How come my A21 Emergency LED Bulb is not as bright when using with a battery as it is when inside a light fixture?

When the A21 Emergency LED Bulb is connected to a light fixture it produces 1100 lumens of bright light for your general purposes lighting needs. When disconnected, the bulb draws power from the built-in, rechargeable battery. It will produce only 150 lumens in order to provide you with up to 4 hours of portable light.