What a tankless electric water heater does in an RV

A tankless electric water heater heats water on demand instead of storing hot water in a tank. When you turn on a faucet or shower, cold water flows through heating coils powered by your RV's electrical system, and you get hot water within seconds. The moment you turn the water off, the heating stops — nothing sits warming in a tank while you're driving or parked.

This matters for RVs because space is tight and weight counts. A traditional tank water heater takes up cabinet space, adds weight to your rig, and keeps running to maintain temperature even when nobody needs hot water. A tankless unit is compact, lighter, and only draws power when someone actually uses hot water.

Key Takeaways

  • Tankless electric water heaters heat water when ready when you turn on a tap, using your RV's 12-volt or 120-volt electrical system depending on the model.
  • These units save space and weight compared to tank models, and they stop using power the moment you stop using hot water.
  • The main trade-off is that they draw significant electrical power, so you need either a large battery bank, a generator, or shore power to run them reliably.
  • Flow rate matters — a unit rated for 2 gallons per minute may not heat water fast enough if two people shower at once or you run a shower and sink simultaneously.
  • Installation requires running 12-gauge or 10-gauge electrical wire from your battery or converter, and some models need plumbing modifications to your existing water lines.

How much electrical power a tankless unit actually needs

Most RV tankless electric water heaters draw between 20 and 40 amps at 120 volts, or between 100 and 200 amps at 12 volts. The 12-volt models are more common in RVs because they run directly off your house battery, but they demand a lot of power — a 150-amp draw for 10 minutes of hot water use pulls 25 amp-hours from your battery, which is significant if you're boondocking without shore power or a generator.

If you have shore power at a campground, a 120-volt model is more practical because it draws from the pedestal hookup rather than your battery. If you're off-grid or moving between spots frequently, you'll need either a large lithium battery bank (at least 400 amp-hours), a generator running while you shower, or a hybrid setup where you use propane for most heating and electric only as backup.

Check your RV's electrical panel and battery capacity before buying. If your current system has a 100-amp-hour lead-acid battery and no generator, a tankless electric heater will drain your battery faster than you can replace the charge while dry camping.

Flow rate and temperature rise — why specs matter

A tankless heater's flow rate is how many gallons per minute it can heat. Most RV models are rated between 1.5 and 3 gallons per minute. A 2-gallon-per-minute unit sounds fine until you realize a typical shower head flows at 2.5 gallons per minute — so the heater can't keep up, and your water comes out lukewarm.

The temperature rise is how many degrees the unit can raise incoming water. If your fresh water tank is 50 degrees and you want 110-degree shower water, you need a 60-degree rise. A unit rated for only a 40-degree rise won't get you there, especially in winter when tank water is colder. Check the spec sheet for the temperature rise at your expected flow rate — manufacturers often list different rises at 1.5 GPM versus 2.5 GPM.

For a single shower or sink, a 2-GPM unit with a 50-degree rise works. For simultaneous use (shower plus sink), you need at least 3 GPM or you'll run out of hot water mid-shower.

12-volt versus 120-volt models and where each makes sense

A 12-volt tankless heater connects directly to your house battery and runs whenever you have battery power. It's simpler to install because you don't need a separate 120-volt circuit, but it drains your battery quickly. These work best if you have a large lithium battery bank, a generator you run during showers, or shore power most of the time (in which case you'd run it on 120 volts instead).

A 120-volt tankless heater plugs into your RV's 120-volt converter or runs off shore power. It draws less current than a 12-volt model doing the same job, so it's gentler on your battery. If you're stationary at a campground with a 30-amp or 50-amp hookup, a 120-volt unit is the practical choice. If you boondock, you'll need a generator running to use it.

Some RVers install both — a small 12-volt unit for quick hand-washing when boondocking, and a larger 120-volt unit for showers when they have shore power. This hybrid approach gives you hot water in both situations without oversizing your battery or running a generator constantly.

Installation basics and what you need to know beforehand

Installing a tankless electric heater means running heavy-gauge electrical wire from your battery or 120-volt panel to the unit, installing a breaker or fuse rated for the heater's amperage, and connecting the water inlet and outlet to your existing plumbing. For a 12-volt unit drawing 150 amps, you need at least 2-gauge wire and a 150-amp breaker. For a 120-volt unit, the wire size depends on the amp draw and distance from your panel, but typically 10-gauge wire and a 30-amp breaker.

You'll also need to decide whether to replace your existing tank heater or install the tankless unit alongside it. Replacing it saves space but requires disconnecting the old unit's gas line (if it's propane) and removing the tank itself. Running both lets you use propane for everyday heating and electric as backup, but takes up more room.

Before you start, verify that your RV's fresh water tank, pump, and plumbing can handle the unit's flow rate. Some older RVs have low-pressure systems that won't work with tankless heaters. Check your water pump's output and your tank's capacity — if your pump only delivers 1 gallon per minute, a 3-GPM heater won't help you.

Common problems and what causes them

The most frequent issue is insufficient hot water during simultaneous use — someone showers while another person runs the sink, and the heater can't keep up. This happens when the unit's flow rate is too low or the temperature rise is too small for your incoming water temperature. Solution: upgrade to a higher-flow model or reduce simultaneous demand by asking people to shower one at a time.

Battery drain is the second major problem, especially with 12-volt units. If you're boondocking and using hot water for showers, dishes, and laundry, a standard RV battery depletes within hours. Solution: add a second battery, switch to a 120-volt model with a generator, or use propane for most heating and reserve electric for emergencies.

Water pressure fluctuations cause the third common complaint — the heater cycles on and off, making the water temperature jump between hot and cold. This usually means your water pump is struggling or your incoming pressure is too low. Check that your pump is functioning and your fresh water tank has adequate pressure.

Propane versus electric — when each makes sense

A propane water heater heats water continuously and doesn't depend on your battery or shore power, so it's reliable for boondocking. It takes up more space, adds weight, and costs more upfront. An electric heater saves space and weight, costs less initially, but requires either a large battery, a generator, or shore power to function.

If you boondock frequently and want hot water without running a generator, propane is the better choice. If you stay at campgrounds with hookups most of the time, electric is simpler and cheaper. Many RVers keep their propane heater and add a small electric unit for situations where they want to avoid running the furnace or generator.

Hybrid systems — propane as primary, electric as backup — give you flexibility. You use propane for routine showers and dishes, and switch to electric only when you're at shore power or running the generator anyway. This spreads the load and extends the life of both systems.

Frequently Asked Questions

Can I run a tankless electric heater on battery power alone while boondocking?

Only if your battery bank is very large — at least 400 amp-hours of lithium, or 600+ amp-hours of lead-acid. A single 10-minute shower on a 12-volt unit can draw 25 to 40 amp-hours. Most RVers with standard batteries run a generator during showers or use propane instead.

What size wire and breaker do I need for a 12-volt tankless heater?

A 150-amp unit needs 2-gauge wire and a 150-amp breaker. A 100-amp unit needs 4-gauge wire and a 100-amp breaker. The wire must run directly from your battery to the heater with no other loads on that circuit. Undersized wire causes voltage drop and overheating — don't guess on this part.

Will a tankless heater work if my RV's water pump is weak?

No. If your pump delivers less than 1.5 gallons per minute, a tankless heater won't function properly. Check your pump's specs first. If it's underpowered, upgrade the pump before installing the heater.

Can I use a tankless electric heater with a small generator?

A 120-volt tankless heater drawing 30 amps needs a generator rated for at least 4,000 watts continuous. Smaller generators (2,000 to 3,000 watts) can't sustain that load. Check your generator's continuous wattage rating, not just peak output.

Should I replace my propane heater or keep both?

Keeping both gives you options — propane for boondocking, electric when you have shore power. Replacing propane saves space and weight but means you depend on electricity for all hot water. The choice depends on how often you boondock and how much space you can spare.