For one group of fittings, the calculator returns the annual kilowatt-hour difference and, when money and cost inputs are complete and the money saving is positive, a simple payback in years. Simple payback is the total project cost for the whole group divided by the annual money saving. A blank cost does not become zero. Explicit zero total cost with a positive money saving returns 0.00 years labelled no upfront cost. The result does not say the replacement gives the same light.

Open the calculator. Copy the unit rate from one bill before typing it here. Record a repair or replace decision without using this payback as the decision. Read the green claims guide before publishing a payback as a saving.

What the calculator returns

The control is on this page, after the worked examples. Open the calculator if you want the form first. It does not send the inputs anywhere. It is not a downloadable file. The form stays inactive until the page script starts, so the numbers are not submitted in the address bar.

Annual kilowatt-hours depend on quantity, both wattages and hours. Money also needs the tariff in pence per kilowatt-hour. Payback also needs both costs, under a chosen installation treatment, and a positive money saving. An unknown treatment does not add the two cost boxes.

Wattages are whole-fitting input power, not a lamp-only label assumed to include every load. Record a separate power source for each wattage when the existing and replacement figures have different bases. Use one VAT basis for the costs and the tariff. The page does not calculate tax relief.

Matching the arithmetic does not mean the lighting is equivalent or that a saving will occur.

Inputs, units and the calculation

Quantity must be a positive whole number. Values must be finite. Missing is distinct from invalid.

Hours are an annual total, or hours per day and days per year. Annual hours cannot exceed 8784, the hours in a 366-day generic year of 24-hour days. Hours per day cannot exceed 24, and days per year cannot exceed 366. If the annual total is filled, that number is used and a partial daily pair is ignored. If the annual total and a complete daily pair are all present, the annual total must match the product within 0.005 hours. Otherwise the state is conflicting hours and every result stays blank. Zero hours is a valid no-saving result. A daily pair whose product is above 8784 is invalid.

Installation treatment is:

  • separate_quoted — add the installation number to the purchase total
  • included_in_purchase — use the purchase total only. A positive installation cost is counted twice and is rejected
  • none_expected — enter an explicit 0. A blank box does not become zero
  • missing — do not add the two cost boxes and do not treat installation as already included. Energy and money can still be shown. A blank installation box is a missing installation cost. Filled cost boxes with this treatment are a missing installation treatment, not a payback.

Costs are totals for the whole fitting group.

Display kilowatt-hours, pounds and years to two decimal places, half away from zero. The payback fraction uses the unrounded cost and unrounded money saving, not the rounded display figures. A positive unrounded cost is not a free project even if the displayed pounds round to 0.00. Non-finite derived kilowatt-hours, pounds or years are invalid and are not shown as a result.

Zero, negative, missing and invalid results

When more than one fault is present, the calculator uses this order: invalid input, conflicting hours, missing input, duplicate installation cost, then the energy result. After energy is computed, a negative saving takes precedence over a missing cost and over a zero tariff. No electricity saving takes precedence over a missing cost. Changing a field after a result, or offering an invalid submission, clears the earlier figures so they are not read as belonging to the current inputs.

A tariff of 0.25 is 0.25 pence per kilowatt-hour. It is not converted from pounds.

The fields have visible labels. The result is announced in the page. The calculate control can be reached with the keyboard.

Worked examples that can be checked

The groups are fictional Northgate Studio fitting groups. They are not a survey and not a current tariff.

Group Inputs Result
A 10 fittings, 36 W to 12 W, 2000 hours, 25.00 pence, £200 plus £50 separate installation 480.00 kWh, £120.00, 2.08 years from 250/120
B 12 W to 12 W, same hours, tariff and costs 0.00 kWh, no saving, no year
C 12 W to 18 W, same quantity, hours and tariff −120.00 kWh, −£30.00, negative saving, no year
D Group A with installation blank and treatment missing 480.00 kWh, £120.00, missing installation cost, no year
E Group A with installation 0 and none expected 1.67 years from 200/120

The same Group A energy at 0.25 pence is £1.20 and 208.33 years for a £250 cost. 7.5 hours a day for 200 days is 1500.00 hours.

Limits

The result is not a lighting design, a maintenance saving or a tax allowance. HMRC’s notice on ending enhanced allowances for energy-efficient plant records that the energy-saving first-year allowance ended for expenditure on or after 1 April 2020 for corporation tax and 6 April 2020 for income tax. That notice is not an input to the calculator.

The office walk-through still has no payback row.

Sources

Calculate one fitting group

Enter totals for one group of fittings. Inputs stay in this page. The result is simple payback from those numbers. It does not say the replacement gives the same light.

The calculator is unavailable because the page script did not start. The form is left inactive so the numbers stay on this page.

Enter a fitting group to see kilowatt-hours, money and, when the inputs allow it, a payback year. The calculate control stays inactive until the page script starts.