Occasionally I have to sit in a very cold office. The room is not well insulated and has no built in heating. One option was a radiant panel and another a Electric Foot Warming Pad . The Foot Warming Pads have an electrical element embedded in a floor mat, a thermostat, cable with standard electrical plug and use about 100 Watt of power. Most seem to have a rubber core, with an optional carpet or fake fur cover.
Examples of foot warming pads available in Australia are the the MATFOOT footwarmer (400mm x 550mm, 150 Watt) and the Negergy Energy Efficient Foot Mat Heater (400mm x 600mm, 75 Watt). Note that a unit purchased overseas might not meet Australian electrical safety regulations and not be Australian voltage.
While electrical heaters are not the most environmentally the best, at least the foot-warmers are low power, compared to other portable room heaters (which are about 2,400 Watt).
Hecla Electrics Foot Warma, from 1927.
Pet Bed Warmers
which work in a similar way to the foot warmers, but are a lower power (and usually low voltage in case your dog chews the wires).
Does anyone have experience of using these?