Secondary point: most residential structures are not designed to have 1000s or even 100s of gallons of water stored on the roof. Even 100 gallons is a huge amount of static load to add to a structure.

One minor but interesting exception - farmers have historically often put domestic water tanks on top of towers to ensure even water pressure. Direct pumping leads to cycles where the pressure in the system rises and falls as the pump cycles on and off, which is very unpleasant if you’re trying to take a shower!

I believe modern off-grid home water pumps use more sophisticated motor control to avoid the need for the raised tank.

Off-grid domestic water uses tanks fitted with rubber bladders that keep the water at a roughly constant pressure and reduce the frequency the pump has to cycle.

That's not an exception, that's an entirely different use for a raised tank. And not a very big one either.

I'd say it's a quite big one, "every city" has water towers to provide pressure to the people living in it, or they rely on the water source if it's higher up than the city.

There are probably exceptions, where this isn't true, but raised tanks make modern society possible.

https://youtu.be/yZwfcMSDBHs more on water towers, which are raised tanks of water.

We also pump water into fake lakes to later extract electricity from it. (https://youtu.be/66YRCjkxIcg)

Another good thing with water towers is that we can keep running the pumps at a lower RPM where efficiency than if we were trying to build a system with pumps to keep pressure.

Great channel if you're curious about everything related to humans relationship with water.

> I'd say it's a quite big one, "every city" has water towers

I meant not a big tank is needed for one house. I wasn't talking about the size of the use case.

> We also pump water into fake lakes to later extract electricity from it.

We do, because lakes are much much bigger than tanks on a house.

It depends what you're calling "not a very big one". On our farm we have a 5000 litre galvanised high tank for house-water.

Right, but do you need more than 50 of those for the specific purpose of evening out pressure?

I thought the point of water tanks is that one needs a simpler / weaker pump to maintain the water pressure.

i.e. if you want to maintain water pressure X, with a pump you need something that can push X, but with a tower, you just need a pump that can keep the tower full over the course of a more length period and just has to pump at the average of what it takes to keep it full over the course of that longer period (i.e. the average pressure needed to maintain the tower is less than the pressure you need to provide).

Here's some "slow math" in video form ;)

https://youtu.be/CMR9z9Xr8GM

I find him entertaining. Put a 55 gallon drum on his roof, pumping water up via solar and running lights at night. Closed loop system.

He also "does the math" and mentions exactly what you are saying in entertaining format. Just not worth it if you're not doing it on a massive scale, preferably way out of sight and danger.

Thanks for that dose of information. Basic arithmetic is so lacking in all these discussions. People throw all sort of fantastic pseudo-facts with little or no anchor to reality.

they do it a lot in NZ. Solar hot water heating.

Solar hot water is great, but that's not gravitational storage, e.g. pumping water up a hill with pumps and later using the flow of water back down to generate power.

You're right it isn't, but it is storing the water on top of the house.

Yes, but I'd expect it's much lesser amounts. 80 gallons is a decent sized water heater.

ahhh i understand what you were saying now.

NZ has a lot of hydro and a reasonable amount of geo thermal generation. Solar, not so much.