Have you heard of GravityLight?
It is a gravity-powered lamp designed as an alternative for off-grid families who would otherwise use kersene lamps. It is basically only a 12kg weight, lifted and put on the gravity light. When the weight goes down again it pulls a cord. This cord makes an electric motor which generates electricity for LEDs. If you lift the weight 1.83m, the light lasts for about 20 minutes.
I wondered how much weight I would need to lift (assuming 100% efficiency) to power my computer for 8 hours.
According to the power supply unit, my laptop can consume up to 65 Watt. That is astonishingly low. I think my big one is at about 600-800 Watt.
\begin{align} 65 W \cdot 8 h &= \frac{65 kg \cdot m^2 \cdot 8h \cdot 60 \frac{min}{h} \cdot 60 \frac{s}{min}}{s^3}\ &= 1872 \cdot 10^3 \frac{kg \cdot m^2}{s^2} \end{align}
You might also remember from your physics courses that potential energy is $E_{pot} = m \cdot g \cdot h$ where $m$ is the mass (in kg), $g = 9.80 \frac{m}{s^2}$ is the gravitational acceleration and $h$ is the height in meters.
\begin{align} m \cdot h &= \frac{E_{pot}}{g}\ &= \frac{1.872 \cdot 10^6 \frac{kg \cdot m^2}{s^2}}{9.80 \frac{m}{s^2}}\ &= 191.0 \cdot 10^3 kg \cdot m \end{align}
This means I would have to lift 191 000 packages one liter of milk to a height of 1 meter. Every day. Just to let my small laptop run.
Or lets view it from another angle. I think lifting about 5 packages of milk to a height of about 1.8m each hour would not be too exhausting. This would generate about $E_{pot} = 5kg \cdot 1.80m \cdot 9.80 \frac{m}{s^2} / (1h \cdot 60 \frac{min}{h} \cdot 60 \frac{s}{min}) = 0.0245 \frac{kg \cdot m^2}{s^3} = 0.0245 W$. Lets think what you can power with 0.0245 Watt...
It is amazing about how much energy we have today.