Conversion

Note that: $1 \text{kWh} = 3.6 \times 10^{6} \text{J}$
$1600 \frac{\text{kWh}}{ \text{yr}} = \frac{1600 \times 3.6 \times 10^6 \text{J}}{\text{yr}}$
To find the mass, use the relation: $E = mc^{2}$ where m is in kg.