How much height does a rain tank need to flush a toilet?

There isn’t one tank height that guarantees a good flush: the available gravity head has to overcome losses through the pipe and fittings. A conventional toilet cistern may fill more slowly at low flow, but its fill valve still needs to operate within its pressure range. A pressure-dependent flush valve needs adequate pressure and flow at the valve. I’d check the toilet’s requirements and judge the setup by whether it refills and flushes reliably, rather than height alone.

If the tank can be placed high enough, I’d lean toward gravity for a system meant to work during outages: it avoids depending on power or maintaining backup batteries. Where that isn’t practical, a pump may be worth the placement flexibility, but I’d favor a simple manual way to pour stored rainwater into the cistern over adding backup power just for occasional flushing.

Keeping the rainwater line physically separate from potable plumbing is the key starting point; labeling helps make its non-potable status clear, but it does not replace protection against a cross-connection. The required backflow arrangement depends on how the systems connect and on local rules, so I wouldn’t assume one device fits every setup. That safety requirement would matter whichever supply method you choose.

For a hypothetical low-placed, pump-fed tank, keeping a small amount of rainwater accessible for pouring into the cistern could provide an outage fallback without maintaining backup power. That seems manageable if it’s only for occasional emergencies; if outages are frequent, repeatedly handling water may be less convenient than a more dependable backup.

For emergency use, I’d lean toward a conventional cistern: a slower refill is inconvenient, but the flush can wait until it has filled. A pressure-dependent flush valve seems less forgiving if gravity flow is marginal, since it needs adequate pressure and flow when used. That preference would still depend on the chosen cistern’s fill valve tolerating the available pressure.