Japt, 60 40 bytes

Ox`nàÀÓ&.getBÂjry().È(x=>O.q(L*x.¤vel))

Test it

Excel VBA, 150 bytes

Restricted to 32-Bit Windows Installs of office, because the windows Kernel32 call is not 64-bit pointer safe.

Outputs to the cell A1 on the ActiveSheet.

Declare Sub GetSystemPowerStatus Lib"kernel32"(f As t)
Type t
i As Integer
b As Byte
End Type
Sub d
Dim e As t
GetSystemPowerStatus e
[A1]=e.b
End Sub

-26 bytes thanks to flexible output

-7 bytes thanks to @Neil for using an Int instead of 2 bytes

Java, 546 bytes (Windows XP or Higher)

The powercfg utility came into play for WindowsXP, I have no idea what utilities were available prior to this. I must also be run as administrator. This is god-awful and I didn't make an extreme golfing attempt...

import java.io.*;import java.util.Scanner;public class J {public static void main(String[] args)throws Exception{Process p=Runtime.getRuntime().exec("powercfg energy");p.waitFor();Scanner s=new Scanner(new File("C:\\windows\\system32\\energy-report.html"));String x;double a=0,b=0;while(s.hasNextLine()){x=s.nextLine();if(x.contains("Design Capacity")){s.nextLine();b=Integer.parseInt(s.nextLine().replaceAll("\\D+",""));}else if(x.contains("Last Full Charge")){s.nextLine();a=Integer.parseInt(s.nextLine().replaceAll("\\D+",""));}}System.out.print(a/b*100);}}

Formatted / commented...

import java.io.*;
import java.util.Scanner;

public class J {
    public static void main(String[] args) throws Exception {
        Process p = Runtime.getRuntime().exec("powercfg energy"); // Run CMD sys32 app.
        p.waitFor();                                              // Wait for it.
        Scanner s = new Scanner(new File(
                "C:\\windows\\system32\\energy-report.html"));    // Read output.
        String x;
        double a = 0, b = 0;
        while (s.hasNextLine()) {
            x = s.nextLine();
            if (x.contains("Design Capacity")) {                   // Find max capacity.
                s.nextLine();
                b = Integer.parseInt(s.nextLine().replaceAll("\\D+", ""));
            } else if (x.contains("Last Full Charge")) {           // Find current capacity.
                s.nextLine();
                a = Integer.parseInt(s.nextLine().replaceAll("\\D+", ""));
            }
        }
        System.out.print(a / b * 100); // Calculate %.
    }
}

Honestly I was more interested to see if it was even possible in Java.

Emacs Lisp, 9 bytes

(battery)

Shows current battery status in mini-buffer at the bottom (or to stdout when using --batch) in format such as: Power N/A, battery Unknown (99.1% load, remaining time N/A).

Should work on Windows, OSX, BSD , and Linux.

Bash+acpi, 43 4 bytes

New rules

acpi

output on form

Battery 0: Charging, 92%, 00:05:37 until charged

not sure if the rule change makes things more interesting, couldn't you dump any text that contains all strings 0 to 100 which would be valid output? Seems to lose the original intent if so.

Old rules

set `acpi`;echo Remaining battery is ${4%,}

output on form

Remaining battery is 92%

Don't ask me why set works like this, but it does.

PowerShell, 28 bytes

-26 (!) thanks to AdmBorkBork. Previous version -3 thanks to colsw.

((gwmi win32_battery)|% e*g)

gwmi is short for Get-WmiObject

|% takes the pattern e*g and finds the only matching property; estimatedChargeRemaining

C (win32, gcc i686-w64-mingw32), 78 bytes

#include<windows.h>
main(s){GetSystemPowerStatus(&s);printf("%hhu",s>>16);}

Abuses the win32 API by letting GetSystemPowerStatus() write "somewhere" on the stack, the interesting member is in the third byte, according to the SYSTEM_POWER_STATUS struct.

Unfortunately, the #include seems to be needed, probably because of calling conventions.

Example output:

> bat.exe
90

Displays 255 on systems without a battery.

JavaScript (browser), 78 77 50 bytes

(-1 byte thanks to Benoit Esnard)

(-27 bytes since the output format is flexible now)

navigator.getBattery().then(b=>alert(b.level*100))

JavaScript (browser), 46 bytes, by returning a promise

(Suggested by Shaggy, requires a header and footer)

f=
_=>navigator.getBattery().then(b=>b.level*100)
f().then(alert)

Note: Only works on Google Chrome >= 38 (desktop and Android) or any other browser that supports Battery API. FireFox has removed this API due to privacy concerns.

Example on Android (old output format):

Example on a Windows laptop (old output format):

Bash, 70 67 50 46 43 16 bytes

`</*/*/*/B*/c*y`

Outputs: <battery 0-100>: command not found

Reads the file /sys/class/power_supply/BAT1/capacity.

Tested using Ubuntu 18.04 on a LENOVO ideapad 500.

Takes a while (since it searches the entire filesystem) - but it saves a byte!

Old answer:

echo Remaining battery is `</*/*/*/B*/c*y`%

QPython for Android, 133 bytes

from androidhelper import*
d=Android()
d.batteryStartMonitoring()
d.dialogCreateAlert(str(d.batteryGetLevel()[1])+"%")
d.dialogShow()

The +"%" isn't strictly necessary since it uses the newer more flexible output format anyway, but it makes for slightly nicer output at just +4 bytes.

Python 3.6 + psutil, 80 bytes

import psutil
print(f"Remaining battery is {psutil.sensors_battery().percent}%")

Try it online!

If the machine has no battery (like on TIO), the program will throw an error, because psutil.sensors_battery() returns None.

This requires Python 3.6 for string interpolation.