Electricity usage was a bit higher in February and March when we had several fans and vaporizers running 24/7 to remove the smell coming from the stains applied to the fiberglass door. Usage was low in April when we were on vacation for 10 days.
We use a gas boiler as a backup heat source for domestic hot water. Primary source of heat for the hot water is the sun. We have a solar thermal system (to be explained in a separate post) that pre-heats the hot water. We also have a natural gas barbecue grill out on the deck. That accounts for the little blip in gas usage in the summer -- 2 Therms in June, 1 Therm in August.
Our house is technically heated by gas because PG&E does not have a category for homes heated by the sun. How is our house heated? When the temperature in the hallway thermostat dips below 68F a pump in the mechanical room turns on to circulate hot water (mostly heated by the sun) through the hydronic coil in the house. The hydronic coil is used to warm up the air in our ventilation system. For a typical house this amount of heat is not enough to make it comfortable. But Midori Haus is a passive house that is super-insulated and super-airtight. So we only need the equivalent energy of half a hairdryer to heat our home in the winter. Should the temperature in the hot water tank dip below 120F then the backup gas boiler turns on, thus the house is technicall heated by natural gas. But as you can see from the graph above this doesn't happen regularly. The higher usage of natural gas in January and February reflects the start-up condition for the hot water heater. The storage tank and the backup gas boiler were installed and turned on in January when it was cold outside and the sunlight duration short. So the gas boiler did bulk of the work to heat up 165 gallons of water to 120F. It's amazing that we only used only about quarter of natural gas that similar homes used in January and the house was comfortable.
The amount of energy reflected in the utility bill is not only for heating the house. A good portion is attributed to appliance choices and our behavior. At Midori Haus we use electricity for cooking, clothes washing, lighting and handful of gadgets plugged into the wall. When we lived in the 1300 sq ft condo we had lots of gas appliances -- wall furnace, standard hotwater heater, stove and oven. So I expected our electricity usage to be a bit higher at Midori Haus than at our condo. And it is a bit higher but not by much. To compare the energy usage between similar seasons I grabbed a few screen shots of the energy usage at our condo in 2012:
It's hard to compare the energy bills of homes with different types of appliances (e.g. gas stove top vs. induction cooktop, gas clothes dryer vs. electric condensing clothes dryer, gas wall furnace vs. hydronic coil). To do an apples-to-apples comparison I took the 6 months data (Mar - Aug 2013) for Midori Haus and the 6 months data for the condo (Mar - Aug 2012) and converted the gas usage (measured in Therms) to equivalent electricity usage (measured in kWh). 1 Therm is equivalent to 29.307 kWh. Plotting this combined usage data on a graph I found the Midori Haus total energy usage to be much less than the total energy we used at the condo. It's the same people with same occupant behavior but living in a different space. This means the dwelling itself is much more energy efficient and the space is much more comfortable. Passive house is amazing.