It is the question that stops people setting a live wallpaper at all, and the honest answer is less dramatic than either side of the argument suggests. A live wallpaper does use more power than a still one. The amount is small, and it is almost never the reason a phone runs out.
Here is what actually happens, and what to do if you want the moving version without paying for it.
What a live wallpaper is doing most of the time
Nothing. That is the part people get wrong.
Nothing on an iPhone lock screen loops continuously in the background. Motion happens at specific moments: when you wake the phone, when you unlock it, when you tilt it, or when a shuffling wallpaper is due to change. In between, the file sits on disk and the screen shows a still frame.
So the cost is not a video playing all day. It is a second or two of decoding each time you wake the phone, and how often you wake the phone is the number that matters.
The home screen has never animated on iPhone, on any version, so it contributes nothing at all.
What actually drains the battery
In rough order, and none of these is the wallpaper.
Screen brightness. By a large margin the biggest controllable factor. The display is the single hungriest component in the phone, and brightness scales its consumption directly.
Screen-on time. How long the display is lit, which is mostly a function of how much you use the phone rather than what is on it.
Cellular signal strength. A phone in a weak signal area works considerably harder to stay connected than one on strong wifi.
Background activity. Apps refreshing, location services, and anything doing work while you are not looking.
A live wallpaper sits well below all of these. If your battery life changed noticeably the day you set one, the wallpaper is probably not the cause and it is worth checking Settings, Battery to see what actually consumed the charge.
Where the wallpaper genuinely does matter
Not motion. Brightness and colour.
On an OLED iPhone, each pixel produces its own light and a black pixel is switched off rather than dark. So a mostly black wallpaper genuinely costs less than a mostly white one, and the difference is real rather than theoretical.
This is why a dark aquarium scene is a better choice than a bright reef one if power is your concern, and it has nothing to do with whether either of them moves. A bright still wallpaper costs more than a dark live one.
On an older LCD iPhone the backlight is always on across the whole screen, so the image makes no difference to power at all. Only brightness does.
The always-on display is the real cost, not the animation
On Pro models from iPhone 14 onward, the always-on display keeps a dimmed version of the lock screen visible while the phone is idle. That is a genuinely different situation from a wallpaper you see only when you pick the phone up, because now the image is on screen for a large part of the day. Nothing animates in that state, it is dimmed and static, so the live half is irrelevant. What matters is how bright the picture is. If you use always-on and you care about battery, a dark wallpaper is worth far more to you than turning off motion.
How to check it for yourself
You do not have to take anyone's word for it, including this article's, because the phone keeps the record.
Open Settings, then Battery. You get usage broken down by app for the last day and the last ten days, along with a graph of screen-on time against battery level. Look at two things.
First, whether anything in that list is consuming a share that surprises you. A wallpaper does not appear as an app, so if one particular app is dominating, that is your answer and it has nothing to do with the lock screen.
Second, compare screen-on time between a day before you changed the wallpaper and a day after. If the screen was on much longer on the second day, the battery difference is explained without any reference to the image.
The useful habit is to change one thing at a time. People usually set a new wallpaper during a general tidy-up of the phone, alongside new apps, new widgets and an iOS update, then attribute the result to the most visible change rather than the most likely one.
If you want to reduce it anyway
Four things, in order of how much they do.
Lower the brightness, or leave auto-brightness on, which is in Settings, Accessibility, Display and Text Size.
Pick a darker image. On OLED this is the second biggest lever and it costs you nothing in function.
Turn on Reduce Motion in Settings, Accessibility, Motion. This suppresses the tilt effect across the system, so it affects more than the wallpaper.
Use Low Power Mode when you need to. It reduces some motion and visual effects alongside a number of other things, and the wallpaper is a minor part of what it changes.
Turning off the live wallpaper itself is the smallest of the four, which is why it is last.
Quick answers
Does a live wallpaper drain battery when the phone is in my pocket?
No. The screen is off, and nothing about the wallpaper runs with the display off.
Is a live wallpaper worse than Photo Shuffle?
Not meaningfully. Shuffle changes the image on a schedule rather than animating, and both cost far less than screen brightness.
My battery got worse after I set one. What happened?
Check Settings, Battery for what actually used the charge over the last day. A wallpaper change often coincides with an iOS update or a new app, and one of those is usually the real answer.
Does a moving wallpaper make the phone warm?
Not in normal use. Sustained warmth points at background activity, charging, or a demanding app rather than a wallpaper that plays for a second on wake.
Where to get it
Aquarium 3D: Live Wallpapers has both bright reef scenes and dark deep-water ones, previewed in a phone-shaped frame so you can judge how much of the screen will actually be lit before you save anything.
If battery is the concern, pick the dark scene and keep the motion. That is the trade that costs you the least.