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http://www.simplifydiy.com/plumbing-and-heating/radiators/power600 x 900 single is approx 735W
600 x 900 single with fins is approx 1080W
600 x 900 double is approx 1050W
600 x 900 double with fins is approx 1650W
600 x 900 double with double fins is approx 2010W
but lot depnds on rad temperature and room heat loss.
Is there another room thats similar (eg similar size, similar outside wall area, similar windows) that does get warm enough? What power of rad does that have in it according to the chart?