How to Size a Hot Water System for Your Caravan

The single biggest driver of hot water system sizing is the number of people who need to shower. A solo traveller has very different needs from a family of four. As a starting point, allow roughly 8–12 litres of hot water per person per shower at a comfortable temperature. For two people showering daily, that's 16–24 litres minimum. For a family of four, you're looking at 32–48 litres per shower cycle. If you're running an instant (tankless) gas or electric system, this matters less — the unit heats water continuously and doesn't run out as long as gas or power is available. If you're using a storage tank system, the tank must hold enough hot water for everyone to shower before the next heating cycle completes.

Shower duration directly multiplies your water and energy consumption. A 3-minute rinse uses roughly 15–24 litres at a flow rate of 5–8L/min. A 7-minute shower uses 35–56 litres. Most caravan and camping hot water systems are sized for 3–5 minute showers — if your household is used to longer showers, factor that in. A useful rule of thumb: multiply your flow rate (L/min) by your average shower duration (minutes) by the number of people showering. That gives you your minimum daily hot water volume. For example: 6L/min × 5 minutes × 2 people = 60 litres per day. This figure drives every other sizing decision — tank capacity, gas bottle size, battery bank, and solar panel output.

Flow rate — measured in litres per minute — determines how strong your shower feels and how quickly your hot water system must heat water. Most instant gas hot water units deliver 5–8L/min, which feels similar to a standard home shower. 12V electric units typically deliver 3–5L/min — adequate for a comfortable shower but noticeably lower pressure. Solar shower bags deliver whatever gravity allows, usually 1–3L/min. When sizing a system, match the flow rate to your water pump output. A 12V water pump rated at 11–13L/min (35–55 PSI) will comfortably supply most portable hot water units. If your pump delivers less than the unit's minimum activation flow rate (typically 1–2L/min), the unit won't ignite or activate. Always check the minimum flow rate specification before buying.

Hot water systems fall into two categories: storage (tank) systems and instant (tankless) systems. Storage systems heat a fixed volume of water and hold it ready — common in residential caravans and motorhomes. Instant systems heat water on demand as it flows through the unit — common in portable camping setups. For caravans with a built-in hot water system, tank sizes typically range from 10 to 22 litres. A 10-litre tank is adequate for one person showering quickly. A 22-litre tank suits two people with moderate shower durations. The limitation of storage systems is recovery time — once the tank is depleted, you wait for it to reheat before the next person can shower. Instant systems eliminate this problem entirely, which is why they're preferred for families and longer trips.

Gas systems are generally well suited to off-grid camping because they heat water without placing a large load on the battery bank. Electric hot-water systems need more careful power planning. Small 12V pumps and controls use relatively little electricity, but actually heating shower water requires substantial energy. Electric water heating is usually done with a storage tank or a 240V heating element rather than a low-wattage 12V instant heater. If you plan to heat water electrically off-grid, make sure your battery bank, inverter and solar system are sized for the heater's actual wattage and daily hot-water demand.

Your 12V water pump is the link between your water tank and your hot water unit, and its output must be matched to the system you choose. Most portable hot water units require a minimum inlet flow of 1–2L/min to activate, and perform best with 5–8L/min of supply pressure. A pump rated at 35–55 PSI (2.4–3.8 bar) and 11–13L/min will comfortably run any portable hot water unit on the market. Undersized pumps — common in basic caravan setups — can cause ignition failures, temperature fluctuations, and premature pump wear. If you're upgrading your hot water system, check your existing pump's rated output first. Pumps are inexpensive to upgrade and a correctly matched pump makes a significant difference to shower quality and system reliability.

Recovery time is the time it takes for a storage hot water system to reheat a depleted tank to operating temperature. For a 10-litre gas storage unit, recovery time is typically 10–15 minutes. For a 22-litre electric storage unit on 12V power, recovery can take 30–60 minutes depending on the element wattage. If you have multiple people showering in sequence, recovery time determines whether the second person gets a hot shower or a cold one. Instant (tankless) systems have zero recovery time — they heat water continuously as long as gas or power is available. For families or groups, instant systems are almost always the better choice. For solo travellers or couples who shower at different times of day, a storage system with adequate tank capacity works perfectly well.

Understanding the energy requirements of different hot water systems helps you plan your gas and battery setup correctly. Instant gas units: 200–280g of LPG per hour of operation; a 4kg bottle gives approximately 14–20 hours of use. 12V electric instant units: 100–150W (8–12.5A) draw; a 5-minute shower uses 8–10Ah. 12V storage units: 60–100W element; a full reheat of a 10-litre tank uses 15–25Ah depending on starting water temperature. 240V storage units: 1,200–2,400W element; fast recovery but requires shore power or a generator. Solar shower bags: zero energy cost but dependent on sunshine and ambient temperature. For off-grid use, gas instant units offer the best balance of performance and energy independence. For solar-heavy setups with large lithium banks, 12V electric units are increasingly practical.

Different hot water systems suit different camping styles. For caravans with a dedicated bathroom, a permanently installed instant gas unit or a 12V/240V dual-input unit is the most practical choice — it integrates with your plumbing and delivers consistent performance. For camper trailers and rooftop tents, a portable instant gas unit that packs away between trips offers the best flexibility. For off-grid properties and extended remote stays, a gas instant unit with a large LPG supply (9kg or 45kg cylinder) provides reliable hot water without dependence on battery charging. For weekend campers and those who prefer simplicity, a portable 12V unit or solar bag may be all that's needed. The key is matching the system to how you actually camp — not buying for a worst-case scenario you'll rarely encounter.

The most common hot water sizing mistakes come down to underestimating demand and ignoring system constraints. Mistake 1: Buying a unit sized for one person when two or more people need to shower. Always size for peak demand, not average use. Mistake 2: Ignoring minimum flow rate requirements. If your pump can't deliver the minimum activation flow, the unit won't work reliably. Mistake 3: Choosing a storage system without accounting for recovery time. If two people need to shower within 20 minutes of each other, a storage system may leave the second person cold. Mistake 4: Underestimating battery draw for 12V electric systems. A 12V hot water unit pulling 12A for daily showers needs to be factored into your battery and solar sizing. Mistake 5: Buying the cheapest unit without checking gas regulator compatibility. Australian LPG regulators use a specific fitting — always confirm compatibility before buying. Mistake 6: Not checking whether the unit is approved for use in enclosed spaces if you plan to use it inside a caravan bathroom.