Estimate appliance electricity from watts and time
Estimate an appliance’s energy use in kWh and its added cost using your bill, an official tariff option, or a custom unit rate.
Back to calculator ↑Find input watts and realistic use
Read electrical input watts from the product label or manual; cooling BTU, storage capacity or heat output are not interchangeable with electrical watts. Enter the number of identical units, hours per day and days in the period. For variable-power equipment, use a reasoned average-power fraction or measured energy use.
A 100% average-power setting assumes the entered watts are drawn throughout the stated hours. At 50%, the arithmetic uses half that power on average; it is not a universal efficiency claim. Air conditioners, fridges and pumps cycle or vary output with conditions, so their real kWh can differ from nameplate arithmetic.
The energy and cost formula
Daily kWh = watts × quantity × hours per day × average-power fraction ÷ 100 ÷ 1,000. Period kWh = daily kWh × entered days. Bill-average and custom methods multiply that energy by the selected rate; official methods compare the household bill before and after adding the load. The annual figure repeats the same daily schedule for 365 days, regardless of the period length selected.
Example: five 10 W lights used eight hours a day for 30 days at 100% average power consume 10 × 5 × 8 ÷ 1,000 = 0.4 kWh/day, or 12 kWh for the period. At an illustrative ฿4.5/kWh, that is ฿54. The ฿4.5 input is an example for demonstrating the formula, not a current Thai tariff.
Compare a second wattage carefully
The optional comparison keeps quantity, hours, days, average-power fraction and rate unchanged, replacing only watts per unit. Five 6 W lamps on the example schedule use 7.2 kWh in 30 days, costing ฿32.40 at the same illustrative rate. The arithmetic difference is ฿21.60 for that period.
A lower wattage is not automatically a like-for-like replacement. For lighting, compare delivered lumens and colour; for other equipment, compare the required output and duty pattern. If the replacement changes usage hours or performance, rerun the estimate with realistic inputs before treating the difference as a saving.
Why the household bill can differ
The bill method uses your bill’s average cost per kWh. Official MEA/PEA and TOU options estimate the added charge under their displayed billing-period tariff, Ft and VAT; check the tariff class and billing period against your bill. Custom rate is a simple multiplier.
A suitable plug-in meter can provide measured kWh for compatible plug-in appliances over a representative period. Confirm the meter’s electrical rating before use. High-power or hard-wired equipment may need a qualified person and another measurement method.
Common questions
Is ฿4.5/kWh the current tariff?
No. Enter that as a custom example, or use your bill or the listed official tariff option.
Should a fridge set to 24 hours use 100% power?
No. The compressor cycles; measured kWh or a supported average-power assumption is better.
Does the annual figure use my entered period days?
It uses the entered daily usage pattern multiplied by 365 days.
Sources and calculation guidance
Check these against your chosen product’s label and instructions.