This grosses to around 363,000 tonnes across all charity shops in the UK; based on 2010 landfill tax value at £48 per tonne, the value of textiles reused or passed for recycling by charity shops in terms of savings in landfill tax is £17,424,000 p.a.

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How do you read "p.a."?

1. pi: ei
2. per annum

Most people would use ‘per annum.’