Is it possible to use a power plant-like machine to create small amounts of electrical (or other) energy directly inside the human body, making it such that the human has energy for activity without the need to eat? Something like directly giving the person heat or electricity for human energy or something like photosynthesis using solar panels and such.

Yes and no.

You can invert the glucose cycle using a machine performing electrosynthesis - converting carbon dioxide and water back into glucose. Then you beam energy to the machine, and Bob's your uncle.

But it's not enough - human beings don't eat just to replace spent energy, they also require basic substances to perform maintenance on their organisms, and there are several more chemical processes that can't be inverted in the lungs. Even if you succeeded into tapping into the lower intestine and recovering basic matter from there, a lot of volatile compounds - starting with water vapour - would still be lost (plus the ordinary wear and tear on skin, hair, nails and so on).

However, with the basic energy needs covered by the machine, you could do without food and maybe even water for substantial periods.

You probably should expand on the question to make it clear what you actually need but...

Recycling with implants

Your body converts sugars and oxygen to energy and carbon dioxide (plus some water). You can implant a machine that takes water and carbon dioxide from the blood stream and converts them to sugars using energy supplied by an external source. This would reduce the need to eat and breathe as long as energy is supplied.

The above is for the aerobic metabolism, you can do a variant for the anaerobic metabolism but it is probably a bad idea unless your implants are nanomachines inside every cell and can boost the cellular metabolism directly.

It is very possible to sustain a human being (even a physically active one) without them having to eat at all.

Gastrostomy tubes go from outside of the body directly into the stomach. But what you put in there is basically identical to food (with some restrictions). Someone can get all of their nutrition and fluid needs this way. They'd only need some water to moisten their mouth and throat, but that's optional.

Central lines aka central venous catheters are IVs placed in a large vein so they can deliver a lot of material. They can provide total parenteral nutrition.

Parenteral nutrition (PN) is the feeding of specialist nutritional
products to a person intravenously, bypassing the usual process of
eating and digestion...The person receives
highly complex nutritional formulae that contain nutrients such as
glucose, salts, amino acids, lipids and added vitamins and dietary
minerals. It is called total parenteral nutrition (TPN) or total
nutrient admixture (TNA) when no significant nutrition is obtained by
other routes, and partial parenteral nutrition (PPN) when nutrition is
also partially enteric. It may be called peripheral parenteral
nutrition (PPN) when administered through vein access in a limb rather
than through a central vein as central venous nutrition (CVN).

A friend of mine lived for years with a central line. She drank some water and ate tiny amounts to keep her gut going (it was severely damaged) but otherwise, got 100% of her energy and nutritional input from the central line. She was very active and could go sailing or on hikes. The port of the line disconnects easily so she was not tethered to it. As long as she connected for a few hours day and night, she was fine.

If you're asking if a human could survive off of some sort of electrical energy, well no. Energy for humans (and most animals) is from calories.

The Calorie (large calorie or kilocalorie — symbols: Cal, kcal), also
known as the food calorie, is defined as the heat energy involved in
warming one kilogram of water by one degree Celsius.

Although calories are heat energy, delivering heat to people doesn't do the same thing. If it did, no one would starve in tropical countries. Our bodies work as transformers, the interface between materials in the outside world and producing energy within our bodies. There isn't a substitute.

So, yes, you can create systems where humans do not have to eat. Meaning they do not have to take anything by mouth or even through their stomachs and digestive tracts. But you can not bypass the body's systems for breaking down calories and nutrients.

Eliminating the need to eat is really hard with any sort of real science(ish). You're basically into teleportation, super powers, and/or magic.

Now wireless charging does exist, can be made reasonably small, and presumably would work for broadcast energy. So in theory cyborg implants or just robots could be (re)charged.

If we extend that to "brain in a robot body" then we're pretty close.

https://en.wikipedia.org/wiki/Inductive_charging
https://en.wikipedia.org/wiki/Conductive_wireless_charging

You could certainly severely reduce the amount of food people would need to consume. It would have to be nutrient-packed and not very energy-dense.

If you were trying to reduce people's reliance on food crops, for instance, or allow people to go weeks without food (military activities?), you could implant a network of nanobots that could do glucose metabolism in reverse, and you could wirelessly power that.