Questions are good, it’s how you learn. You can prime with sugar or you can just inject CO2. Priming with sugar is just the same for a keg as it is for bottles, think of the keg as one big bottle. Generally, you will need to occasionally top up with CO2 as you drink the beer and create more head space or the pressure will drop and your beer become less carbonated. In extreme situations you’ll not be able to serve beer without allowing air into the keg - don’t do that, it will oxidise your beer and ruin it. Topping up the CO2 can be done by re-priming or by injecting CO2.
If you prime with sugar and don’t drink very often you may just be lucky and not have to top up the CO2 at all because the yeast does still produce CO2 over time from residual sugars but only very slowly.
If on the other hand you get a taste for the beer and drink three or four pints in succession you may have to inject CO2 more often to fill the head space.
I used to use small 8g CO2 bulbs, many people use the larger soda steam CO2 bottles. I’ve now moved to fitting Schrader valves to my kegs, as shown in the picture, and I inject CO2 from my big CO2 bottle (6Kg) because it’s so much cheaper.
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