Overland Water System Guide: Tanks, Pumps and Weight

A vehicle water system is a storage container, a way of moving water out of it, and a plan for what happens to the water afterwards. That is the whole thing, and the reason it gets hard is that the storage container is the heaviest object in the build. A Ameri-Kart 20 Gallon RV Fresh Water Tank at $145.00 holds 20 gallons, which is 167 lb of water once it is full. Everything on this page follows from that number. The water capacity calculator does the sizing arithmetic for your own trips, days and reserve; this page is how you actually build the system and live with it once the gallons are decided.

How much water do two people actually need per day?

Between 1 and 1.5 gallons per person per day covers drinking, cooking and minimal washing, and 2 to 3 gallons per person per day covers the same plus a shower. That is the number that sets tank size, and it is worth choosing it honestly before you buy anything, because the difference between the two ends of that range doubles the weight you are about to bolt into the vehicle.

For two people, the lower figure is 2 to 3 gallons a day and the upper is 4 to 6. Over a five day trip that is the difference between 15 gallons and 30, or between 125 lb and 250 lb of payload spent on one item. Drinking water is the part nobody should economise on, particularly in heat or at altitude. Washing is where the savings live: a basin instead of a running tap roughly halves washing up, and a foot pump uses noticeably less than an electric demand pump for the simple reason that it stops the instant you stop pushing.

Rather than reproduce the full consumption grid here, run your own people, days, dogs, showers and reserve through the water capacity calculator. It carries the per-use breakdown, the days-per-tank figure and the weight against your remaining payload, and it will tell you plainly when the water alone has eaten more than half your headroom. Bring its answer back here to decide what shape that capacity should take.

One planning habit is worth more than any tank you can buy: assume you will refill more often than you think. Water is the only heavy item in the build that a tap can replace for free. A battery bank does not refill from a hose, a rooftop tent never gets lighter, but water can be topped up at nearly every town, campground and trailhead in the country. Sizing a tank for the longest trip you might ever take means carrying that weight on every trip you actually take.

Why is water the heaviest thing in the build?

Because of a single physical constant that no amount of gear selection can argue with. Fresh water weighs 8.34 lb per US gallon. There is no lightweight water, no premium version that weighs less, and no way to spend money to improve the ratio. Twenty gallons is 167 lb. Thirty gallons is 250 lb. Forty gallons is 334 lb.

Put that next to the things people agonise over. A 100Ah LiFePO4 battery is roughly 25 to 30 lb, and with a tray and hold-down it is comfortably under 45 lb. So a single 20 gallon water tank outweighs a 100Ah lithium battery and its case by a factor of more than three. It outweighs a 400W solar array. On many builds it outweighs the battery bank, the array, the charge controller, the inverter and every foot of cable put together, and it is the one item that gets bought without a spreadsheet.

This matters because payload is a fixed budget and water is the largest single line item in it. Payload is GVWR minus curb weight, and it covers occupants, cargo, fluids in cans, the fridge, the tent, the recovery gear and the entire build. A vehicle can be inside its GVWR and still over its rear gross axle weight rating, or the reverse, because GAWR is a separate per-axle limit. The door jamb sticker is the authority on all of it, never the brochure and never a forum post. The payload and GVWR guide works through those limits properly, and the payload weight calculator will show you what is left after the water goes in.

There is one saving grace, and it is genuinely useful. Water is the only heavy item that gets lighter as the trip goes on. A tank that puts you at the edge of your rear axle rating on the first afternoon is 80 lb lighter by the third day. That is not permission to leave the driveway overloaded, because the first day is exactly when the worst roads and the least practised loading tend to coincide, but it does mean the peak load is short-lived and predictable. Plan for the full-tank case, then enjoy the improvement.

Fixed tank, portable cans or a bladder?

All three store water. What separates them is how much control you keep over the weight, and that is the axis most people never consider.

A plumbed fixed tank is the comfortable answer. It feeds a pump, a sink and a shower properly, it hides under a bed platform or a bench where it is out of the way, and you interact with it by turning a tap like a person in a house. Its weakness is not plumbing, it is commitment: once the tank is installed and filled, that weight is in the vehicle. You can travel with it half full, but then you are carrying a container of water that sloshes and shifts under braking, which is its own problem.

Portable cans let you carry water only when you need it, which is a payload strategy as much as a water strategy. A Scepter 5 Gallon Water Can with Easy-Pour Spout at $27.99 holds 5 gallons and weighs 42 lb full, so two cans for a weekend and six for a long trip means the vehicle only ever hauls what the trip requires. Nothing else in the build offers that. You cannot fit half a battery for a short trip. Cans also fill from any tap, spigot or garden hose without an adapter, they can be carried to the water rather than the vehicle driven to it, and if one develops a leak you have lost a fifth of your capacity rather than all of it.

A collapsible bladder is the specialist option. It fills a cavity no rigid tank will fit, it packs flat when empty, and it is usually the cheapest gallons per dollar. The trades are real: a bladder is puncture-prone in a vehicle full of hard edges, you cannot get a hand or an eye inside it to check what is growing in there, and its shape changes as it empties, which means it needs a container or a cradle rather than just a strap.

The full head-to-head, with dollars, gallons and dry versus full weight for a plumbed tank against four cans, is in water tank versus jerry cans. The short version is that most experienced builds end up running both.

Where should the tank go in the vehicle?

Low, and between the axles. Those two rules cover almost every real decision, and they both come from the same place: 167 lb positioned badly changes how a vehicle handles far more than 167 lb positioned well.

Low means as close to the floor as the layout allows, because centre of gravity is what decides how a vehicle feels in a corner, on a side slope and in an avoidance manoeuvre. Between the axles means inside the wheelbase, ideally slightly ahead of the rear axle, because weight behind the rear axle acts on a lever. It loads the rear axle more than its own mass, unloads the front, and makes the steering feel light exactly when you need it not to. A tank pushed to the back of a cargo area is one of the more common ways a build ends up over its rear GAWR while still under GVWR overall.

Never on the roof. This should be obvious and it still happens, usually because the roof is the only empty space left. Putting the single heaviest item in the build at the highest point in the vehicle is the worst available choice for stability, and on most vehicles it also exceeds the dynamic roof load rating on its own. Dynamic roof load, the limit that applies while the vehicle is moving, is commonly 150 to 220 lb on a standard unibody vehicle, so a full 20 gallon tank at 167 lb has already consumed all of it before the rack, the mounts and anything else. Check your own owner manual, and treat the rack rating and the vehicle roof rating as two separate limits where the lower one governs.

Slosh is the other half of placement. A tank that is neither full nor empty has several hundred pounds of liquid moving inside it, and that mass arrives a fraction of a second after the vehicle changes direction. Baffles are internal walls with openings that let water pass slowly while stopping it moving as a single slug, and they are the reason purpose-built tanks behave better than an improvised container. If your tank is unbaffled, keeping it either near full or near empty is a meaningful improvement over half.

Then secure it as though it were 167 lb of anything else, because that is what it is. A tank restrained only by gravity and friction is a projectile in a collision and a wandering load on a corrugated road. Build a cradle that supports the tank across its whole base rather than on edges, strap it or bolt it to structure rather than to plywood, spread the load with backing plates or spreader washers, and use the same discipline you would apply to a battery. The tray and hold-down logic behind a battery tray and hold-down applies identically here, and the tank is three times heavier.

How do you plumb a 12V demand pump system?

A demand pump system is a pump that sits idle until a tap opens, senses the pressure drop, runs until pressure is restored and then stops. That is what makes it feel like plumbing rather than like camping. It also makes it a 12V motor circuit, and motor circuits are where DIY builds get the wiring wrong.

The parts, in the order water passes through them. Tank outlet, then a strainer, then the pump, then either an accumulator or the pump's own internal bypass, then the runs to the taps. Every joint uses food-grade hose and fittings, which matters because ordinary vinyl tube can impart a taste and is not made to sit against drinking water for years. Half inch is the usual size on a vehicle system and it is what pump ports are threaded for; going smaller strangles flow and going larger buys nothing but volume you have to fill.

A SHURFLO Revolution 4008-101-E65 RV Fresh Water Pump (3.0 GPM, 55 PSI) at $127.05 is the default in RV and van builds for good reasons. It moves 3.0 GPM at 55 PSI, which is more than enough for a sink and an outdoor shower, and it has an internal bypass that stops it hunting on and off at low flow, which is the behaviour that makes a cheap pump unbearable. It draws roughly 7 to 8A while running.

That 8A figure is the entire electrical story, and the duty cycle is a trap. A pump runs for a few seconds when a tap opens, so a whole day of ordinary use is well under 2 amp-hours, which is nothing next to a fridge. It is tempting to conclude that a thin wire will do. It will not. The conductor and the fuse are sized for the current that actually flows while the motor turns, not for the daily total, because a conductor does not average anything. It heats at 8A whenever 8A is passing through it.

Those figures come from Vdrop equals 2 times L times I times R, where L is the one-way run length in feet, I is the current in amps and R is the conductor resistance in ohms per foot. The factor of 2 is there because current travels out on the positive and back on the negative, so the copper in the circuit is twice the distance between the ends. Copper resistance is 0.000999 ohms per foot for 10 AWG and 0.000628 for 8 AWG. The 3 percent target on a 12V system is 0.36V, and a pump counts as a critical circuit because it contains a motor: a motor fed low voltage draws more current to do the same work and runs hotter for it.

Ampacity is the other test and it is not the binding one here. Ten AWG copper with 105C insulation carries 60A outside engine spaces under ABYC ratings, which is many times the pump's 8A, and 12 AWG carries 45A. Run length is what forces the gauge up, not current. Confirm your own run with the wire gauge calculator, which applies both tests at once.

The pump needs its own fuse. Not a share of a bigger one, and not a fuse at the pump end of the wire. The fuse protects the conductor from the source of power, so it goes within a few inches of that source, and it is sized above the running current and at or below the ampacity of the smallest wire it protects. A 15A fuse on 10 AWG satisfies both. Give it a dedicated way on a FASTSTORM 12 Way Blade Fuse Block with LED Indicators (12V) at $16.98 so that a jammed pump or a chafed wire takes out one circuit and not the lights as well, and so that you can isolate the pump for maintenance without pulling the main. The fuse sizing chart gives the fuse for every conductor size, and the wiring and fusing guide carries the full ampacity table and the derating rule for engine spaces.

Mechanically, three details separate a system you forget about from one you keep fixing. Mount the pump on rubber feet or a compliant pad, because a pump bolted hard to a plywood panel turns the panel into a drum and you will hear it in the small hours. Fit the strainer before the pump, always, because grit in the pump valves means it cannot hold pressure, and a pump that cannot hold pressure runs continuously and turns a 2 amp-hour a day system into a 20 amp-hour one overnight. And use flexible hose for the first foot on each side of the pump so vibration does not travel into rigid plumbing and work a fitting loose. Sleeve the wiring in Split Wire Loom Conduit, 3/8 in x 120 ft where it crosses metal, seal the terminations with adhesive lined Adhesive Lined 3:1 Heat Shrink Tubing Kit (400 pieces) , and crimp proper tinned lugs and ring terminals rather than twisting strands under a screw.

An accumulator is a small pressure vessel with an air bladder that stores a cup or so of pressurised water. Its job is to stop the pump starting for every brief draw, which is quieter and slightly kinder to the motor. If your pump has a good internal bypass you can skip it; if it cycles annoyingly at a half-open tap, an accumulator is the fix.

Can you build a water system with no electricity at all?

Yes, and for a lot of builds it is the better answer. Two approaches work.

Gravity feed puts the tank higher than the tap and lets physics do the work. It is completely silent, has no electrical circuit to fuse, cannot fail in a way that floods the vehicle, and adds nothing to the power budget. The cost is flow rate, which is modest, and the fact that the rule about mounting water low is in direct conflict with the rule about mounting it high enough to feed a tap. A small day tank fed by hand from a low main tank is the usual compromise, and it keeps the bulk of the weight where the vehicle wants it.

A foot pump or a hand pump is the other option, and it is quietly excellent. It uses no power, it is nearly unbreakable, it works when the battery is flat, and it is the single most effective water conservation device there is, because water only flows while you are actively pumping. The difference in daily consumption between a foot pump and a demand pump is real and it shows up as days of extra range from the same tank. Plenty of experienced builds fit a foot pump at the galley and a demand pump only for the outdoor shower.

What filtration and treatment does a vehicle water system need?

This is where honesty matters more than product selection, because there are two completely different problems and they are constantly conflated. Making water taste better and making water safe are not the same job, and the equipment for one does not do the other.

Read the second row twice. An activated carbon filter is not a pathogen barrier. It removes chlorine, it improves taste and odour, and it makes treated municipal water pleasant. It does not make questionable water safe, and an inline carbon filter on a tank outlet gives a false sense of security to anyone who assumes otherwise. If water comes from a source you are not confident about, it needs a purifier carrying a published rating against bacteria, protozoa and viruses, or it needs boiling. Those are the options.

The practical layout for a vehicle is to separate the two streams. Fit a sediment filter and, if you like, a carbon stage on the main system, so the whole tank tastes fine and the pump valves stay clear. Then handle drinking water separately, either with a dedicated container filled from a source you trust or with a point-of-use purifier at one tap. That way you can fill the main tank from a wider range of sources without having to trust all of it equally, and you never have to decide whether a whole tank is drinkable.

A plain caution, because this is one of the few places on a gear site where the stakes are health rather than money. Nothing here is medical advice and nothing here is a guarantee of safety. Read the certification on any filter or purifier you buy, note the specific organisms and reduction levels it claims, replace cartridges on the schedule the manufacturer publishes rather than when flow drops, and follow local guidance on the water sources you use. A filter past its rated volume can be worse than no filter, because it is trusted and no longer working.

How do you sanitise the system and stop biofilm?

Biofilm is the slippery layer of microorganisms that forms on any wet surface left alone, and a warm plastic tank of standing water in a vehicle is close to ideal conditions for it. It is the reason a system that was fine in spring tastes wrong after it has sat.

The routine is straightforward. Sanitise the whole system at the start of a travelling season, after any period of storage, and any time the water starts tasting of the tank rather than of water. Fill the tank, add a sanitising solution appropriate for potable systems at the concentration its own label specifies, run it through every tap so the pump, the hoses and the fittings all see it, leave it to dwell for the stated time, then drain and flush repeatedly until there is no smell. Do not improvise a concentration from memory, and do not mix products.

Prevention beats treatment. Keep water moving, because a tank that turns over every few days rarely develops a problem and one that sits for two months reliably does. Store the system dry rather than full. Keep light off the tank, since sunlight through a translucent container feeds algae. And use food-grade hose throughout, because the tank is not the only place biology lives.

This is also the strongest argument for a particular kind of tank, and it is worth weighting heavily when you choose one. A tank you can get a hand inside is worth choosing. An inspection port large enough for a forearm and a cloth means you can physically clean the interior, see whether the last flush worked, and recover a tank that has been neglected. A sealed tank with two threaded ports gives you no such option: chemistry is your only tool, and when chemistry does not fully work your only remaining move is replacement. Ask what the access is before you buy, not after.

How do you winterise a water system?

By emptying it, and the reason is worth stating precisely. A full tank is what splits when it freezes. Water expands as it turns to ice, and that expansion happens inside a closed container with nowhere to go, so the container gives way. The casualties are a cracked tank wall, a split pump head, a fractured plastic fitting or a burst run of hose, and every one of them is discovered as a flood rather than as a crack.

Drain properly rather than approximately. Open the tank drain and every tap including the outdoor shower, run the pump briefly with the taps open to clear the pump head, then drain or blow out the lines so no low point holds a slug of water. A pump head with water still in it is one of the most common freeze failures, because it is small, it is metal and plastic together, and it is easy to forget once the tank is empty.

If you travel in the cold rather than storing for it, the layout matters more than any single part. Keep the tank, the pump and the runs inside the heated envelope wherever possible, insulate any under-floor tank, and remember that a tank frozen solid is not only a plumbing problem: it is several hundred pounds of ice you cannot drink. The winter camping power guide covers cold weather systems as a whole, including the fact that a LiFePO4 battery cannot accept charge below freezing without self-heating or a heat pad, which is the constraint that tends to govern a cold weather build.

What do you do with grey water?

Grey water is the sink and shower water on its way out, and it is a legal and etiquette question at least as much as a plumbing one. Dumping it on the ground is prohibited in a growing number of areas, the rules differ by land manager and jurisdiction, and they have been getting stricter rather than looser. Near any watercourse it is poor practice regardless of what the local rule says, because soap, grease and food residue do not belong in water other people and animals drink from.

Plan for roughly 70 to 90 percent of your fresh usage to come back as grey, since drinking and cooking consume some of the total. It weighs the same 8.34 lb per gallon on the way out as it did on the way in, so a build that carries grey water carries that weight twice on the same trip, which is a payload fact worth putting in the calculator. Fit a container with a real capacity rather than a bucket under the sink you will forget to empty, and know where you intend to dispose of it before you leave.

The cheapest grey water strategy is producing less of it. Wash up in a basin rather than under a running tap, use biodegradable soap sparingly, scrape plates before washing, and take rinsing outdoors where it belongs. A build that halves its grey water halves both the container it needs and the number of times it has to find somewhere legal to empty it.

Is on-board hot water and a shower worth it?

Briefly, because the answer is usually decided by power rather than by desire. Hot water is lovely and it is the single most expensive comfort in a vehicle.

An electric water heater or a heated hose is a large continuous load, frequently 300W or more, which is roughly 23 amp-hours for every hour it runs. Against a 100Ah LiFePO4 bank with 85 usable amp-hours, that is a serious fraction of your storage for one shower. This is why nearly every off-grid build that wants genuine hot water uses a diesel or propane heater instead: the energy comes from fuel, which is far denser than any battery you can afford to carry. Run any electric option through the power consumption calculator before you buy it, because the number usually decides the question on its own.

The cheap version works better than it sounds. An outdoor shower fed from the demand pump costs almost nothing in hardware, uses cold water or water warmed by the sun in a black container, and puts the grey water outside where a container can catch it. A navy shower, meaning wet down, stop the water, soap, then rinse, uses 2 to 3 gallons where an ordinary short shower uses 4 to 8. On a 20 gallon tank that difference is days.

What does a water system add to the build?

Here is the whole thing in dollars, pounds and amps, using real listed prices. The plumbing sundries line is a budget allowance rather than a listed product, because hose, elbows and clamps depend entirely on your layout.

Two things stand out. The first is that the money is modest by the standards of a 12V build: for the price of one mid-range solar panel you get 25 gallons of capacity and a working sink. The second is that the weight is not modest at all. Two hundred and twenty pounds is a passenger, and it is the reason the water decision belongs in the payload weight calculator alongside the battery bank and the tent rather than being treated as a plumbing detail.

Notice the current column too. The daily energy cost of the whole water system is under 2 amp-hours, which is less than a couple of hours of lighting, and yet the peak is 8A. That gap between average and peak is the single most important electrical fact about water in a vehicle, and it is why the pump gets its own fuse and a properly sized conductor rather than whatever wire was left over.

Weight limits, roof load ratings and road legality vary by vehicle, model year and jurisdiction. Payload, GVWR and GAWR figures must be read off your own door jamb sticker, and aftermarket rack or tank mounting ratings are separate limits where the lower one always governs. Everything on this page is researched guidance based on published manufacturer specifications and verified owner reviews, not an engineering sign-off, not a legal opinion and not a substitute for the installation instructions supplied with your tank and pump. On water safety specifically, nothing here is medical advice: read the certification on any filter or purifier you buy, follow local guidance on the sources you use, and treat a carbon filter as a taste device rather than a pathogen barrier.

Where to go next

• Water capacity calculator, for your own gallons, days and reserve.
• Water tank versus jerry cans, the head-to-head with real prices and weights.
• Payload and GVWR guide, where the 167 lb actually lands.
• Payload weight calculator, to check the whole build against the sticker.
• Wiring and fusing guide, for the pump circuit and the full ampacity table.
• Fuse sizing chart, for the fuse that matches your conductor.
• Power consumption calculator, before you consider electric hot water.
• Winter camping power guide, including freeze protection.