A flexible heater is a device which can conform to the surface which requires heating. There are many varieties of flexible heaters which include silicone rubber heaters, Polyimide Film heaters, heating tapes, heating tapes with thermostats, rope heaters,wrap around tank heaters, gas cylinder heaters and custom sizes. 
Silicone Flexible Heaters are rugged, reliable, accurate, and moisture and chemical-resistant. They can be easily bonded or adhered to other system parts. Flexible heating elements have a wide range of industrial, commercial, and military applictions where reliability, cost of effectiveness, minimum cross section, resistance to deterioration and basic flexibility are critical. 
1. How much wattage is required?
2. What voltage will be used?
3. If it is a flat surface, what size will be needed?
4. How long do the heater leads need to be?
5. Will you require a thermostat or a digital controller with sensor?
6. What is maximum temperature?
7. For flat surfaces will require wire wound or etch foil?
8. What is the time required to reach your temperature?
9. What is the ambient temperature?
10. Is insulation being used?

Ohms's Law

Ohm's law shows the relationship between voltage, wattage, amperage, and resistance. It is important to know Ohm's law in order to calculate the load you will be switching with your thermostat or digital controller. For example if you select a flexible heater which is rated 200 Watts and apply 120 Vac, the current will be 1.6 Amps.
A is equal to W / V where: A = Amperes; W = Wattage; V = Voltage 
Using our example above 200 Watts/120 Vac = 1.6 Amps 
Some other variation of Ohm's Law: 
Volts = Watts/Amperes 
Volts = Amperes x OHMS 
OHMS = Volts/Amperes 
Amperes = Volts/OHMS 
Watts = Volts x Amperes 

Killowatt Requirement

To determine the kW required to be heat up a solid. The following formula is used. 
kW = (W_T x C_px Δ T)/3412 x h 
Where: 
W_T= Weight of Material, lbs
C_p=specific heat, BTU/lb-°F
Δ T = Temperature Rise, °F
3412= Conversion Factor, BTU/kWh 
h = heat-up time, hrs