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Use actual water volume after displacement, not the advertised tank size. Enter your bill’s unit rate. Volume labels your scenario; it does not determine power consumption.
1. Setup and equipment
Enter purchase price per item. For a kit, count its price once. Included components can have £0 purchase cost while retaining their power use. Untick a row to exclude both its cost and power. Zero watts is valid.
2. Recurring costs
Enter the cost for the selected period. Weekly costs use 365 ÷ 7, not 52 rounded weeks. Avoid counting initial consumables twice. Replacement reserve is an optional annual allowance, not a prediction.
3. Your ownership estimate
Net water volume: enter a value greater than zero.
Explore uncertainty
Change power assumptions above, or compare electricity-only sensitivity below. These percentages are editable scenarios, not confidence intervals. Consumables and upfront costs stay fixed.
Basis: 365 days/year; month = annual ÷ 12; GBP; full precision until display. Heater duty and actual draw can differ from rated power. Excludes livestock, structural/electrical work, delivery, finance, inflation and unentered costs. Standing charges are excluded. Formula ownership-1.0; prepared 12/10/2026.
How the calculation works
For each device: watts × quantity × hours/day × duty percentage ÷ 100 ÷ 1,000 = daily kWh. Annual electricity is daily kWh × 365 × tariff in p/kWh ÷ 100. Upfront purchases use unit price × quantity. Annual running cost adds electricity, annualised consumables and any replacement reserve. First year adds upfront purchases; average month is annual running ÷ 12.
A worked example you can check
An illustrative 100 W device running 24 hours/day at 100% duty uses 876 kWh/year. At an illustrative 25p/kWh that costs £219/year or £18.25/month. With £1,000 upfront and £120 annual consumables, annual running is £339, average monthly running is £28.25, and the first year is £1,339. This is an arithmetic example, not a recommended setup or current tariff.
Check the assumptions
Enter the unit rate from your own electricity bill. Ofgem explains how price-cap rates work; a cap is not a maximum bill. A plug-in energy monitor can help replace nameplate power with measured consumption. Do not combine measured average wattage with a second duty adjustment for the same variation. Follow each manufacturer’s safety instructions.
Common questions
Does this prescribe equipment for my tank?
No. It prices the items you choose. Water volume is a scenario label, not a safety or equipment-sizing rule.
What if my kit includes the light or pump?
Put the full kit cost in one row. Set the included component’s purchase price to £0 and enter its power separately. Excluding a row removes both its purchase cost and electricity.
Are monthly costs exact bills?
No. They are annual costs divided by 12. Actual use, seasonal heating and replacement timing vary. Costs not entered remain excluded.
Build a better shortlist
Use the UK setup-cost guide, equipment checklist and tank-kit comparison. For power alone, keep using the electricity calculator. Read our affiliate disclosure; commercial links never change these calculations.
PROPOSED / NOT AVAILABLE YET
A plan you can come back to
We are exploring Reef Companion at £29 billed annually: one saved cloud tank profile, your own recurring maintenance tasks, completion history, spending records and optional reminder emails. Demand and launch timing are unconfirmed. Free calculators would remain free.
See the proposed Companion and interest form preview →