To calculate the efficiency of the space heater, we can use the formula for efficiency:
\[ \text{Efficiency} = \left( \frac{\text{Useful energy output}}{\text{Total energy input}} \right) \times 100 \]
In this case, the useful energy output is the thermal energy produced, which is 896 joules. The total energy input is 1,400 joules.
Now, we can plug in the numbers:
\[ \text{Efficiency} = \left( \frac{896 \text{ J}}{1,400 \text{ J}} \right) \times 100 = \left( 0.64 \right) \times 100 = 64% \]
Therefore, the efficiency of the space heater is 64%.