Off-Grid Sanitation, Incinerating/Composting Toilets & Heat-Pump Water Heater Solar Load Guide
How much solar and battery capacity does it take to run a Cinderella Incinerating Toilet (1,800 Wh per flush!), a Sun-Mar / Nature's Head composting toilet evaporator heater, freeze-proof PEX heat trace cable, and a hybrid heat-pump water heater?
📐 Step-by-Step Off-Grid Engineering & Sizing Rules
Composting Toilet Fan vs Evaporator Heater Math: A 12V DC exhaust fan on a Nature's Head or Separett urine-diverting toilet draws only 1.8 Watts (43 Wh/day—almost nothing!). However, turning on the 120V thermostatic liquid evaporator element inside a Sun-Mar Excel draws 300 Watts intermittently (~3,600 Wh/day)—requiring an extra 1.5 kW of winter solar!
Incinerating Toilets (Cinderella Comfort / Incinolet): Electric incinerating toilets burn 1,200 to 2,000 Wh of battery energy per single use (a family of 3 uses 8 to 10 kWh/day just for the toilet!). Off-grid cabins should either use a propane-fired incinerating toilet (Cinderella Freedom, which uses only 15 Wh of 12V DC per cycle) or a urine-diverting composting toilet.
Smart-Load Water Heating Dump Load: Wire a 50-gallon Heat-Pump Water Heater (Rheem ProTerra / AO Smith Voltex, which uses only 400W in Heat-Pump-Only mode!) to your inverter's Smart Load port so it heats water exclusively when battery SoC exceeds 85% in the afternoon.
Self-Regulating Pipe Heat Trace Warning: 50 feet of 5W/ft freeze-protection pipe heat tape running 24/7 in sub-zero weather burns 250W × 24h = 6,000 Wh/day! Always put pipe heat trace on a 38°F pipe-sensing thermostat wrapped inside closed-cell foam pipe insulation.
⚡ Conductor Sizing, Fusing & Conduit Schedule
Open Full Single-Line Wiring Schematics →- 12V Critical Sanitation Bus: Connect composting toilet exhaust fan and 12V PEX water pump directly to the 48V-to-12V DC fuse block so they remain online even if the 120/240V AC inverter is turned off.
- Afternoon Opportunity Load: Configure Inverter Smart-Load relay ON at 85% SoC / OFF at 75% SoC to feed the Heat-Pump Water Heater.
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Frequently Asked Questions: Off-Grid Sanitation & Hot Water Loads
Can I run an electric tankless water heater on an off-grid solar system?
NEVER install a whole-home electric tankless water heater (18,000W to 27,000W / 75–112 Amps @ 240V!) in an off-grid cabin—it will instantly overload your inverter. Use either a 400W Hybrid Heat-Pump Storage Tank Water Heater or an on-demand propane tankless water heater (which uses only 45W of electricity for ignition and fan).