Off-Grid Tiny House Basics (Australia): Power, Water, Waste

Off-grid is a site-specific system, not a package label

An off-grid tiny house must balance electricity demand, seasonal generation, water collection, storage and wastewater treatment at the same time. A package described as “off-grid ready” is useful only when its assumptions match the location, occupants and appliances.

Victorian buyers comparing suppliers can use the off-grid tiny home builder guide for Victoria alongside this systems primer. It separates the builder's factory scope from solar, water, wastewater, approvals and licensed site work.

Start with measured or defensible demand. Then design for the difficult period — cloudy winter weather, a long dry spell, guest turnover or a failed component — rather than an ideal summer day.

General information only: This guide is not electrical, plumbing, wastewater, water-quality or engineering advice. Stand-alone systems can create fire, electric-shock, drinking-water and public-health risks. Use appropriately licensed and accredited designers and installers, obtain required approvals and follow current standards.

The four linked design questions

System	First calculation	Failure plan
Electricity	Daily energy use and peak simultaneous load	Low-state-of-charge response, backup generation and critical-load circuit
Water	Daily demand versus monthly collection and storage	Low-tank trigger, lawful cartage/refill access and demand reduction
Wastewater	Design flow, site-and-soil capacity and approved land application	Alarm, service access, overflow response and reduced use
Building fabric	Climate, orientation, glazing, insulation, shading and ventilation	Safe heating/cooling strategy during extreme weather

A larger battery cannot compensate indefinitely for poor thermal design. A larger rainwater tank cannot collect water that the roof and local rainfall do not supply.

Step 1: make an honest appliance load audit

List every electrical load, including items that do not appear in a styled floor plan.

energy used (kWh) = appliance power (watts) × hours used ÷ 1,000

Load	Information to record
Fridge/freezer	Energy label or measured daily use, plus hot-weather conditions
Hot water	Technology, tank size, heating schedule and recovery demand
Cooking	Induction, oven, microwave, kettle and likely simultaneous use
Heating/cooling	Rated input, expected hours and extreme-day use
Pumps	Water pressure, wastewater, transfer and fire pumps; starting current matters
Ventilation	Bathroom, rangehood and composting-toilet fan; some run continuously
Laundry	Washer cycle and any dryer use
Work and communications	Computers, monitor, modem, mobile booster and security
Other	Lighting, entertainment, tools, vehicle charging and guest appliances

Use the Australian Government Energy Rating Calculator for labelled appliances and a plug-in meter where appropriate. Record both daily energy and maximum simultaneous power. Inverter sizing is governed by peak and surge loads as well as total kWh.

Create separate weekday, weekend and extreme-weather profiles. If the home will be a short-stay rental, include guests using heating, hot water and cooking at the same time.

Step 2: size solar for the location and season

The Australian Government’s solar system sizing guidance says suitable capacity depends on electricity use, sunny roof area, shading, local sunshine and budget. A stand-alone design also needs to cover seasonal demand without the grid as fallback.

Give the designer:

site coordinates and shading information
roof orientation, pitch and usable area
monthly and peak-day load profiles
whether the home is occupied all year
required autonomy during poor weather
acceptable generator runtime
critical loads that must remain powered
any planned future load, such as air-conditioning or an electric vehicle.

The YourHome photovoltaic guide notes that stand-alone systems commonly include panels, inverter/charger, batteries, controller and backup generation, and should be designed for annual and seasonal needs. Do not size from annual-average solar output alone.

Step 3: distinguish battery capacity from usable energy

Battery labels can refer to nominal or usable capacity. The useful amount is affected by permitted depth of discharge, conversion losses, temperature, age and reserve settings.

Ask the designer to show:

nominal and usable kWh
continuous and surge power
expected days of autonomy for your audited critical loads
lowest design state of charge
round-trip and inverter losses used in the model
performance assumptions at site temperature
cycle and throughput warranty limits
replacement, isolation and end-of-life plan.

The Australian Government’s battery guidance recommends checking approved products, usable capacity, efficiency, warranty and installer credentials. Battery siting is constrained by safety requirements; it is not simply spare storage under a bed or stair.

Backup generation is an operating choice

A generator can reduce the amount of battery capacity needed for rare poor-weather periods, but it brings fuel storage, noise, fumes, servicing and start-reliability issues.

Document:

automatic or manual start
fuel type and safe storage
outdoor or correctly ventilated location
charging rate and expected runtime
noise impact on occupants and neighbours
maintenance test schedule
what happens if nobody is present when the system needs backup.

Never run a fuel-burning generator inside the home, a closed shed or another space where exhaust gases can accumulate.

Reduce demand before buying more equipment

The building envelope is part of the power system. Specify insulation, glazing, shading, airtightness and ventilation for the climate. The YourHome climate-design guidance provides Australian climate-specific principles.

Practical demand controls include:

avoid unshaded west-facing glass in hot climates
design useful cross-ventilation without relying on it during smoke or extreme heat
insulate exposed floors and treat thermal bridges
select an appropriately sized efficient heat-pump system
heat water when solar production is available where the design permits
interlock or schedule large loads so they do not all run together
make battery state and tank level visible to occupants.

Gas appliances can reduce electrical load but introduce fuel logistics, combustion safety and ventilation requirements. Treat that as a design trade-off, not a default solution.

Step 4: build a monthly water balance

Rainwater storage depends on roof catchment, local rainfall timing, losses and demand. Annual rainfall alone hides the dry season.

A useful first-pass collection calculation is:

litres collected = roof area (m²) × rainfall (mm) × collection-efficiency factor

For illustration only, 50 mm falling on a 40 m² effective roof with an assumed 0.85 efficiency yields about 1,700 litres. Actual collection is reduced by first flush, splash, evaporation, leaks and tank overflow.

Build the model month by month:

Obtain long-term monthly rainfall data for the nearest representative Bureau of Meteorology station.
Measure the effective connected roof area, not the floor area.
Apply a defensible loss factor.
Subtract realistic daily demand for each occupant and guest.
Carry the tank balance into the next month, capped at physical storage.
Test a drier sequence and the longest plausible no-rain period.
Add fire-fighting water separately where required; it may not be available for household use.

Audit water demand

Record drinking, cooking, showers, basins, toilet flushing if any, washing, cleaning, garden use, evaporative cooling and system backwashing. Use fixture flow rates and actual durations rather than a national per-person guess.

Drinking-water quality needs a management plan

Roof water can collect animal droppings, dust, ash, leaves, chemicals and microbes. The design may need:

suitable roof and gutter materials
overhanging-branch and animal controls
screens and first-flush diversion
a sealed, vermin-resistant tank
sediment management
treatment matched to the hazards and intended use
accessible sampling and maintenance points
a response after bushfire, contamination or long stagnation.

Follow current state health guidance and council conditions. A clear-looking tank is not evidence of safe drinking water.

Step 5: approve wastewater before fixing the site plan

Off-grid does not mean outside public-health rules. The wastewater system must suit the soil, slope, groundwater, flood exposure, setbacks, design flow and available land-application area.

The system may include:

septic and land application
secondary or aerated treatment
an approved composting toilet plus a separate greywater system
connection to an existing lawful system with adequate capacity.

Do not place the house, driveway, tanks and gardens before reserving the wastewater area and its required setbacks. NSW Health says onsite facilities and land application require council involvement; WA Health says every onsite wastewater system requires approval before installation. Start with the composting toilet guide and then obtain local site-and-soil advice.

A worked planning example

Consider a two-person fixed tiny home used year-round, with electric cooking and hot water on an unsewered rural site. A sound brief would:

measure appliance energy and create winter and summer profiles
identify the largest simultaneous and motor-starting loads
model monthly solar output and several low-generation days
set critical circuits and acceptable generator runtime
model rainwater collection using local monthly data and a dry sequence
reserve water for any fire-safety requirement
complete a wastewater site-and-soil assessment
test the design again with visitors and equipment degradation.

This process produces a system size. Starting with “5 kW solar and a 10 kWh battery” produces an assumption.

Quote-comparison checklist

Electricity

load and seasonal-generation report
panel, inverter and usable battery ratings
protection, earthing, switchboard and monitoring
battery location and enclosure
generator, fuel and automatic-start scope
installation credentials, commissioning and warranties
remote support and replacement availability.

Water

effective catchment and monthly balance
tank usable volume, overflow and structural base
pumps, pressure vessels and freeze protection where relevant
filtration/treatment and replacement schedule
low-level alarm and backup supply method.

Wastewater

site-and-soil report and approval
designed daily load and occupancy
treatment unit, land application and alarms
service contract, access and power demand
replacement area and flood response.

Use the questions to ask a builder, land guide and site preparation checklist when comparing tiny home builders or modular builders.

Official sources

Sources were checked on 22 July 2026.

FAQ

What size solar and battery does a tiny house need?

There is no safe universal size. Audit daily energy, peak power and seasonal use, then model local solar conditions, autonomy and backup. Hot water and heating or cooling can dominate the result.

How big should the water tank be?

Model monthly collection and demand using connected roof area and local rainfall, then test a dry sequence. The tank must bridge the gap between rainfall events; annual rainfall totals are insufficient.

Does a composting toilet make the site fully off-grid?

No. Greywater and any liquid or finished material from the toilet still need an approved management path.

Is off-grid cheaper than connecting to services?

Sometimes, but only a site-specific comparison can show it. Compare distributor or utility connection proposals with the full stand-alone installation, maintenance, fuel and replacement costs over the intended ownership period.