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A power label tells you how quickly an appliance uses energy, not what it will cost over a month. Start with electrical power, estimate how long the equipment actually runs, and apply a rate that matches the electricity being purchased. Keep these inputs separate so that you can see which assumption drives the result.
For a useful comparison, keep the period and tariff consistent. The energy tools show a 30-day month and a 365-day year as fixed comparisons, alongside the period you enter. These are arithmetic projections, not forecasts of weather, appliance cycling or your next utility bill.
From watts to kilowatt-hours
Divide watts by 1,000 to get kilowatts. Multiply kilowatts by hours of use to obtain kilowatt-hours (kWh), then multiply kWh by your price per kWh. A kW is power; a kWh is energy. Confusing the two can make a plausible-looking estimate fundamentally wrong.
Use an effective rate appropriate to the question. This model applies one flat rate and does not separately add standing charges, time-of-use bands, taxes or demand charges. Do not treat its energy cost as a complete bill unless your inputs genuinely account for the relevant charges.
Worked example
Suppose a 1,000 W load runs for 4 hours daily over 30 days at a hypothetical 0.20 currency units/kWh.
1,000 ÷ 1,000 = 1 kW. Daily energy is 1 × 4 = 4 kWh; period energy is 4 × 30 = 120 kWh. Multiply 120 by 0.20.
4 kWh per day; 120 kWh and 24.00 currency units over 30 days.
Choose an appliance usage assumption carefully
The appliance tool uses rated watts with the same energy-cost arithmetic. A nameplate rating is not necessarily an average draw: equipment may cycle, change modes or spend time on standby. If you have a suitable measured average, use it with a matching duration rather than mixing a maximum rating with a misleading running time.
Keep the comparison focused on the appliance and usage entered. The tool does not identify the device, retrieve its specification or measure it remotely. A change in assumed daily use changes every projected period.
Worked example
Use an illustrative 150 W appliance for 8 hours per day, for 30 days, with the same hypothetical 0.20/kWh rate.
0.15 kW × 8 hours = 1.2 kWh daily. Over 30 days, 1.2 × 30 = 36 kWh; 36 × 0.20 gives the cost.
36 kWh and 7.20 currency units over 30 days.
Separate AC cooling output from electrical input
If electrical input is known, enter that power directly. If you use COP, the tool divides cooling capacity in kW by the dimensionless COP. If you use EER, it divides cooling capacity in BTU/h by EER, then converts the resulting watts to kW. It supports the displayed EER/COP definitions; do not substitute a seasonal efficiency label without checking that it means the same thing.
Cooling capacity is useful output, not electricity consumed. After deriving input power, the tool applies your hours and rate. A single efficiency and daily duration simplify changing temperature, part-load behavior and compressor cycling, so this is not a measured seasonal consumption forecast.
Worked example
Assume 3.6 kW cooling capacity, COP 3, 4 running hours per day, 30 days and a hypothetical price of 0.20/kWh.
Electrical input is 3.6 ÷ 3 = 1.2 kW. Energy is 1.2 × 4 × 30 = 144 kWh; cost is 144 × 0.20.
1.2 kW electrical input; 144 kWh and 28.80 currency units over 30 days.
Value solar energy used and exported separately
The simplified solar model multiplies system size in kWp by peak sun hours and a performance factor. Peak sun hours are equivalent full-sun hours, not daylight duration. The performance factor is the fraction remaining after assumed losses. Choose inputs appropriate to the period and avoid counting the same loss twice.
Self-consumption is the share of generated energy used on site, not the share of household demand covered. That energy is valued at the import rate; the remainder is valued at the export rate. The calculation assumes the remainder can be exported. Its gross bill benefit excludes installation costs, financing, battery operation and degradation, so it is not investment profit or a payback calculation.
Worked example
Assume 5 kWp, 4 peak sun hours/day, a 0.80 performance factor and 60% self-consumption for 30 days. Use hypothetical import/export rates of 0.20 and 0.05 per kWh.
Generation is 5 × 4 × 0.80 = 16 kWh/day. Of that, 9.6 kWh is used on site and 6.4 kWh exported daily. Value each share at its own rate before multiplying by 30.
480 kWh generated; 57.60 avoided import cost + 9.60 export credit = 67.20 currency units over 30 days.
Compare lighting on equal usage
The LED tool calculates the old and replacement bulbs separately using the same count, hours, period and rate, then subtracts their energy and cost. Compare suitable replacements for your lighting need; this tool does not compare brightness, fittings or product quality.
The displayed saving is operating energy cost only. Purchase and installation costs are not included. A replacement using more watts can produce negative savings; the label is not a promise that every proposed replacement saves money.
Worked example
Compare ten 60 W bulbs with ten 9 W bulbs, used 4 hours daily for 30 days, at a hypothetical 0.20/kWh.
Old energy is 10 × 60 ÷ 1,000 × 4 × 30 = 72 kWh. Replacement energy is 10.8 kWh. Subtract the energy and multiply by the rate.
61.2 kWh and 12.24 currency units saved over 30 days.
Common mistakes
- Treating cooling output as AC electrical input.
- Using daylight hours as peak sun hours, or valuing all solar generation at the import rate.
- Comparing bulbs over different hours or treating operating savings as profit after purchase costs.
- Assuming a fixed 30-day comparison is a forecast for every calendar month.
Before you act
- Record the power source, units, usage period and tariff used.
- Compare a lower-usage and higher-usage scenario when daily hours are uncertain.
- Check actual bills, equipment specifications and export arrangements before making a purchase.
Read each calculator’s formula and limitations alongside this guide. For significant or safety-critical decisions, verify the inputs and seek appropriate professional advice.
Read the calculation disclaimer