Pay attention to how you're breathing when you sing. Vocal power comes from using just the right amount of breath. You can increase your vocal power and volume by using less with the low notes, more with the high ones.
Work on using your diaphragm. Try bending over when you practice your song or speech. Push your stomach out when you breathe in before singing or speaking to warm up your diaphragm.
Practice articulating the words you're saying or singing. This helps project your voice so everyone in the room can hear you. It also improves your vowel sounds.
Notice how much of your vocal work comes from your chest. In singing these tend to be notes in the lower ranges. You may have a voice power problem if you do more singing in a low key or speak with a low-pitched voice.
Do exercises to improve your head voice in singing. Often associated with higher notes, you can train yourself to add head vocal quality to notes you sing from your chest.
Perform a singing exercise using the letter "F." Hold the "e" sound of this diphthong and feel the vibration in the back of your nose. Start as high as you can and move slowly down your vocal range, noticing when the vibration shifts from your head to your chest.
Play games with your voice once you feel confident with your head voice. Hit a high note, then a low note and move gradually towards that middle point where you shift from head to chest. Notice how your vocal power increases with those low notes.