Your monthly running cost is a year’s electricity plus a year’s supplies, divided by 12. Tank size alone can’t tell you the bill: you need what your equipment really draws, your tariff, your water-change plan and what you spend on supplies. Work each one out separately, then check the estimate against what you actually buy.
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Keep buying and running costs apart
The tank, cabinet, lights, pumps and starting equipment are upfront costs. Electricity, test reagents, salt, water, food, additives and filter media are running costs. Your first-year total adds the two together, which is a different number from your monthly running budget.
Check what a kit actually includes. Count the kit price once, then enter anything that came with it at zero cost while keeping its power use. Otherwise a light that came in the box can end up counted twice. Keep delivery, livestock and any building or electrical work as separate lines if you want a complete personal budget.
Work out electricity one device at a time
Yearly electricity in pounds = watts × quantity × hours a day × duty ÷ 1,000 × 365 × your rate in pence ÷ 100. A duty of 0.5 means the device runs at the entered wattage for half the hours you entered. The wattage on the label is a starting point, not proof of what it really uses.
Use the unit rate on your bill. Ofgem publishes reference rates by region and payment method, but the household price cap isn’t an aquarium allowance. We leave out your standing charge, because you’d pay it with or without a tank.
For a heater, write down how often you expect it to cycle rather than counting it as always on. For a dimmed light or adjustable pump, a measured reading beats the label maximum. Running low, middle and high scenarios shows how much your budget moves when your assumptions change.
A worked example you can check
This is a made-up example to show the maths: one 100 W total load running 24 hours a day, a 25p/kWh tariff, £1,000 of upfront equipment and £120 a year of supplies. The 100 W stands in for the example’s combined load. It isn’t a recommendation for a 200 L tank or an estimate for every reef.
| Part | Calculation | Result |
|---|---|---|
| Energy per year | 0.1 kW × 24 × 365 | 876 kWh |
| Electricity | 876 × £0.25 | £219/year |
| Supplies | Your entered allowance | £120/year |
| Running total | £219 + £120 | £339/year |
| Average per month | £339 ÷ 12 | £28.25/month |
| First year | £1,000 + £339 | £1,339 |
Add salt and water without counting them twice
Your water-change budget depends on your real system volume, how much you change, how often, your salt’s published or measured mixing ratio and its pack price. The saltwater planner turns those into a yearly cost. Save that plan and import it into the running cost calculator, where it replaces the default or previously linked salt and water lines. Then check any custom lines you’ve added for duplicates.
RODI water isn’t automatically free. You can add an allowance for tap water, filters and resin, or enter a price per litre if you buy water. Keep the RODI unit itself separate from its replacement filters, and don’t count the same replacements on two lines.
Compare the estimate with what you really spend
The free Reef Companion records purchases by date and category. Its monthly total is simply what you’ve logged that month, while the calculator’s monthly figure is a yearly average. Buy a big bucket of salt this month and your spending will look high, even if your usage over the year matches the forecast.
Save your plan, attach it to the Companion and keep your assumptions visible. After a few months of receipts and any electricity readings, refine the estimate. Records stay in this browser only once you save them, so download a copy as your own backup.
Common questions
Is a smaller reef always cheaper to run?
Size is only one part of it. Your equipment, what it really draws, the room, your water changes and your supplies decide the cost. Compare specific setups rather than giving each tank size one price.
Does the monthly figure include livestock?
Only what you enter is counted. The worked example leaves out livestock and anything else not entered. If you add them, label them as separate lines.
Can I use my own tariff?
Yes. The calculator asks for your rate in pence per kWh. Use your own, and update the plan when your rate or usage changes.
Sources and limits
Last checked 12 October 2026. Our worked examples use made-up inputs to show the maths. They aren’t measured costs or care targets for your tank.
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Calculate your reef running costs · Free Reef Companion · All calculators