Choose the energy basis first
A figure in kWh is energy. Watts and kilowatts are power. Use electrical input, not heat output, cooling capacity or a microwave’s cooking output. An energy-efficiency letter or ratio alone is not an annual consumption input.
| Source basis | Annual or period energy |
|---|---|
| kWh per stated year | Use that annual figure directly |
| kWh per 100 cycles | Figure ÷ 100 × cycles per week × 52 |
| kWh per cycle | Figure × cycles per week × 52 |
| Watts and use time | Watts ÷ 1,000 × hours per use day × use days |
| Measured energy over complete days | Sample kWh ÷ complete sample days × use days |
Annual label tests retain their stated programme and assumptions. We do not rescale an annual test figure to household hours. Power mode assumes constant input during the entered time; no generic duty factor is added. Measurements include only the scope and conditions captured in the sample.
Electricity cost and household planning
Electricity cost = kWh × entered price per kWh. Enter pounds, dollars or euros, not pence or cents. Currency selection changes formatting and does not convert prices.
The household planner multiplies each row’s annual energy by quantity, adds the rows and applies one unit rate. An empty plan totals zero. Monthly averages divide annual cost by 12; daily annual averages divide by 365. These averages do not represent seasonal bills. Power and measured rows use your chosen use days, up to 366.
Do not add overlapping measurements. Some US washing and dishwasher figures include associated water heating. A whole wash-and-dry cycle already includes its washing stage. Keep these scopes with the result.
Two tariffs and fixed charges
Off-peak energy = annual kWh × off-peak percentage ÷ 100. Peak energy is the remainder. Energy cost = peak energy × peak rate + off-peak energy × off-peak rate. Daily fixed charge × 365 or 366 is added once, separately.
Compare the same annual energy on both tariffs. A shifted-use scenario changes the split while holding energy constant. Off-peak percentage describes energy, not hours. Tiered and demand charges, discounts, exit fees, export payments and other charges are excluded. Enter both quotes with a consistent tax basis; no tax is added by the calculator.
Purchase comparison and replacement payback
For two purchase options, total cost = purchase price + annual electricity cost × selected years. Extra price for B ÷ positive annual electricity savings gives simple payback. If B has no extra purchase cost, or no positive savings, the tool reports that condition separately.
Replacement uses keeping the current working appliance as its baseline. Net upfront replacement cost = purchase + installation + disposal − confirmed credits. Credits cannot exceed those costs. Keep total = current annual electricity × years. Replace total = net upfront cost + replacement annual electricity × years.
Replacement payback = net upfront cost ÷ positive annual savings. A zero net upfront cost is separate from a positive-savings payback. No positive savings means no electricity-savings payback. The old purchase price is a sunk cost and is excluded. A broken appliance requires a different baseline.
Both models hold tariffs and energy constant. They exclude repairs, failures, financing, degradation, resale and environmental impacts. A chosen period is a scenario, not a service-life forecast.
Shopping price thresholds
The purchase comparison normalizes both energy inputs to annual kWh first. If B uses less annual electricity, its maximum electricity-funded extra purchase price = (annual kWh for A − annual kWh for B) × entered tariff × chosen years. Paying exactly that extra amount produces equal purchase-plus-electricity totals on this scenario.
Break-even price per kWh = (purchase price for B − purchase price for A) ÷ (annual kWh for A − annual kWh for B) ÷ chosen years. When B uses less electricity and costs more upfront, a higher tariff favors B. When B is cheaper upfront but uses more electricity, a lower tariff favors B. Equal energy, no extra price or no financial advantage is reported separately instead of dividing by zero or presenting a negative crossover as useful. Displayed thresholds are rounded; the calculation retains full precision.
These thresholds use a constant tariff and compatible energy boundaries. They do not include maintenance, failures or a forecast of appliance life, and they do not establish a product's market value.
Measurement records and projections
The measurement journal subtracts the starting accumulated kWh counter from the ending counter. Average measured kWh = sample kWh ÷ entered complete-day or completed-cycle count. A lower ending counter is rejected; a reset needs a separate uninterrupted interval. Days require the same local clock time after the stated number of calendar days. Dates and times are retained as entered, without assigning a time zone; daylight-saving changes can alter elapsed hours within a calendar day.
Sample cost = measured sample kWh × optional entered tariff. Annual projections require your own routine: average daily kWh × use days, or average cycle kWh × cycles per week × 52. A missing tariff or routine remains unknown. The original counters and context remain observations; the routine and constant tariff are scenario assumptions. No extra duty-cycle factor is applied to measured energy.
Saving is explicit and device-local. CSV and print output retain readings and context beside separately labelled projections. A calculator transfer uses total kWh and complete days in measured mode, or average kWh per completed cycle in cycle mode. A planner transfer starts with one compatible row. Source notes identify a user-entered sample; they do not establish a standardized label test or a Watt & Home product test.
Cooking and what-if scenarios
Cooking power mode counts cooking time for every batch and preheat time for the entered number of sessions. A measured whole-meal figure is used directly; preheating and batches are not added again. Compare the same amount of food and result. Annual differences use meals per week × 52.
The three-scenario tool changes an annual baseline by a user-entered percentage and applies each scenario’s tariff. Replacement mode applies that same percentage to both appliances and uses a separately entered net price. The columns are assumptions, not confidence intervals or predictions.
Rounding, input limits and examples
Calculations retain precision internally. Money is displayed to two decimal places; energy and tariff details can show up to six. Totals are calculated from unrounded values, so adding rounded display lines can occasionally differ by a cent or penny. A label can round energy differently from a product sheet; retain the precision and unit of the source you choose.
Enter valid numbers within the limits shown by the controls. Only fields in your selected calculation mode are used, and invalid entries clear the corresponding result. Starting calculator values are illustrative examples. Guides identify manufacturer or registry declarations, attributed research measurements and derived quantities, with tariff, routine and purchase-cost assumptions explained separately. External source values are not measurements collected by Watt & Home or live shopping data.
In the published dryer field case, one laundry load can include several machine runs. Annual inputs are projected from reported kWh per load and the assumed load count. That preserves the source boundary without equating a complete laundry load with one standardized programme.
A worked-example link loads its stated inputs, including the currency and price assumptions. It does not convert your tariff, confirm a rebate or establish that a reference model is currently available.
Markets and documented model inputs
A label market, currency and electricity price are separate choices. Changing the market in the preference panel suggests a currency and clears the draft price for your own entry. It performs no exchange-rate conversion. A remembered market supplies the label reader's starting market; changing an active label task requires its own compatible energy input.
Links in the documented examples retain the source energy, unit, model and programme. They also load explicitly illustrative prices and routines. Shared and worked-example inputs take precedence over a remembered tariff. A dated manufacturer price observation remains a past listing, rather than live shopping data.
Sources and reporting a problem
Relevant sources appear on tool and guide pages. Check the source policy for reference dates and editorial boundaries. Save or print the active inputs with your result. Use the correction page for a reproducible calculation or source issue.
Calculations are for informational purposes only and may be incorrect, outdated or inaccurate for your situation. Read the calculator disclaimer before relying on an estimate for a decision.