My wife uses a different technique for some canning.

She takes tomatoes and put an "X" in the bottom. Puts them into boiling water. When the "X" pulls apart its ready to peel. Peel the skin. Cut them up. But them in a sauce pan without water. Bring to a boil without burning over low heat.

Put your cannig jars in the oven at 200F. Lids are in hot water in a sauce pan.

Dump tomatoes into jars. As contents cool it will seal the lid.

You can do this and not require all the energy boiling a dozen jars for an hour would normally take. Something which may come in handy post SHTF when there isn't a lot of fuel available.