As title. I'll need a 400mm x 400mm heating pad for the printer I'm planning. I'll be using 24V DC power. I can get a 24V 450W silicone heater. Will this be sufficient or should I go the AC mains / SSR route? The bed will be well insulated and I'll be using an aluminium heat spreader but probably with a removable glass print surface on top. Thanks.

450W is about the absolute minimum that you need for a bed of that size. If you want to reach ABS temperatures easily and quickly, then I suggest you aim for 700 to 800W.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thank you. Looks like 240V AC and a SSR then.