It seems you want more heat energy out than electrical energy you put in. Conservation of energy means that this therefore requires some other form of energy to be put in, which is then controlled electrically.
The question then comes down to what other form should this energy take. This depends on your application. One obvious way to get a lot of heat is via chemical reaction. It's easy enough to initiate a exothermic chemical reaction with electricity, but controlling it roughly proportional to some electric signal is more difficult. One possibility may be electrically controlling a valve that lets more or less fuel into a combustion chamber. It gets more tricky when the total output needs to be able to go to 0, since then you need a way to start up the chemical reaction, which can be quite different from sustaining it. Traditional solutions include pilot lights or spark ignitors. Or, you can chose chemicals that react exothermically on contact. These will be harder to procure and may attract unwanted attention from the authorities.
If you put enough uranium or plutonium close enough with some carbon rods separating things, you can have the electric signal control how much the carbon rods inserted into the pile or not. This controls the magnitude of the chain reaction. The advantage is that this method can produce large amounts of heat for small amounts of material, but getting the material will be difficult. The system can also go into exponential runaway if carbon rods are removed too far. This will have bad consequences, so overall I don't recommend this method.
