr is the actual cooling capacity. HP is a marketing legacy from refrigeration (1 HP ≈ 12,000 BTU/hr). Buy by BTU, not HP — different manufacturers' '1/4 HP' chillers vary by 800 BTU.

Yes — go one notch up. An oversized chiller short-cycles less, holds temp tighter, and lasts longer. Undersized chillers run continuously, overheat, and fail in 12–18 months.

Inline (plumbed in from sump return) is more efficient and quieter. Drop-in is for emergency use or display-only systems where you can't run plumbing. Inline almost every time for permanent reef installs.

Calculator

Aquarium Chiller BTU Calculator

Name: Aquarium Chiller BTU Calculator
Author: reefcalcs

Reef tanks need chillers when (a) your room reliably runs above target tank temperature, or (b) lights and pumps dump enough heat that fans alone can't keep up. The math: equipment wattage × 3.412 BTU/W gives the equipment heat load; ambient gain adds ~5 BTU/hr per gallon per °F of room→tank drop. Add a 25 % safety factor for duty cycle and cabinet heat, then round up to the next common chiller nameplate.

JavaScript is disabled in your browser, so the interactive calculator below won't run. The math, formula, and worked example are all written out further down this page under “How this is calculated.” Enable JS (or open this page in another browser) to use the live calculator.

Whether you compute by hand or in the calculator, the same caveat applies: verify against your test kit and the linked primary source before dosing. reefcalcs is reference information, not veterinary or engineering advice.

Chiller sizing

Tank volume (US gal)

Target tank temperature (°F)

Hottest ambient room temp (°F)

Use your summer high, not winter.

Equipment heat input (W)

Sum of lights, return pump, powerheads.

Total heat load

4,348BTU/hr

In watts

1,274W

Buy this size or larger

6,000BTU/hr

~0.5 HP nameplate

HP equivalent

0.50HP

Heat load = equipment W × 3.412 BTU/W + 5 BTU/hr per gallon per °F of ambient→target drop, then × 1.25 safety factor. Common sizes round up to JBJ / TECO / Aqua Euro nameplate capacities.

How this is calculated

Heat load (BTU/hr) =
  equipment_W × 3.412
  + tank_gal × 5 × max(0, ambient_°F − target_°F)

Recommended BTU/hr = ceil(heat_load × 1.25 → next size)

Common nameplate sizes (BTU/hr):
  645   ≈ 1/15 HP    JBJ Arctica Mini
  950   ≈ 1/10 HP    Aqua Euro / JBJ
  1500-2200          JBJ Arctica 1/10 HP+
  3000  ≈ 1/4 HP
  4000  ≈ 1/3 HP     JBJ DBE-200
  6000  ≈ 1/2 HP
  9000  ≈ 3/4 HP
  12000 ≈ 1 HP

The 5 BTU/hr per gallon per °F figure is the conservative end of the TECO/Aqua Euro sizing tables — they actually publish 4–6 BTU depending on cabinet ventilation. Equipment heat is exact: every watt of lighting and pump power eventually becomes heat in the water (the only path out is evaporation, which is why fans help so much).

Before buying a chiller, try fans first. A pair of 120 mm USB fans blowing across the sump surface drops tank temperature 3–4 °F via evaporative cooling — that's enough for most reefs in most climates. Chillers earn their place above 86 °F room highs, basement metal-halide rigs, or 250 W+ LED bars in low-airflow rooms.

FAQ

Why include equipment wattage — doesn't the chiller only fight the room?: Lights dump 80–90 % of their input power as heat into the water column. A 200 W reef-bar over 50 gal of water can hold the tank 4 °F above ambient even in a cool room. Always count equipment.
What's the deal with HP vs BTU/hr?: BTU/hr is the actual cooling capacity. HP is a marketing legacy from refrigeration (1 HP ≈ 12,000 BTU/hr). Buy by BTU, not HP — different manufacturers' '1/4 HP' chillers vary by 800 BTU.
Should I oversize?: Yes — go one notch up. An oversized chiller short-cycles less, holds temp tighter, and lasts longer. Undersized chillers run continuously, overheat, and fail in 12–18 months.
Inline vs drop-in?: Inline (plumbed in from sump return) is more efficient and quieter. Drop-in is for emergency use or display-only systems where you can't run plumbing. Inline almost every time for permanent reef installs.

Share / embed this calculator

Paste this snippet into any blog or forum post that allows raw HTML to embed the calculator.

<iframe
  src="https://reefcalcs.com/embed/chiller-btu/"
  width="100%"
  height="720"
  style="border: 1px solid #1C2D34; border-radius: 8px; max-width: 720px;"
  loading="lazy"
  title="ReefCalcs Aquarium Chiller BTU Calculator"
></iframe>

Sources & references

01
JBJ Arctica chiller sizing chart
https://www.jbjlighting.com/arctica-chiller/
02
Engineering Toolbox — Convert watts to BTU/hr (3.412 conversion factor)
https://www.engineeringtoolbox.com/electric-power-energy-d_605.html
03
Aqua Euro USA — chiller sizing tables
https://www.aquaeurousa.com/aquarium-chillers

Last reviewed 2026-05-25