How much battery does NFC actually use?
Very little. An idle phone with NFC enabled emits short detection pulses that cost a small fraction of what the screen consumes, and modern devices reduce that further when the screen is off. Reading a tag draws more, briefly. Battery is not a good reason to keep NFC disabled.
Two very different regimes
A phone with NFC enabled is doing one of two things. Most of the time it is polling: emitting a brief field pulse, listening for an answer, going quiet again. Occasionally it is actually powering a tag through a full exchange.
These cost very different amounts, and conflating them is why the question gets confused answers.
What idle polling costs
Polling has to happen, otherwise the phone could never notice a tag. But each pulse is short and the interval between them is long by the standards of a processor.
Manufacturers also throttle it aggressively. Polling typically slows or stops when the screen is off, and further still when sensors suggest the phone is in a pocket or face down. The result on a current handset is a background cost that disappears into the noise of everything else the device does.
What a read costs
During an actual exchange the phone is supplying the energy for two chips rather than one, and it does so continuously for the duration. That is a real draw, but it lasts a fraction of a second.
You would need to tap tags continuously for a long time to notice it, which is a workload that essentially only occurs when encoding a production batch. Even then, the screen being on the whole time costs more than the NFC hardware does.
The real reasons to switch it off
There are reasons to disable NFC, and battery is not among them. Wanting to avoid a tag triggering something unexpectedly is one. Preferring that a card in the same pocket not be interrogated is another.
What has changed is that the battery argument was more defensible on older hardware, before polling was throttled the way it is now. Advice written for that era still circulates and no longer matches how current devices behave.
Frequently asked questions
Should I turn NFC off to save battery?
Not on a reasonably recent phone. The measurable difference over a day is small enough to be hard to detect against normal variation in usage. If you want to disable it, do so for control or privacy reasons rather than for endurance.
Does NFC keep polling when the screen is off?
Usually at a much reduced rate, and on many devices it stops entirely in certain states. This is a manufacturer decision rather than a standard, so it varies between handsets and between versions of the same operating system.
Does a tag drain the phone that reads it?
Only for the fraction of a second the exchange lasts. The phone supplies the tag's entire power budget during that time, but that budget is microwatts against a battery measured in watt hours.
Is card emulation more expensive than reading?
Emulating a card involves listening rather than generating a field, so the phone is not powering anything external. The cost profile is different and generally lower, though the secure element and the operating system add their own overhead.