1 BTU is the amount of energy required to raise 1 pound of water by 1°F. Take 5 gallons (~42 pounds) at say 70°F (21°C). Raising that 42 pounds of water by 25°F/14°C will take just over 1K BTU.
That's maybe 2 hours of cooling per person and it's not at all obvious that during such a heat event that you'll have access to a continued source of 70°F/21°C water. If you only have access to 82°F/28°C water, 5 gallons only lasts you an hour or less.
That's maybe 2 hours of cooling per person and it's not at all obvious that during such a heat event that you'll have access to a continued source of 70°F/21°C water. If you only have access to 82°F/28°C water, 5 gallons only lasts you an hour or less.