You can multiply the required voltage by amperes (current) to obtain the wattage.
watts = volts × amps
Or you can divide the lumens, i.e., the brightness of the LED strip, by its luminous efficacy to obtain the wattage.
η = lm / W
where:
η – Luminous efficacy;
lm – Brightness in lumens; and
W – Power in wattage.
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