Water supply vs sewage costs
Range in cost per gallon
This sounds like a very dull question, but here goes anyway: when does your tap water in Portland cost a third of a penny per gallon, when does it cost 1.3 pennies, and when does it cost 3.1 pennies? Short answer: if you're irrigating your garden in the summer, that water probably costs you .37¢ per gallon. During some of the winter, that water probably costs you 1.29¢ to the gallon. But during a certain portion of the winter, your water use will cost you 3.14¢, or possibly 4.06¢ per gallon, due to the way the Portland Water Bureau (PWB) estimates your sewer usage through the year.
You can read about this process directly from PWB, but I'll take a shot at summarizing what I find to be somewhat confusing language.
In the following discussion, "one unit" means 100 cubic feet of water, which equals 748 gallons. PWB measures and bills based on these "units."
PWB bills you separate prices for water supply ($2.733 per unit) vs sewer usage ($6.92 per unit), but only measures the water you pull in from their supply; they don't measure how much water you actually put down the sewer. They assume that during the winter and early to mid spring months, you aren't washing your car, watering your lawn, or irrigating your garden, so any water you use from the supply is going down the sewer. So they use your water usage during the billing cycle which falls into that timeframe to establish your "winter average water use." Any water you use above and beyond that amount during the rest of the year is only billed for the cost of the water supply, not for the sewer usage.
For example, if during the key winter months, you use 5 units of water, then during those months you'll be billed for 5 units worth of water supply, and 5 units of sewer usage. If during the summer irrigation period you use 15 units of water, you'll be billed for 15 units of water supply, but still only 5 units of sewer usage. If in late fall and early winter you only use 4 units worth of water, you'll be billed for 4 units for water supply and sewer usage.
Since the sewer charge costs more than twice as much as the water supply cost, this matters a great deal to your bill! Every unit of water you conserve during the crucial "winter average water use" period saves you not only the cost of the water and sewage for that period ($2.733 + $6.92), but most likely saves you $6.92 for the late spring/early summer sewage charge, and another $6.92 for the summer through fall period. In total, you save $23.49 per unit conserved over the course of that year (3.14¢ per gallon). If you wind up using more water during your fall through early winter period than you did during the "winter average water use" period, then you'll save another $6.92, bringing your savings up to $30.41, or 4.06¢ per gallon.
Two notes on this phenomenonIf you move into your residence after (or too late during) the "winter average water use" period, then PWB assumes you're a standard super wasteful household and sets your value to 15 or 18 units (!!!). Considering that we normally use 2 or 3 units in the winter, that really screwed us the first year we moved here as we irrigated a lot during the summer.
If you use 0-2 units during the key period, the city sets your default to 7 (!?!). So if you're super frugal, you'll want to keep an eye on your meter towards the end of the key period, and run a lot of extra water if needed, to get yourself to 3 units.
Winter average water use months
The PWB webpage says "For residential accounts billed quarterly, the city calculates the winter average on water readings taken between February 1 and April 30." Some people may have their billing cycles fall clearly into the middle of this range. ie, if each year your quarterly billing cycle looks something like June 2nd - September 1st, September 2nd - December 1st, December 2nd - March 1st, March 2nd - June 1st, then you'll know that your key period is always going to be roughly the months of December, January, and February.
Our billing cycle falls right around February 1st, sometimes a day or few before, sometimes a day or few after. So it's not clear to us each year whether we need to focus on November, December, and January, or on February, March, and April.
If your billing cycle offers you similar ambiguity, or if you just plain find all this confusing, you can call PWB customer service at 503-823-7770 with your account number and ask them which billing period will determine your winter average water use.
Payback CalculationsA good price on a 55 gallon rain barrel is $10 used (let me know if you have information on cheaper sources, since that changes these calculations a lot!). (I've seen similar deals on 250 gallon totes, which go for $50 as a good price.)
Each time you fill and drain your 55 gallon rain barrel during the crucial winter average water use months, you save $1.73 - $2.23 in that year (the range based on whether you save on sewage charge for all four quarterly bills that year, or only three). If you use 2 gallons a day (say, washing off roots, tools, and your hands) over 90 days, you save $5.65 - $7.30. Payback in less than two years! But of course, it's raining more or less all the time, and you're probably not doing much outside that demands water use, so you probably only need to put in one barrel for this payback. For that matter, you can go pretty far with a few five gallon buckets under your downspouts for no cost at all.
Each time you fill and drain your barrel during the rest of the year, you save 20 cents. You need to fill and drain the barrel 50 times before it'll pay for itself. During what I call the "swing months", of April, May, June, and September, you may get decent usage from your rain barrel, as we often go a week or more between rainfalls, and plants can benefit from the water you've stored in your barrel, which then gets refilled in the next rainfall. A barrel may get drained and filled 8-12 times across those months and during the few summer rains, saving you $1.60 - $2.40 in water costs, thus paying for itself in four to six years. Here a rain barrel clearly wins out over dealing with a bunch of five gallon buckets, especially if you integrate the rain barrel into a gravity-fed irrigation system so you're not hauling the water around.
Optimal Storage CapacityHere are some rough ideas on how much storage capacity will be usefully used during these months. Looking at some historic precipitation data I would target at least enough storage to hold the water from a .5" rainstorm on your roof. A 1000 square foot house footprint would yield 280 gallons of water (1000 ft * .5" / 12 gives you the cubic feet of water, times 7.48 to convert from cubic feet to gallons, times .9 to account for the water lost to evaporation and other loss.) So about five rain barrels per 1000 square feet of roof catchment area. I estimate from looking at the last few years of rainfall, that you'll get spells of .75" or more maybe three or four times a year. So you could have another 140 gallons of storage, say two or three barrels, each of which would save you 60 or 80 cents per year, and pay for themselves in 12 to 16 years. Beyond that and you get only one or perhaps two uses of your barrel per year, so it takes 25 to 50 years to pay for itself.
A single 250 gallon tote would come close to my targeted 280 gallons of storage per 1000 square feet of catchment, assuming that you can direct all that water to the tote. Multiple rain barrels gives you more flexibility in positioning them in different areas, though of course it then requires additional work to visit them all to drain them into garden areas.
Other considerationsOf course, water costs will continue to go up, especially if the city moves ahead with its plan to give tons of money to private contractors with good connections to create treatment plants the system doesn't really need. (Look into this and get active if you plan to remain a PWB customer.) So you could assume water costs will double, and thus bring the payback period way down for the barrels. You do the math.
If you can get rain barrels for less than $10 each, or find another way to store water for less than 18 cents per gallon, then again that improves the payback time on the water storage, in a direct proportional relationship. (Cutting the cost in half cuts your payback time in half.)
If you're planning a multi-barrel system, odds are good that you'll be buying some hardware to connect the barrels together, maybe lumber to support them, and perhaps irrigation pipe above and beyond what you would otherwise purchase. Be sure to include that in your total costs to determine your payback time.
If your garden area to be irrigated by the rainwater is too small to allow you to usefully apply all the water from your storage, then your payback time increases. Be sure to position your barrels to give you the greatest likelihood of actually using them between rainfalls. Place them uphill from your garden beds, and make them convenient to use.
Storage can start to take up a lot of space, fast. We originally wanted to have one or two multi thousand gallon storage tanks, but we realized they would each displace a small tree or large shrub. If you have a two story house and can fit the storage against your shady north side and still get the water to useful areas, losing that space doesn't hurt your food production all that much. Storing water under your house can save a lot on space, but then you need to figure out how to get the water out and up to your growing areas! A hand pump, or a small solar panel with a pump may work, but of course add to the cost and thus to the payback time.