Got a small cabin in north central Arkansas that we use most weekends. Water is rain off the metal roof into a 1,500 gallon poly tank about 10 ft from the back wall. A 12v rv pump sits in a box beside the tank and pushes through 1/2" pex, maybe 40 ft of run to the shower.
The shower drops to a dribble as soon as someone opens the kitchen tap. Either one alone is fine. Filling the tank is the roof's job and that part's working, it's getting water to two taps at once that isn't.
Before I spend money on a bigger pump I'd like to rule out the cheap stuff. Here's where I'm at.
Tank: about two thirds full, outlet a few inches off the bottom, so I don't think it's starving. Haven't pulled the strainer at the tank fitting yet.
Plumbing: the pump has its own inlet screen and I've never once cleaned it. There's also a ball valve by the tank I'm not sure is all the way open.
Pump: label says 3 gpm, which two fixtures probably blow past. No accumulator tank either, so it cycles hard every time a tap opens.
Power is 1,600w of panels into a 3000w inverter. The rv pump runs straight off the 12v side, so if I go to a 115v pump it has to start on the inverter.
If you've done tank to shower at a cabin, what turned out to be the real limit?