TL;DR. Here’s how to start collecting rainwater: Step One – calculate how much water you can collect. UK homes receive an average of 885 litres per square metre of rain annually. Step Two – decide what you’re going to do with it. Toilets and washing machines offer biggest savings, gardens are easiest to start with. Step Three – choose basic setup (£30-£70), or go full throttle with a pumped system (£750-£3000+).
Reduced water bills will happen within one quarter (3 months) of adding a basic water butt. Garden watering costs zero pounds immediately after installation. Payback on systems that include toilet flushing or washing machines takes 10-15 years.
You can spend as little as £30 on a 200 litre water butt with diverter for basic garden use. £200-£500 will get you enough storage to seriously garden or keep an allotment fully stocked. £2000-£15000 buys you enough tanks to run an entire house, complete with pumps and underground installation.
Rainwater harvesting is simply redirecting water falling on your roof into storage tanks or butts, instead of draining away into the ground. You can then use this water for flushing toilets, feeding into washing machines, or irrigation. Rainwater collecting has been practiced for centuries, but rooftop collection systems became common in the UK during the 1990s.
The reasons why rainwater collection matters start with typical water usage. The average UK household uses 142 litres per person per day (Ofwat). Toilets account for approximately 30% (Waterwise), washing machines approximately 15% (Energy Saving Trust), with garden watering being as high as 30% of household consumption during summer months (Environment Agency).
Water rates have increased by over 40% in the last five years nationally, and even more where water scarcity issues have developed local shortages. Putting water butts under downpipes and using rainwater to flush toilets can cut mains demand by up to half (WRAP) depending on your household water usage patterns.
Just how well rainwater harvesting really works? Let’s look at the numbers…
Disclaimer: I used links to rainwater butt suppliers, of which there are hundreds. But this blog post was written with two companies in particular at the top of mind. Both ecoRainwater Harvesting and Save Water Save Money supplied useful information and products during my research into setting up this article. If you’re in the market for any rainwater harvesting supplies feel free to visit those pages or Google for more options.
## Rainwater Collection Math
I’ve been collecting rainwater for eight years, starting with a single water butt and ending with what has become an obsession. The easy part was buying that first water butt. After that initial impulse purchase I sat down with a calculator, tape measure, and spreadsheet.
By measuring my roof, checking the annual rainfall in my area (Met Office), factoring out roof areas that don’t run off into gutters (inside corners, shaded roofs mounts, etc) I worked out how much water we can expect to collect each year. Then I compared those volumes to our typical annual household water usage.
Doing the math was a huge eye opener. We could easily collect more rainwater than our annual water bill, yet never fully fill more than two water butts. Turns out there’s a lot more to rainwater collection than buying a few cheap barrels and calling it a day.
Here are the numbers that proved the concept for me, and probably will for you too.
### What Is Rainwater Harvesting?
Rainwater Harvesting simply refers to collecting rainwater that would otherwise flow off your roof. For centuries we’ve been doing this without thinking twice; using rain chains to fill barrels at backyard kitchens or farms. The first widespread introduction to rooftop rainwater collection systems in the UK were published in two manuals (by Welsh Water) during the early 1990s.
This practice still matters today because toilets alone account for nearly a third of household demand. Washing machines require another 15%, with garden watering making up to 30% of summer months (Environment Agency). Replace those elements with harvested rain and you can easily cut mains demand by half.
A resource as valuable as water should be treated with respect. Every litre captured and reused is one less litre entering wastewater streams, with treatment and pumping costs. Every litre not pulled from lakes, rivers, and reservoirs helps keep those ecosystems healthy and resilient during dry seasons or extended droughts.
### Collection Potential is MASSIVE
For those interested in sustainability and cutting consumption where you can, the greenhouse gas emissions impacts alone are reason enough to give rainwater harvesting a try. The average rainwater harvest system offsets 0.3 tonnes CO2 equivalent per year (WRAP).
The real key number is annual rainfall. The average UK house sits idle and collects about 885mm (Met Office) of rain each year. The reason 885mm is important is because every millimetre of rain that falls on 1 square metre of roof is equal to 1 litre of water.
Given an average UK roof size (78sqm) we can expect most homes to collect approximately 69,000 litres of rainfall every year. Here’s where collection potential really blew my mind.
Roof size is irrelevant when you’re gathering water from only one butt. It doesn’t matter if you have the roof area to fill twenty water butt…if you’re only using one you still only have one barrel.
A UK property with 100 square metres of roof collecting just 800mm of rain annually would still gather 80,000 litres of rainwater that can be used around the home. To put that in perspective, the average UK family uses approximately 145 litres of water per person per day (Ofwat), which comes out to about 53,000 litres for a family of four.
This fully stocked 80,000 litre rainwater harvesting system could supply our entire households non-potable needs with less than a metre of rainfall. Heck, we could even do ourselves a solid and start watering the garden too!
How much roof runoff can you actually collect? Every millimetre of rain on 1 square metre of roof translates to 1 litre of water you can collect, store, and use (SparkCalc). But we don’t collect all the rain that hits a roof due to evaporation and inefficiencies in guttering.
How much water is lost to evaporation? Not much (SparkCalc). Although you should factor in some degree of system inefficiency; what percent of rainfall actually winds up in storage? Most guttering will collect water from 75-90% of the roof surface area it drains, depending on material (SparkCalc). Let’s call it 80%.
Assuming 80% runoff coefficient (amount of rain that your gutters actually collect), and system efficiency of 80% we can expect to collect about 56,640 litres from a 100 square metre roof in an average year. Taking into account variation in annual rainfall based on location in the UK the safe estimation for calculation purposes is 88,500 litres.
### Payback Time is REASONABLE
An average family with dual flush toilets and decent modern washing machine can expect to use roughly 25,000 litres of water per year just for toilets and laundry (Carbon Trust). Remember how I said you could easily supply a whole house with rainwater based on the numbers we crunched above?
I wasn’t kidding! Now we know how much water the average family can expect to consume and collect let’s look at real examples.
| System Size | Annual Water collected | Installed Cost | Avg. Annual Savings | Regional Multipliers |
|---|---|---|---|---|
| 200 litre water butt | 880 litres | £30 – £100 | £10-15 | Rainfall: x0.5Southern England |
| 1000 litre system | 4,400 litres | £200 – £400 | £30-40 | Rainfall: x1Southern England |
| 3000 litre system + Whole house distribution | 13,200 litres | £1000 – £1500 | £100+ | Rainfall: x1Northern Ireland |
Did you notice what time frame these systems take to pay for themselves? Zero to fifteen years with saving starting at £10 per year for a basic setup. Multiply any of those averages by 0.5 if you live in Southern England, by 1.5 if you’re in Northern Ireland.
Factoring water butt dimensions those savings come out to be roughly 3-7 years to get your initial investment back through lower water bills. If your heart set on a full blown rainwater harvesting setup you’ll payback costs in 10-15 years, after which water bills drop by £100+/year.
The numbers work out pretty well when you take into account water usage stats from earlier. Remember our family of four using just over 53,000 litres per year? The largest harvesting systems installed throughout the UK collected an average of 58,629 litres annually (Carbon Trust).
Just over 55% of rainfall collected in funded projects. With really efficient system designs you could conceivably set yourself up to use rainwater for almost all household demands. To put that number into perspective, the majority of new eco builds in the UK now include rainwater harvesting capabilities (RICS). This technology isn’t just proven effective, it’s quickly becoming standard practice.
### EVERYTHING Took Longer Than I Thought it Would
Patience grasshopper. Here are my learnings about what took me far longer to accomplish than necessary:
**Size your storage properly. ** I installed my first water butt next to the backdoor because it was easy. Big mistake. By the time I moved it close to where most roof runoff was collecting I’d gone through two additional water butts due to overflow.
Unless you plan on using everything outside or have an unusually small roof, garden watering alone will never fill your barrels. Garden usage took me far longer to realize would not meet my water butt expectations.
Efficient Use Needs some Planning
When harvesting rainwater you can’t just stick a water butt under any old downpipe and call it a day. Position storage close to where most rain is collecting on your roof. Once you determine that, trace your guttering and figure out where the water flows relative to downpipe placement.
When positioning water butts use the guidelines below to size system appropriately.
| Roof Area | Estimated Annual Collection | Suggested Storage Capacity | Suggested Usage | Install Cost |
|---|---|---|---|---|
| Up to 50 square metres | Up to 44,000 litres | 800 litres+ | Toilet & laundry | £150 – £1000+ |
| 50-100 square metres | 44000 – 88000 litres | 1500 litres+ | Toilet, laundry, garden | £300 – £2000+ |
| 100+ square metres | Over 88,000 litres | 3000+ | Whole-house filtration | £600 – £5000+ |
**Quick tips for properly planning your system include: ** Flush toilets and external taps can be just as easy to plumb into as standard mains feeds (rainwateronline.co.uk). Watering vegetable gardens just got a whole lot easier if you’re only collecting outside tap (aplusbwater.co.uk).
I Hate Wasting Water, Now What?
Gardening takes watering year round here in the UK, so don’t feel limited to using collected rainwater outdoors. By connecting a rainwater harvesting system to your toilets and washing machine you can expect to cut total household demand by half.
**Got kids? ** Rainwater is ideal for flushing toilets, and even washing hands (Reckitt). Setting yourself up for laundry should be a no-brainer; rainwater is naturally soft which means you can cut detergent use by half with no worries about detergent builder deposits inside your machines (Green Building Store).
If your just going to install one water butt use it for Toilet Flushing
Just want to get started collecting rainwater, but not sure where to begin? I know I’ve thrown a lot of information at you so far. Here’s the shortlist if you need somewhere to start.
1. Sit down with a calculator and do the math. How much rain falls in your area? How much roof area does your house have? Are you planning on feeding washing machines, toilets, or watering the garden? Find those answers first.
2. Based on answers from step 1, get your butt…er, rainwater butt started. Garden only systems should be sized to capture enough rainwater to get you through dry spells without overflow. Systems including toilet feeds will likely overflow without proper storage, so plan accordingly.
3. Monitor your household water consumption so you know how much water you actually use.
| Use Case | Rainwater Harvesting Supplies Needed | Suppliers | Increased Complexity |
|---|---|---|---|
| Garden use only | Water butt, diverter valve | Water Butt Direct | |
| Toilet flushing | Pump,header tank(diverter valve, but bigger) | PlumbRight | |
| Washing machine feed | Filtration system. | Utility Warehouse | |
| Toilet flushing + washing machine feed | All of the above | Direct Rainwater Storage |
**Monitor rainfall too. ** Rainfall can vary dramatically throughout the year, and even within seasons. Throughout three years of monitoring my systems water collection has varied as much as 32% year over year.
What will this tell you? Unless you tailor your rainwater harvesting system towards garden usage only, chances are water butts will overflow repeatedly throughout the year.
By winter you’ll understand how much water your family uses and have somewhat of a baseline for year-round consumption. Summer months will shine light on how much water you could potentially be collecting. Use these insights to tweak and improve system efficiency over time.
Better Homes and Buildings Blog via rainwaterharvesting.org
There are great resources online to walk you through everything. Links to suppliers above will get you started if you’re in the UK, otherwise Google is your friend.
Realistically speaking DIY rainwater harvesting systems can be built by most homeowners on weekends, and installed within a month if done right. Seek out suppliers who offer support and guidance when needed – no sense reinventing the wheel!
**Doing This On Purpose starts here**
* Want Toilets That NEVER Run Out of Water?
* Can’t stand hosepipe bans but live in Southern England?
* Hate water waste and paying increased prices each year?
This blog post alone isn’t going to magically turn you into an expert overnight but it will get you thinking about rainwater harvesting in ways you may not have considered before.
**What you CAN learn here: **
- How to properly size storage
- How much rainwater you can realistically expect to collect
- Comparing water butt suppliers to find the best deal
- Ways to use rainwater around your house
The most important takeaways from this entire guide are the ideas of balancing rainwater harvesting collection capacity with realistic year-round usage, and accounting for UK seasonal rainfall distribution.
You’ll never fill more than two or three water butts without adequate storage, so plan ahead. You also don’t need to use everything outdoors. Rainwater collected for toilets and laundry can cut overall household demand by up to 50%.
**Also? Its cheap. ** I paid £30 for my first water butt. Reducing your mains consumption by half using rainwater starts at £30. Grab yourself a water butt and get started collecting rain.
Want more details?
Look into rainwater collection offsets. Want to get technical about real rainwater harvesting systems installed around the UK? These posts will walk you through everything.
| Using Harvested Rainwater for… |
|
| Gravity Fed vs Pumped Systems | Pros and Cons of each |
| DIY Rainwater Harvesting Setup | on a budget |
| Rainwater Butts vs Underground Tank Options | which suits your property best? |
| Rainwater Harvesting For… |
|
| Urban Homes | how to setup a rainwater harvesting system in a small space |
Real examples, from rainwater.harvesting folks just like you.
| Suburban semi-detached home. Hampshire. | 75 square metre roof feeding 1,200 litres of storage used for vegetable garden and greenhouse irrigation. |
| Annual rainwater collection | ~52,000 litres |
| Annual usage | ~15,000 litres for vegetable production |
| Water bill savings | £35-40 per year. Plus zero mains water usage during hosepipe ban periods. |
| Installed cost | £280, payback period 7 years. |
| Victorian terrace. Manchester. | Underground 2500 litre tank with pumped distribution servicing two toilets and washing machine. Roof collection area is 45 square metres providing approximately 32,000 litres annually. |
| Percentage of non potable household demand supplied by rainwater | 60% ( toilet flushing and laundry) |
| Savings on annual water bills | £85-95 pounds annually, or 18,000 litres off yearly consumption. |
| Installed Cost | £4200, payback through savings of 14-16 years. |
| New build detached house. Lincolnshire. | Integrated system with 6000 litre underground storage. Whole-house distribution including toilets, laundry, outdoor taps. |
| Roof feeding system | 120 square metre roof collects rainwater totalling around 78,000 litres per year. |
| Percentage offset | System supplies 45% of total household water demand. Includes automatic mains plumbing to use when rainwater runs out. |
| Installed Cost | £8500 savings of £180-220 per year, payback period 12-15 years. |
| Urban flat with balcony. Bristol. | 100 litre slimline water butt. Limited to container gardening and occasional powerwashing of balcony due to space restrictions. Annual rainwater collection is around 800 litres. Annual usage is closer to 400-600 litres. |
| Install cost £45 because water butts are crazy cheap. Not much impact on water bills without laundry or toilets connected, but I love it when hosepipe bans are announced. |
TLDR Version.
**What I Actually Noticed Once I Started Collecting Rainwater**
Rainwater. Harvesting.
Changed my life.
Probably won’t do the same for you, but here’s some observations that have become automatic since keeping better watering.
I don’t stress about garden watering during drought
Because I do it anyway during rainy months. The water quality is vastly superior to mains water too. Plants love it. We’ve eliminated lime scale deposits in our greenhouse since connecting the washing machine to rainwater. It’s just soft water straight from the sky.
Knowing how much water we’re actually using around the house is probably the biggest change. I look at weather forecasts way differently now, and I’ve started to recognize subtle clues that show when our water butt levels are getting low.
Automatic habits outside of just watering the garden with harvested rainwater bleed into how much mains water we waste throughout the rest of the house. I simply think about water as a resource we shouldn’t waste.
Years ago I remember thinking my rainwater harvesting system wasn’t saving us money. Now? Water bills have gone up by almost 50% since we installed our first water butt eight years ago. Now they pay for themselves.
The water butt pictures featured in this post were provided by Waterbutts Direct. Thanks for the gear!
