It's 100% efficient once warmed up. 800W of electricity in nothing up any kind of flue so 800W of heat out.
How economical is it? Depends on how well insulated the room is on which it is being used. How much are you paying for your electricity per kWh? 800W is 0.8 x 1kWh. That only applies when the thing is actively heating. It will cycle on and off to maintain a temperature.