Lowtech Graph lists low-tech options for every need of a home. Here you can compare them, find tutorials to build them, and link them together into a circular system. Why low-tech?
Solar power diverter / router (surplus-PV to resistive load)
Passive
One of 4 ways to meet this need. Cost, carbon, footprint and rankings are relative to other solutions for the same need, not absolute.
What your site needs
Pairs with a PV array and a resistive load such as an immersion tank; works grid-tied or off-grid.
A small controller that watches how much solar power is being sent to the grid and diverts the surplus to a water heater instead. Electricity that would have been given away heats water for free. It gets more out of solar panels without a battery, and open-source builds exist.
10-15 years (power electronics); the resistive load (immersion element) follows its own longer cycle.
Embodied carbon
Low
Space / material use
Low
Output timing
Weather/time-dependent
Skill to repair
Intermediate
Regulation (FR example)
Fixed electrical installations must meet NF C 15-100, and a Consuel certificate is required before a new connection is energised. Grid-connected generation requires an Enedis connection agreement.
Levels are relative to the other options for this need, not absolute. Fewer dots is lighter.
Solar power diverter / router (surplus-PV to resistive load) (This page) Can I use it? Works in a flat · No rebuilding needed What does it cost me? Upfront cost: Medium · Skill to build: Intermediate · Upkeep: None · Change to daily life: None · Works without the grid: Yes Choose it if you want your solar surplus to heat water instead of being given away.
Dual-bus wiring (DC essentials + AC general) Can I use it? Works in a flat · No rebuilding needed What does it cost me? Upfront cost: Medium · Skill to build: Intermediate · Upkeep: None · Change to daily life: None · Works without the grid: Yes Choose it if you want lights and the fridge to keep working even if the inverter fails.
Inverter (DC to AC) Can I use it? Works in a flat · No rebuilding needed What does it cost me? Upfront cost: Medium · Skill to build: Specialist · Upkeep: Light · Change to daily life: None · Works without the grid: No Choose it if you want a solar system to run ordinary sockets and appliances.
Direct solar power (battery-free direct-PV supply) Can I use it? Works in a flat · No rebuilding needed What does it cost me? Upfront cost: Low · Skill to build: Intermediate · Upkeep: None · Change to daily life: Daily · Works without the grid: Yes Choose it if some tasks can wait for the sun, such as pumping or heating water, and you want no battery to buy.
We don’t list sizing or quantities, as the yield of each technique depends on the quality of its build. The catalogue compares options but doesn’t dimension them. Regulatory notes are indicative, with a French example, and need to be confirmed locally.