Heater Construction Set
600W modular system is used in 3D printers. Can be scaled readily to multiple elements. 1800W is used in D3D_v20.04.27_Build_Instructions#Heater.
Practical limits are as much power as is available from outlets, in units of 600W. Other power units can be developed readily, the limit being the melting temperature of the fiberglass sleeving.
V=IR -> I=V/R -> P=IV=V^2/R
Modular Heatblock - remember there already being a page for this or something? found it: "Scalable Heater Block"