is that £20 ontop of your general electric use or £20 total a week??
your 300w light is only consuming 136 watts, the fans are also inexpensive to run,
i run a 600w, 6'' extractor, 18'' oscillating fan and a dehumidifier, my electric use a week is nearly £20
but that includes general house electric use also,
yours should be cheaper since your light is only pulling 136 watts, maybe your general electric use is high
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