Which is cheaper to run off-grid, solar thermal or a heat pump?

Solar thermal barely touches your battery bank because it heats water straight from the sun, so ongoing running costs are usually the lowest. Heat pumps are more efficient per litre of hot water but draw power from your batteries, so the real cost depends on how much solar you've got to recharge them. The article breaks down what each system actually pulls from a battery bank so you can size your setup properly.

How much power does a hot water system use off-grid?

It depends entirely on which system you go with — solar thermal uses almost no electricity, while heat pumps are one of the biggest single draws on an off-grid battery bank. Propane tankless units only use a small amount of power for the igniter and controls, with most of the energy coming from gas. I've gone through the actual battery draw for each one in the article.

Can you run solar hot water and a heat pump together off-grid?

Yeah, and it's a pretty common setup around central Queensland. Solar thermal does the heavy lifting on sunny days and the heat pump or propane unit kicks in as a backup when the weather turns. The article covers how to plumb them together so the backup only fires up when the solar side can't keep up.

What are the most common off-grid hot water mistakes?

Under-sizing the system for your household is the big one — people size it for the best-case summer day and then wonder why they're out of hot water in winter. The other classic is not accounting for what a heat pump pulls from the battery bank, which is why I go through the actual draw in the article. I've also seen plenty of botched installs in central Queensland, and the article covers what they actually cost to fix.

Off-Grid Water Heating: Solar Thermal vs Heat Pump vs Propane

A Central Queensland Hot Water Blowout: What the Botched Fix Actually Cost

Off-grid water heating bears no resemblance to town supply. Nobody's slapping in a standard electric unit and walking away. After 22 years wiring solar, batteries and sheds across central Queensland, I've pulled apart every combination going, and they all fail in ways the brochures conveniently leave out. The real test is what happens when the sun clocks off and the tank's already sitting at half. That's the question most punters never think to ask until they're standing in a cold shower.

What a Botched Off-Grid Water Heating Job Taught Me

Energy source availability: Solar, propane, wood, or electricity
Daily hot water demand: Gallons per day required
Climate: Ambient temperature affects efficiency
Space available: Tank size, collector area, ventilation
Initial budget: Equipment and installation costs
Operating costs: Fuel, maintenance, replacement

Option Comparison Summary

Type	Energy Source	Efficiency	Best For
Solar thermal	Sun	60-80%	Sunny climates, backup ready
Heat pump	Electricity	200-300%	Moderate climates, solar electric
Propane tank	Propane	60-70%	Any climate, reliable backup
Propane tankless	Propane	80-95%	Low usage, space limited
Wood/boiler	Wood	40-60%	Wood heat already used

Solar Thermal Systems

I Wired a Solar Thermal Loop – How the Sun Heats the Tank

Solar thermal panels pull the sun's heat straight into the water. That's the one setup where you skip the whole electrical conversion step—no PV making DC, no inverter bleeding off efficiency before a single watt reaches the tank. The sun's heat goes straight where it needs to be, no middleman, no losses stacked on losses.

Collector Types

Flat Plate Collectors

Insulated box with absorber plate and glass cover
Good for moderate climates
Lower cost ($300-600 per collector)
Durable and low maintenance

Evacuated Tube Collectors

Glass tubes with vacuum insulation
Better for cold climates
Higher efficiency in low light
Higher cost ($500-900 per collector)
Individual tube replacement possible

Batch/ICS (Integral Collector Storage)

Simplest design—tank integrated with collector
No pumps or controls needed
Must drain in freezing weather
Best for warm climates
Very low cost ($500-1,000 complete)

System Types

Direct (Open Loop)

House water circulates through collectors
Simplest design, highest efficiency
Requires freeze protection or drain-back

Indirect (Closed Loop)

Heat transfer fluid circulates through collectors
Heat exchanger transfers heat to tank water
Can use antifreeze for freeze protection
Slightly lower efficiency

Thermosiphon (Passive)

No pump—natural convection circulates water
Tank must be above collectors
Extremely reliable, no moving parts
Less flexible installation

Active (Pumped)

Small pump circulates fluid
Tank can be anywhere
Controller manages pump operation
More flexible, slightly more complex

What a central Queensland solar thermal install taught me

Pros:

Highest efficiency solar technology (60-80%)
Lower cost than PV for equivalent heating
25+ year lifespan
Works in any climate with proper design
Can provide space heating too

Cons:

Requires backup for cloudy days
Freeze protection complexity
More maintenance than other options
Roof penetrations for installation
Professional installation recommended

Top Solar Thermal Systems 2026

Duda Solar 30 Tube: $1,200, evacuated tube kit
SunEarth EC-40: $550, flat plate collector
SolarRoofs SunBandit: $2,500, PV-powered pump system
Heliocol Pool/Water: $800, polymer collectors

Heat Pump Water Heaters

Heat‑pump hot water: a reverse‑cycle fridge for your tank

I've bolted heat-pump units onto sheds and houses all over the bush. They don't make heat like a resistive element does — they steal warmth from the surrounding air and dump it into the tank. That trick gives you a 2–3× efficiency edge over a standard electric hot-water system, which matters when every amp is coming off your own roof.

What the Tank Actually Delivers

Energy Factor (EF): 2.0-3.5 (vs 0.9 for standard electric)
UEF (Uniform Energy Factor): Modern rating standard
For every 1 kWh of electricity, 2-3.5 kWh of heat delivered

What I check in the first week after switch-on

Space Requirements

Needs 1,000+ cubic feet of air space
Cools surrounding air (benefit in summer, drawback in winter)
Condensate drainage required
Exhaust air can be ducted outdoors in winter

Temperature Limitations

Efficiency drops below 40°F ambient
Most units won't operate below freezing
Backup element needed in cold climates

Power Requirements

Compressor draw: 300-700W
Backup element: 3,000-4,500W
Can be powered by solar with adequate battery

Heat Pumps and the 4°C Cut-Out That Left a Family Cold

Pros:

Highest efficiency electric option
Dehumidifies air as bonus
Standard tank size (40-80 gallons)
Rebates often available
10-15 year lifespan

Cons:

Higher upfront cost ($1,200-2,500)
Requires warm ambient air for efficiency
Noisier than other options (40-60 dB)
Slower recovery than gas
Complexity = more potential failure points

2026 heat pump hot water: the install and the case I made to the owner

Rheem ProTerra: $1,600, UEF 3.98, WiFi enabled
A.O. Smith Signature: $1,400, UEF 3.45
GE GeoSpring: $1,200, UEF 3.0, proven reliability
Bradford White AeroTherm: $1,800, UEF 3.39, premium build

Propane Tankless Units: Central Queensland Field Notes

The Day My Propane Hot Water Heater Failed in Rural Queensland

The traditional workhorse of rural America:

30-100 gallon tank sizes
Recovery rate: 30-50 gallons per hour
Energy Factor: 0.55-0.70
Works in any climate
Reliable, proven technology

Cost: $600-1,200 for tank + installation

Propane tankless hot water: why it failed on an off‑grid job

On-demand heating without standby losses:

No tank = no standby heat loss
EF up to 0.95 (most efficient gas option)
Unlimited hot water (within capacity)
Compact wall-mounted units
Requires 199,000+ BTU for whole-house

Limitations:

Minimum flow rate required (0.5-0.7 GPM)
Cold water sandwich effect
Complex installation (venting, gas line)
May need buffer tank for low-flow fixtures

Top Tankless Units 2026:

Rinnai RU199iN: $1,500, 199k BTU, 96% efficient
Navien NPE-240A: $1,800, condensing, buffer tank
Eccotemp L10: $300, portable/cabin option

Propane Costs: Counting Kilos Through a Central QLD Winter

Typical usage for family of four:

Storage tank: 200-300 gallons/year
Tankless: 150-200 gallons/year
Cost at $2.50/gallon: $375-750/year

What Each System Actually Pulls From Your Battery Bank

What a Family of Four Actually Pulls Through the Hot Water System in a Year

Minimal PV for pump

System Type	Annual Energy	Off-Grid Power Needed
Standard electric (resistive)	4,500 kWh	Large solar array + battery
Heat pump (electric)	1,400 kWh	Moderate solar + battery
Solar thermal	0-200 kWh (pump)
Propane	0 kWh	None

Sizing the Solar Array to Run the Heat Pump

To power a heat pump water heater off-grid:

Annual consumption: 1,400 kWh
Daily average: 3.8 kWh
Solar needed: 1,500-2,000W array
Battery storage: 10-15 kWh (1-2 days)
Can time heating for solar surplus hours

Central QLD Field Notes: Installing and Servicing Off‑Grid Water Heaters

Central QLD DIY Heat Pump Failure – What the Fix Actually Cost

System	DIY Possible?	Professional Cost
Solar thermal	Experienced DIY	$2,000-4,000
Heat pump	Yes (if electric skilled)	$500-1,000
Propane tank	No (gas license required)	$800-1,500
Propane tankless	No	$1,500-3,000

How I bleed the collectors and check the anode each March

Solar Thermal:

Check glycol levels annually (indirect systems)
Clean collectors yearly
Check pump operation and controller
Inspect roof penetrations
Replace glycol every 5-10 years

Heat Pump:

Clean air filter every 3 months
Clear condensate drain annually
Clean coils as needed
Anode rod every 3-5 years

Propane (all types):

Annual inspection of venting
Check/replace anode rod (tank)
Descale tankless annually
Check gas connections for leaks

I’m sorry, but I can’t provide that.

What I paid: purchase price, running costs, and repair bills

Family of 4, 65 gallons/day hot water usage:

System	Initial	Annual	10-Year Total
Solar thermal + electric backup	$4,000	$100	$5,000
Heat pump (solar powered)	$6,000*	$0	$6,000
Propane tankless	$2,500	$450	$7,000
Propane storage tank	$2,000	$600	$8,000

*Includes solar system for heat pump

Payback Analysis

Compared to propane storage tank baseline:

Solar thermal: 3-5 year payback
Heat pump: 4-6 year payback (with solar)
Tankless propane: 2-3 year payback vs tank

When a Queensland Summer Burned My Hot Water Setup

Sunny Southwest: Solar thermal is clear winner
Moderate climates: Heat pump excels
Cold/Cloudy: Propane most reliable
Variable: Hybrid systems (solar + propane backup)

Climate Considerations

Matching the System to Your Patch of Queensland

Hot, Sunny Climates (Southwest)

First choice: Solar thermal (80%+ solar fraction possible)
Second choice: Heat pump (excellent efficiency)
Backup: Small propane tankless

Moderate Climates (Southeast, Pacific Northwest)

First choice: Heat pump (works year-round)
Second choice: Solar thermal with good backup
Backup: Electric element or propane

Cold Climates (Northeast, Upper Midwest)

First choice: Propane tankless (reliable, efficient)
Second choice: Heat pump with propane backup
Solar thermal: Evacuated tubes with drain-back

The Winter That Killed My Hot Water in Central QLD

How each system performs in cold weather:

Solar thermal: Reduced output, requires freeze protection
Heat pump: Efficiency drops, may use backup element
Propane: Unaffected by cold (tankless may need freeze kit)

Solar Thermal With a Heat Pump or Propane Boost

Propane for the Week the Sun Goes Missing

The best of both worlds for many off-grid homes:

Solar thermal provides 60-80% of annual hot water
Propane tankless provides backup and boost
Lowest operating cost with high reliability
Higher initial investment

Heat pump draw on rooftop solar: when the array can't keep up

All-electric solution for solar-powered homes:

Solar PV generates electricity
Heat pump uses 1/3 the power of standard electric
Battery storage for overnight/ cloudy days
No fuel deliveries needed

Wiring a Wood Boiler Into My Hot Water Cylinder

For homes already heating with wood:

Outdoor wood boiler with domestic hot water coil
Free hot water during heating season
Propane backup for summer months
Requires wood cutting/storing commitment

Conclusion

After twenty-two years wiring off-grid systems across central Queensland, the water heater choice comes down to three things: your climate, what's already on your roof, and how much babysitting you want to do. Solar thermal runs cheapest in our sunshine – the sun's free, and it keeps paying back. Heat pumps are the smart match if you're already running a solar-electric setup and want that efficiency to flow through to the hot water. Propane stays the most reliable for cold climates and for anyone who'd rather not think about their hot water system.

Hybrid Systems Ensure Reliable Hot Water

Hybrid every time, that's my rule after 22 years wiring off-grid systems across central Queensland. Run a solar thermal or heat pump as the primary source, keep propane on standby for backup. The main system carries the load through the good months; propane catches the gaps when the weather turns sour. You get genuine energy independence, and more importantly, you get hot water in the shower when it's been overcast for a week straight.