# Gravity & energy storage

Thinking about Energy Vault and the maths behind it, I thought about large heavy objects in many Australian homes, namely those water tanks.

We were discussing this at dinner some time ago, imagining say a 4,000 litre water tank raised by perhaps 3 meters as a potential energy storage device for your home.

How much potential energy could it store?

We can start with the gravitational potential energy : PE = mgh (mass x gravity x height)

So let’s do the maths:

• weight: = 4 metric tonnes (1lt = 1kg, 1000 lts = 1000kg = 1 tonne)
• gravity = 9.81m/s
• height = 3.00 metres

This offers you 117,720 Joules
(see Gravitational Potential Energy Calculator

If we then convert Joules to kWh = 0.327kWh
(see Joules to kWh conversion calculator)
(see explanation for how to convert joules to kWh on Quora)

Looking at my Solar PV import meter and excluding large spikes for cooking, my home at night consumes approximately 500W (0.5kW) between 6pm and 7am (13hrs)

So my 4 tonne energy storage device would last for approximately 45 minutes, or to store 6.5kWh of energy, enough to see me until morning, I’d need either 19.8 tonnes raised 3 metres or 4 tonnes raised 59.6 metres.

(Somehow I don’t think Council would allow me to build this either!)

But looking again at Energy Vault, they claim it can deliver as much as 80 MWh of electricity, enough to power around 60,000 homes for up to 16 hours.

Each concrete block weights 35 tons, so each block dropping 100m will generate 24,525,000 Joules which converts to a mere 6.8kWh.

So if each of the 60,000 homes uses 10kWh over 16hrs (just to keep the maths simple), that means they need 88,000 concrete blocks going up and down.

... have I got my maths right here? That seems like an awefully complicated way of storing energy!