Calculate with your own values
Defaults are illustrative scenarios, not current tariffs or measured savings. Use one currency for every amount; no exchange-rate conversion. Units are litres, °C and kWh.
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Method and example
Cost = watts × hours × days ÷ 1,000 × energy unit price. Add timer watts × 24 × days ÷ 1,000 × price. Example: 20 W × 8 h × 30 days at 0.35/kWh costs 1.68. Four lighting hours plus a 0.5 W timer costs about 0.97: a difference of 0.71, before buying the timer.
Find the actual input power
The maximum output rating on a power supply is not necessarily the consumption of the connected lights. Use the maker’s input-power specification or a meter suitable for small loads. A full-day measurement captures both lighting and standby. Do not add power twice if a measurement already includes an integrated timer.
Include the timer
This calculator compares the same lights with and without an added timer. It assumes the timer consumes its stated power continuously for 24 hours a day. Savings can be negative when little lighting time is avoided. The timer purchase price is not deducted; recovering it needs enough positive savings over the intended use.
Keep or replace?
LEDs often use less power than old filament lights for a similar purpose, but a purchase decision needs both sets’ actual consumption and the full replacement cost. A tiny seasonal saving alone may not justify replacing working lights. Changing hours can be the more direct first step.
Indoor and outdoor ratings
The lights, power supply, connectors and timer must all be approved for their intended location. An outdoor string does not make its indoor power supply weatherproof. Follow instructions for water exposure, mounting and distance from combustible materials. Replace damaged equipment; a timer does not make an unsafe installation safe.
Common questions
Does a timer always save electricity?
No. Its additional consumption can exceed the energy saved by switching the lights off sooner.
Is the power-supply maximum the actual consumption?
Not necessarily. Use actual set input power or a suitable measurement.
Sources and limits
Technical sources below are in German, checked for the original guide on 4 October 2026. These adaptations do not establish local rental law, installation rules or tariffs. Follow local requirements and the instructions for your actual system.