Composting toilet fan load calculator

Understanding your composting toilet fan's load

The direct answer for a typical fan

A typical 2.5-watt composting toilet fan running continuously adds about 60Wh a day, or roughly 21.9kWh over a year. It's a small individual draw, but running 24 hours a day, every single day, without exception makes it add up more than people expect when they first see the fan's low wattage rating. The quick maintenance cost estimator takes the same inputs and works through the next step.

Why continuous runtime matters more than wattage alone

Unlike most loads that run only part of the day, a composting toilet fan runs continuously to manage odor and moisture, every hour of every day without exception. A 2.5-watt fan uses about 1.7 times more energy over a day than a single 9-watt LED bulb running 4 hours, purely because of the always-on runtime rather than any difference in wattage. This is a useful reminder that small, continuous loads deserve real attention in a load list, not just the obviously large appliances. The DOE's solar photovoltaic technology basics has the current version, so check there before you rely on a summary.

Why turning the fan off isn't a good energy-saving option

The fan's continuous operation is what controls odor and helps the composting process by managing moisture and airflow inside the unit, so turning it off defeats the toilet's core function even though it would save a small amount of daily energy. This is one load where the correct answer is to plan around its continuous draw rather than looking for ways to cut its runtime. EPA's overview of alternative wastewater systems covers composting toilets alongside other off-grid waste options if you're still deciding between systems.

Checking your specific fan rather than assuming

A larger, higher-CFM fan drawing 4 watts instead of 2.5 watts uses about 60% more energy over a day and year, so checking your specific fan's actual wattage rather than assuming a generic figure is worth the extra minute. Composting toilet manufacturers commonly list fan wattage in their product specs, or you can check the fan itself for a printed rating.

What to do with your fan load figure

Count this small continuous draw in your total load list with the rest of your appliances. On its own the number looks trivial, but leaving it out, which is easy to do at such a low wattage, understates the continuous baseline your system actually has to carry. The quick 5-minute system size estimator in our system sizing tools is a fast way to see how much this and other small continuous loads add up to, and the quick maintenance cost estimator in our maintenance and lifespan tools helps budget for the fan's eventual replacement. DOE's solar photovoltaic technology basics is a good starting point if you're new to sizing a system around small continuous draws like this one.