LilyM gives good info.
She forgot the disclaimer that you have to be careful with it, if you get a drop of it on you it burns!

Heheh just kidding.
I just wanted to make the comment that I believe there is the ability to replenish holy water with unblessed water as long as it there is a certain amount of the holy water left. I can’t find a link for it at the moment, but when I do I will post it.
When I first started my training I asked about how Holy Water was made outside of the Easter Mass, to replenish it in case of dehydration or if the large container somehow sprung a leak, and there was little left. The priest giving the class got very serious and told us to take notes. He then explained, each of the steps, in a very clear, and slow paced method with all the somlemness he could muster.
Step 1. Find a large stainless steal pot.
Step 2. Fill the large pot it with water. Distilled is better, but tap water will do.
Step 3 Place the pot filled with water on the stove over high heat.
Step 4. Boil the (H-E- double hockey sticks) out of it.
On a serious note, it is good to keep holy water in the house as it reminds us of our baptism when we use it. I will look for the specifics on replenishing it with regular water, as I believe if there is enough holy water, then the regular water becomes blessed when it is mixed in since the blessing maintains it’s power even to the last drop.
Best bet is to just follow what LilyM said above, and if you find yourself running low, just take the bottle to church and refill it there.