Water Butts & Rainwater Harvesting: Cut Your Watering Bills

If there's one bit of garden kit that pays for itself faster than almost anything else, it's a humble water butt. Whilst the rest of us obsess over secateurs and posh compost, a quiet revolution has been happening down the side return of British gardens: people are catching the rain instead of paying for it twice over. And honestly, once you've fitted your first diverter and watched a downpour fill 200 litres in twenty minutes, you'll wonder why you ever dragged a hose out in July.

In this guide I'll walk you through the whole thing properly — how to size your butt to your roof and your garden, how diverter kits actually work, the slightly fiddly business of linking several butts together for serious capacity, and the bit nobody talks about enough: keeping that stored water clean enough that it does your plants good rather than harm. I've spent a fair while with these systems, so expect honesty about the limitations as well as the wins.

Why Bother Harvesting Rainwater At All?

Let's start with the money, because that's what gets most people over the line. If you're on a water meter — and increasing numbers of UK households are — every litre you pour onto your borders is a litre you've paid for. At the 2026 UK average metered rate of around £3.50 per 1,000 litres, that adds up quietly across a growing season.

A reasonably equipped garden can realistically displace around 25,000 litres of mains water per year by leaning on stored rainfall instead. Do the sums and that lands you somewhere in the region of £75 to £100 a year off your watering bill. It's not going to fund a holiday, but against the cost of a butt and a diverter kit, you're typically into profit within a couple of seasons — and everything after that is gravy.

But there's more to it than pennies. Rainwater is naturally soft and slightly acidic, which is exactly what acid-loving plants like camellias, rhododendrons, blueberries and most ericaceous shrubs crave. Mains water in hard-water areas is full of lime, and over time that nudges your soil and your potting compost the wrong way. Catch the rain and you're giving those plants the drink nature intended. There's also the resilience angle: when a hosepipe ban lands in a dry summer, your butts keep working when the tap won't.

Sizing: How Big A Water Butt Do You Actually Need?

This is where most people either over-buy a vast tank they'll never fill, or under-buy a 100-litre starter that empties after one watering. The sweet spot depends on two things: how much roof you're collecting from, and how thirsty your garden is.

Capacities on the domestic market start at 100 litres and stretch right up to 13,000 litres for serious underground harvesting systems. For most of us, though, the action is between 100 and 2,000 litres. A 100-litre slimline is brilliant for a balcony, a small patio, or as a top-up tank by a greenhouse. A 200–227 litre butt is the classic all-rounder for an average garden, and popular real-world choices include the Harcostar 168L and Harcostar 227L. Step up to the 350L Magnum and you're catering for a properly productive veg plot or a thirsty collection of containers.

On physical dimensions, smaller tanks typically stand around 130cm high including their stand and roughly 35cm wide — slim enough to tuck against a wall. Larger tanks scale up dramatically, with some reaching 200cm tall on a 120cm base. That stand height matters more than people realise: it's what gives you enough clearance to slide a watering can under the tap.

Diverter Kits: The Clever Bit That Does The Catching

A water butt sitting under an open downpipe is a crude solution — it overflows constantly and catches debris. A rain diverter is the elegant fix, and fitting one is the single upgrade that transforms a butt from "bucket on a stand" into a proper harvesting system.

The principle is simple. You cut a section out of your existing downpipe and slot the diverter in. As water runs down the pipe, the diverter channels it sideways through a hose into your butt. The genuinely clever part is that once the butt is full, the diverter automatically lets surplus water carry on down the pipe to the drain as normal — so you never get an overflowing mess against your wall.

Most kits fit the two standard UK downpipe sizes: 68mm round and 65mm square plastic. A typical diverter kit includes the diverter unit itself, a 20mm connector, a washer, a nut and a 500mm hose to bridge between the downpipe and the butt. Fitting is a job for a hacksaw, a drill and a steady hand — you measure the height of your butt, mark the downpipe, cut your section out, drill the entry hole into the butt, and connect the lot up. Allow an unhurried afternoon for your first one.

Linking Multiple Butts For Serious Capacity

Here's where rainwater harvesting gets genuinely addictive. Once you've got one butt brimming after every shower, the obvious next thought is: why not catch the overflow into a second butt? Linking kits make exactly this possible, and it's the most cost-effective way to multiply your storage without paying for one enormous tank.

A standard linking kit connects two butts together with a single 500mm connector hose, two 20mm connectors, two washers and two nuts. You drill matching holes near the base of each butt, fit the connectors, and run the hose between them. Because the holes are low down, the butts fill as a single pool of water — when one rises, the water flows through to balance the level in the next.

You can chain several butts in a row using multiple linking kits, and this is how allotmenteers build out a wall of storage from inexpensive individual tanks. It's modular, it's forgiving, and you can add to it season by season as budget allows. My honest take? Two or three linked 200-litre butts is the configuration that suits most real gardens — enough buffer to ride out a dry fortnight without committing to a tank that dominates the plot.

Keeping The Water Clean For Your Garden

This is the part that separates a system that helps your plants from one that quietly breeds problems. Stored rainwater that's full of roof debris, leaf litter and stagnant sludge isn't doing your seedlings any favours — and it certainly isn't pleasant to dip a can into.

Your first line of defence is a downpipe filter. These work by combining a steel or plastic grid or mesh with the walls of the downpipe to collect water, allowing heavier debris — grit, leaf fragments, moss washed off the roof — to fall straight through the middle and continue to the drain, whilst the cleaner water is diverted into your butt. It's a passive, elegant bit of design with no moving parts to fail, and it dramatically cuts the amount of muck that ever reaches your stored water.

The second discipline is maintenance, and it's wonderfully low-effort. The golden rule is to empty and clean the inside of your butt once a year — ideally at the end of winter before the growing season kicks in. Tip out the dregs, give the interior a scrub to lift any settled sludge, rinse, and refit. That single annual job keeps the water sweet and stops a slow build-up of organic gunk that would otherwise sour at the bottom.

What The Different Designs Get You

Water butts aren't all the same green plastic cylinder any more. The market has matured into several distinct styles, and choosing the right shape matters as much as choosing the right capacity — especially in a small or much-loved garden where an ugly drum sticks out a mile.

The traditional round green butt remains the workhorse: cheap, robust, and easy to find. Slimline designs are the answer for tight spaces — tall and narrow, they slot down the side of a house or behind a shed where a round butt simply wouldn't fit. The 100-litre slimline class is the least expensive route in and offers genuinely fantastic value if your space is limited.

Then there are the prettier options. The 210-litre style of butt is a clever middle ground — it manages to keep relatively inconspicuous whilst still holding a useful volume, which is exactly what you want in a garden where the butt is on show. Planter-topped butts hide their function under a planting trough on the lid, and decorative stone-effect models dress the whole thing up to look like a feature rather than a utility. There are even tiered "rainwater terrace" designs that double as staging for pots.

How The Designs Stack Up In Practice

To make the trade-offs concrete, here's how the main butt styles score across the things that actually matter day to day. These reflect my hands-on experience and the consensus from real-world testing rather than any single spec sheet.

The takeaway is that there's no single "best" butt — there's a best butt for your situation. If budget and space are tight, a slimline 100L set is a no-brainer. If you've got the room and want a true buffer against dry spells, the 227L round or 350L Magnum earn their keep. And if the butt is going somewhere you'll see it every day, the 210L-class decorative designs strike a lovely balance between capacity and curb appeal.

Handling, Weight And Installation Reality

One practical reassurance: a 200-litre plastic water butt, when empty, can be lifted and positioned by an average adult without much drama. They're bulky rather than heavy until they're full. Full, of course, is a very different matter — 200 litres of water weighs 200kg — which is precisely why the base it sits on is so important.

Stand any butt on firm, level ground. For smaller butts, the supplied stand on a paving slab is plenty. For the bigger 350L-and-up tanks, you want a properly solid, level base that can take a serious static load without sinking on one side. An off-level tank doesn't just look wonky — it stresses the connectors, under-fills when linked, and in extreme cases can topple. Five minutes with a spirit level at the start is the cheapest insurance going.

How I'd Rate The Rainwater Harvesting Approach

Stepping back from any single product, here's how the whole rainwater harvesting concept scores as a piece of garden kit. It's rare to find something this genuinely useful that asks so little of you in return.

The only marks I'd hold back are for the one-off faff of cutting a downpipe and the inescapable fact that no amount of storage beats a genuinely long drought. Beyond that, this is about as close to a free win as gardening offers.

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