What you will make
In this project you make two things for your plan:
- A water budget: how much water your household uses, indoors and outdoors, month by month.
- A catchment sizing sheet: how much your roof can catch each month, and what tank size gets you through the dry months.
Together they show whether rain can be your main supply, a partial supply, or a garden top-up. Both save to My plan, on this device only.
Gather your numbers
Have these ready before you start. Estimates are fine. You can change them later.
- People: how many live there, and how many are home most of the day.
- Fixtures: toilet type, shower head flow, laundry loads per week (lesson 14).
- Garden: rough bed area and which months you water.
- Roof footprint: length × width at the eaves, in square feet (lesson 16).
- Monthly rainfall: from NOAA climate normals for the nearest station.
- Climate zone: from your homestead brief (lesson 6). It changes outdoor use and freeze risk.
Step 1: your water budget
Fill in your household and fixtures. The tool adds up daily use per person and turns it into a monthly total. Add outdoor use only in the months you water.
Look at the result as two lines: a steady indoor line and an outdoor line that rises in summer. The outdoor line is usually where the biggest savings are.
Step 2: your roof and tank
Now enter your roof footprint and monthly rainfall. The tool shows how much you could catch each month, after losses. It then runs the tank month by month against your water budget.
Try a few tank sizes. Watch for the month where the tank first runs empty. That month, not the yearly total, decides your plan.
Text description
A line chart of the water level in a rain tank at the end of each month, from January to December, with illustrative values. The tank is full from January to April, and extra rain overflows and is lost; these months are marked as overflow. The level falls slowly in May, then quickly in June and July as rain drops and the garden needs water. The tank first runs empty in August and stays empty in September; these months are shaded as shortfall. It starts to refill in October and is nearly full again by December.
Reading the result
Your result usually lands in one of three places:
- Surplus all year: the roof catches more than you use in every month. A modest tank may be enough. Check the dry-year case.
- Seasonal gap: you catch plenty in the wet season but fall short in the dry one. A bigger tank carries the surplus forward. This is the most common result.
- Yearly shortfall: even with a huge tank, the roof cannot catch what you use in a year. You need more roof, less use, or another source.
Text description
Three small charts, each with monthly bars for the water a roof catches and a line for household use, which is steady most of the year and rises in summer for the garden. The values are illustrative. First, surplus all year: the bars are above the use line every month, so a modest tank may be enough. Second, a seasonal gap: the bars are tall in winter and very short in summer, and the summer months fall below the use line, shaded as shortfall; over the year there is enough rain, so a bigger tank can carry the wet-season surplus forward. Third, a yearly shortfall: the bars are below the use line in every month, so the household needs more roof, less use or another source.
Run the numbers again with rainfall about a quarter lower than normal. Dry years happen, and plans should survive them.
When it does not balance
What to take to a supplier or pro
Your plan gives a tank supplier, well driller or water professional a clear starting point. Print or save:
- Your monthly water budget, indoor and outdoor.
- Your roof footprint, roof material and monthly rainfall source.
- The tank size you tested, and the month it ran lowest.
- Whether rain is for drinking, household use or only the garden.
Check
Your roof catches more than you use over a year, but the tank runs dry each August. What does the budget tell you?
Summary and next steps
- Your water budget shows use by month. Your catchment sheet shows supply by month.
- The driest stretch, not the yearly total, sizes the tank. Test a dry year.
- Close gaps by using less first, then adding roof, storage or a backup source.
- Take your budget to suppliers and pros, and test any drinking supply.
Your water budget joins your plan. In the Energy track you will build a matching budget for electricity.
Go deeper: the tank balance method
The tool uses a month-by-month balance. Start with the tank at some level. Each month, add the rain caught and subtract the water used. If the tank goes over full, the extra overflows and is lost. If it goes below empty, that is a shortfall. Run it for a few years of real rainfall and you see how often you would run short. Manuals like the Texas Manual on Rainwater Harvesting describe this method in more detail. Real systems also lose a little to leaks and evaporation that a simple model leaves out.