It depends what you're optimizing for. If you're primarily interested in resilience to power outages, then maybe you are OK with the wasted energy used to freeze/re-freeze the extra water.
Professional cooler packs can have that point down as far as -35 Celsius.
Adding salt to your water will substantially lower the thawing point and it’s still cheap and widely available.
If you want to prep for cases when you need water you’d be better off keeping your freezer cold with a suitable medium and have a few 30liter buckets of water with some sort of additive that prevents spoilage (silver ions for example) or have some packs of bottled water around.
You would have a larger thermal mass but the same surface area so it should keep for longer.