Rainwater is free and abundant. You can get an idea of just how much rainwater you can collect from your roof by multiplying its area (in metres squared) by the average annual rainfall (around 0.56 metres for the Cambridgeshire area). This final figure should then be multiplied by 0.75 to account for evaporation. So for a roof of 50m2 in Peterborough you could expect to collect about 21m3 of rain a year (50 x 0.56 x 0.75). If that doesn’t sound like much, convert this figure to litres by multiplying by 1,000; that would be 21,000 litres, or 4,619 gallons! No surprise, then, that a water butt recharges so quickly after a wet day.
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