It is the first question everybody asks and the one no supplier can answer honestly with a single figure. Here is what the figure actually depends on.

The bag adds nothing

Insulation slows a transfer. It does not create cold and it does not create heat.

So the starting temperature of the contents is doing most of the work, not the panel.

Thermal mass is the real battery

Ice, gel packs and a large frozen load hold a temperature far longer than a small one.

Two packs in a full bag will beat four packs in a half-empty one nearly every time.

Headspace is where hold time goes to die

Air has almost no thermal mass, so a half-full bag has nothing to keep cold but itself.

Fill it, pad it, or buy a smaller bag. This is usually cheaper than more foam.

Open time counts more than people expect

Every opening dumps the conditioned air and replaces it with ambient.

A delivery bag opened at eight stops is a different bag from one opened once.

How to get your own number

Pack the bag exactly as you really pack it, put a probe thermometer in the load, and run your actual route on a warm day.

That number is worth more than any figure printed on a supplier’s page.