Julie
Tomatoes do much better early on with rain water rather than tap water and I’ve found that if you have 100 gallons worth of butts linked to a downpipe from your roof, apart from in drought years like last year, you pretty much have what you need for up to 20 plants.
The refilling is most efficient if the rain is heavy, so you really want heavy showers like we’ve been having this past week rather than a steady drizzle. Four hours of heavy rain one day this week refilled a half-empty 50 gallon butt and more.
If you’re really serious 200 gallons of storage would be great, but you start racking up bills unless you have a friendly tradesman/woman frined who can rustle you up something for peanuts.