The arithmetic
Annual kWh = watts × device count × standby hours per day × days per year ÷ 1,000. Cost = annual kWh × your price per kWh. Standing charges, taxes not included in your entered rate and active-use energy are excluded.
Measure the right thing
A device’s rated maximum or a charger’s output rating is not a measurement of standby draw. Use a suitable meter according to its instructions or a documented standby figure. This calculation assumes a steady load throughout the hours entered.
Choose what can be switched off
Do not turn off safety, medical, security, network or other equipment that needs continuous power just because a calculator shows a cost. Check the manufacturer’s guidance and the consequences first.
Sources: EIA: watts and kilowatt-hours; Natural Resources Canada: standby power.