Skip the heated lunch box. Get a wide-mouth thermos.

A wide-mouth vacuum-insulated food jar is the best “lunch box” for keeping food hot without a microwave.

Most electric lunch boxes are gimmicks. They either need to be plugged in or use weak heating pads that make your food soggy. A good thermos is passive, needs no power, and will keep food above 140°F for hours if you set it up right.

Look for a wide-mouth thermos, not a narrow soup bottle. You want to pack stews, rice, noodles, pasta — stuff you can eat straight from the jar with a fork. Thermos King is the reliable classic. Stanley Master Unbreakable is tougher if you toss your bag around. Zojirushi makes the best insulation per size, but their food jars tend to be smaller.

The trick with all of them: preheat the jar by filling it with boiling water for a couple minutes, then dump it. Pack your food as hot as possible, fill to the very top — air is the enemy — and close the lid. Don’t “check” it at 10 a.m. Every open costs you heat.

If you need hot food plus a side in one bag, use an insulated lunch bag and carry a couple of separate thermos containers. That’s more practical than the fancy stacked “bento thermos” options.

Preheating is the difference between warm-at-noon and cold-at-10.