Model comparison
eufy C10 or E21: which unit needs the battery?
The current eufy C10 and E21 are both one-camera hybrid kits, but their rechargeable batteries sit in different units. Choosing between them starts with whether the camera or the parent display needs to be portable, then moves to screen size, app setup and the exact two-room package you would need.
Start with which unit needs to move
The C10's 5,000mAh battery is in its parent monitor. Eufy states up to 12 hours of screen time in internal testing, with actual performance varying, and says the monitor's built-in battery is not replaceable. The C10 camera has no built-in battery and requires continuous wired power. By contrast, E21's built-in 5,000mAh battery is in its camera; Eufy describes up-to camera use estimates, while its E21 guide does not establish a continuous parent-screen runtime. Do not transfer either system's battery claim to the other device.
For a fixed camera near a safe power source and a caregiver carrying the receiver, C10's screen-side battery can be useful. If a camera-side power interruption or a camera placement away from an outlet is the essential constraint, E21 has the documented camera battery. Neither claim guarantees a particular runtime in every use pattern.
Screen, image and framing are separate choices
Eufy's support comparison identifies C10 as a 2K camera paired with a 4.5-inch 480p monitor. Its current page lists 351-degree pan, 60-degree tilt and 4× zoom. E21 has a 4K camera and a five-inch parent display; the exact camera resolution is not a measured assessment of the screen panel. If you have a minimum screen size, use the display value rather than comparing camera resolution with another model's screen.
The C10 support material gives an up-to 800-foot local-link figure in open conditions. E21's materials have inconsistent range wording, so no numeric range is assigned to that model here. Neither number is an indoor guarantee: walls and interference matter, so test the connection at the planned camera and monitor locations.
Both are hybrids, but app setup is not interchangeable
Each base kit has a direct local parent-screen path that does not require Wi-Fi, plus a separate connected app route for remote phone viewing. C10 uses the Anker eufy app; Eufy's C10 support article says the eufy Baby app is not compatible. E21 uses the eufy Baby app. For either model, the remote path depends on the household's network and internet connection; do not count it as an offline feature.
For a single room, either exact base kit includes one camera. Each supports pairing another compatible camera, but that does not turn the standard box into a two-camera kit. C10's C10-only add-on and E21's E21 add-on are separate products. The C10 support article confirms up to two paired cameras and split-screen use, while its audio guidance says sound follows the selected camera unless event-triggered switching is enabled. The E21 sources likewise do not confirm two continuously open room-audio feeds. If two cameras must arrive in the box or both rooms must be audible continuously, compare a documented twin package instead.
Where the earlier E20 fits
The E20 remains useful as an owner reference, but its former exact U.S. product route redirects to the current collection and it is not included as a current purchase match. E20 and E21 are not interchangeable: the archived E20 guide and product evidence cover its earlier package, while E21 is the current kit with a charging stand and cot-mount set. See the E20/E21 owner and package comparison for that exact distinction.
In short, choose by the device that must run on battery, not by the shared 5,000mAh headline. The C10 review details its wired camera, one-camera box, 4.5-inch screen and app limits; the E21 review covers its battery camera and separate local/app operating plans. Neither model should be selected for a second room unless the exact package and sound behavior meet that household's requirement.
These distinctions come from Eufy's exact product pages, model support material and the linked C10 T6310 and E21 T8354 guides. Manufacturer specifications are reported as such; no range, battery, alert or picture-quality lab test is claimed.