"But if you do the math on something like a whole-house backup, it seems a little tricky. Assume you have a 100 foot (30 meter) tall hill to work with (which is very tall for someone's backyard). And you want to store 15 kWh (about half of a typical US home's daily electricity usage). You would need about 220 US tons (200,000 kg) of mass. Flipping it around a bit: 1 US ton lifted 100 feet is about 75 Wh. I'm not sure if you can even buy a battery pack with an AC outlet that small - they seem to start around 200 Wh. 75 Wh is one common powertool battery that you can buy for ~$50.Math checked with https://www.calculatorsoup.com/calculators/physics/gravitational-potential.php and https://www.omnicalculator.com/physics/potential-energy."