Off-grid tiny house: water, electricity and the real limits

Self-sufficiency is the dream associated with tiny houses, and it is also the subject on which you read the most approximations. Here is what works, what is expensive, what the regulations in France allow, and where the reasonable balance lies between going off-grid and being connected.

What is an off-grid tiny house?

The term covers three very different levels, which are best distinguished before comparing quotes:

1. Back-up self-sufficiency: the house is connected, but can run for a few days without the grid. This is the most common and least expensive level.
2. Partial self-sufficiency: solar generation and a water reserve, with top-up from the grid or a generator for peaks. This offers the best balance between comfort and price.
3. Complete self-sufficiency: no connections at all. Technically possible, but sizing then drives everything else — panel area, battery capacity, water reserve, wastewater treatment.

A useful point of vocabulary: being self-sufficient does not exempt you from any planning formalities. An installation that should have been authorised remains unauthorised, off-grid or not.

How much electricity can a tiny house generate?

The order of magnitude given by ADEME (the French ecological transition agency) helps put things in perspective: a 25 m² photovoltaic installation, or about 5 kWp, generates 4,500 to 6,500 kWh a year in France, whereas the average electricity consumption of a four-person household, excluding heating, is around 2,500 kWh a year.

In other words, everyday uses — lighting, fridge, computer, ventilation, small appliances — are easily covered by a much smaller area. What makes demand soar are thermal uses: electric heating, water heater, hob, tumble dryer.

Two constraints specific to tiny houses come on top of this. The available roof area is limited, and solar generation is at its lowest in December and January, precisely when the need for heating is at its highest. It is this seasonal mismatch, not peak power, that makes complete self-sufficiency difficult.

Do you need batteries, and which ones?

Without a grid connection, storage is essential: solar panels generate during the day, while most consumption is in the morning and evening. Three points determine the cost:

• Usable capacity, not nominal capacity: a battery cannot be fully discharged without being damaged.
• The number of days of autonomy targeted: going from two to five days without sun multiplies the size of the battery bank, and therefore the budget and the weight.
• Lifespan, counted in cycles: this is the item you will have to replace after a few years, and it belongs in the running costs.

Weight is a parameter not to forget on a house on a trailer: batteries and tanks eat into the payload. See our guide on transport, licences and trailers.

Water: the real sticking point

You can generate your own electricity; you cannot generate your own water. There are three solutions, none of them perfect:

• A reserve filled by top-ups: simple, but it has to be filled, and a household quickly uses several hundred litres a week.
• A borehole or well: must be declared to the town hall (mairie), with tests if the water is intended for human consumption.
• Rainwater harvesting: regulated. The decree of 12 July 2024 specifies the permitted uses for water unfit for human consumption. Rainwater does not replace a drinking water supply.

In practice, the most liveable combination is a drinking water supply for drinking and hygiene, supplemented by rainwater harvesting for the uses that allow it.

What about wastewater?

This is the constraint people discover last, and it is often the one that decides the location. There are two flows to treat separately:

1. Greywater (sink, shower, washing machine): it cannot be discharged onto the ground without treatment. Where there is no mains sewer, the installation falls under off-mains drainage (assainissement non collectif) and is therefore inspected by the SPANC (the public off-mains drainage service).
2. Blackwater (toilets): eliminated if you choose dry toilets; otherwise it requires a complete treatment system.

Your municipality’s SPANC is the body to consult before you finalise the project, not after. The drainage characteristics of our houses are set out in the guide on utility connections.

Are dry toilets allowed?

Yes, under certain conditions. The order of 7 September 2009 on the technical requirements for off-mains drainage regulates waterless toilets: they must not cause any nuisance or pollution, the by-products must be composted on the plot, and composting must take place under cover from the weather on an impermeable surface.

An important point that is often forgotten: removing flush toilets does not remove the obligation to treat greywater, which remains domestic wastewater.

And off-grid heating?

This is where off-grid electricity reaches its limit. There are two consistent answers:

• A wood-burning stove: independent of the grid and effective in a small volume, but it requires a compliant flue, a supply of dry wood and regular chimney sweeping.
• An air-to-air heat pump powered by solar in spring and autumn, with back-up in winter.

In every case, insulation is the first investment in self-sufficiency: every kilowatt-hour you do not use is a kilowatt-hour you did not have to generate or store. See our guides on insulation and heating.

How much does going off-grid cost?

There is no single price, because there is no standard level of self-sufficiency: it all depends on the number of occupants, the thermal uses and the number of sunless days you want to cover. The right approach is to ask for a costing based on three scenarios — back-up, partial, complete — rather than a flat “off-grid” package.

For the same budget, money spent on insulation and ventilation goes further than money spent on extra batteries. The budget items for a complete project are on our prices page.

Key points

• Three levels of self-sufficiency, three budgets: back-up, partial, complete.
• Solar easily covers everyday uses, but struggles with heating in winter.
• You cannot produce drinking water: rainwater harvesting is a regulated supplement.
• Greywater must be treated, even with dry toilets.
• Going off-grid does not exempt you from any planning authorisation.