How to measure water usage of rainwater system, using a powermeter

I moved into a house with a rainwater capture system. There is a big tank somewhere under the driveway and a large motor and small sub tank system in the garage. The rainwater system has its own pipework for 4 things in the home. Toilets, Washing Machine, Garden Taps (for watering) and a Random Utility Sink in the garage. System seems to work fine, although the pump is a little noisy when in use, and the water tank level is broken, which will be another job for me one day soon, but other than that it’s a great setup. If the rainwater tank is empty then the mains water system fills the sub tank system and then mains water is pumped into the system.

I know it all works because after it rains, for the next few days the flushing water in the toilet is always a little green and the mains water metre doesn’t move. When the tank is empty the mains water system meter runs. .

What I would like to do is measure how much rainwater I am using at the device level (Toilet, Garden Watering, Washing Machine etc.). Currently what I have at my disposal is a power meter socket that the pump is connected to. This has been connected for a year now and works really well. I have two toilets in the house (1x 7 liter cistern and 1 x 10 liter cistern, measured from mains water use when the rainwater was empty), when you flush and the pump kicks in, I get a power reading.

How can I use this data to calculate water used over time but the toilets knowing they are 10liter and 7liter. And same for washer and then random for the garden watering (which would be everything else as its not fixed like the toilet and washer)??

Before you can convert power usage on the pump to anything with water you need to know its pump effect.
It is probably running at full capacity when it is running, so it should be a somewhat even amount of water it pumps up per time unit.

But you might be hitting some obstacles like washing machine using different amount of water depending on program and also on amount of cloth put in, and also what happens if washer is running and a toilet is used or both toilets at the same time.

I think we could find those measurements, we pretty much just use an ECO setting on the washer, or at least 49 loads out of 50. We pretty much always have a full load (kids in the house) so again 49 times out of 50, so not bothered about a missing 1 load being non standard.

The washing machine also has a power meter plug so I’d know when its on and when its running or in standby.

I could calculate the water requirements of each wash by reading the water mains meter when there is no rainwater.

With toilets both running at the same time and one being a 7 liter and the other a 10 liter, depending how long the pump runs, you should be able to extrapolate is it pumping 7, 10 or 17 from the duration its drawing power.

Hope that helps, it may not be exact but its a start.

I do a very similar thing for my water well pumps.

The first step is to determine how many Gallons that pump produces per minute. First I’ll tell you how I approached it for my well pump, then we can discuss your application.

A well pump has a holding tank with a pressure switch. When the pressure in the tank drops below 40 psi the pressure switch turns the pump on. When the pressure goes above 60 psi it turns the pump off.

So this is the process to determine the pump GPM.

  1. Trip the pressure switch to force the pump to run to 60 psi (e.g. have the pressure tank be at its max fullness).
  2. Turn off the breaker for the well pump. Make sure no one is going to use water!
  3. Open a facuet(e,g, a sink), and measure the water coming out (like a 1 gallon container). Make
  4. Do this until you hear the pressure switch trip, turn faucet off.
  5. Write down how much water came out.
  6. With a stop watch in one hand, flip the breaker and start the timer. Record how long it takes to fill the tank back up
  7. Divide the gallons by time to produce pump GPM. Write this down, ideally repeat the test a few times.

With this information it’s easy to calculate gallons by multiplying the pump run duration times this constant. You may already have this information for your 7 liter and 10 liter tanks. So calculate this and see if it is consistent. Note, that if your power reading are every 15 seconds and the run time is a minute - that’s not a great level of accuracy. 15/60.

With that you can get the total gallons used across those fixture. Apportioning it will be challenging as it’ll be hard to differentiate the sink running vs the toilets and if multiple are active at the same time…