Energy Usage: First 8 Months

September 1, 2013

Are you curious how much energy Midori Haus uses and how it compares with other houses in the area?  The proof is in the utility bill.  When I get the utility bill from Pacific Gas & Electric (PG&E) each month via email I always log in to their site to look at the “My Usage” tab to see how our electricity and natural gas usage compare with similar homes in the area.  Take a look at these screen shots:

Our electriity usage averaged 227kWh per month for the last 6 months (Mar-Aug).  You can see from the graph that our usage (blue line) is pretty low, tracking closely to the efficient homes (green line) in the area.  The house was in construction through January and we started living full time in the house on March 15, 2013 when the blinds for the windows were installed.

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.

 Our natural gas usage averaged 1.8 Therms per month for the last 6 months (Mar-Aug).  Our gas usage (blue line) is just a fraction of similar homes in the area and even lower than the most efficient homes in the area.

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.

You may wonder what homes are compared in these graphs.  PG&E does a good job of giving us an apples-to-apples comparison.  The definition of similar homes for us is single family houses with an average of 1572 sq ft using natural gas heat within 0.6 miles.

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:

 The monthly electricity usage averaged about 147 kWh for the condo for the same period last year (Mar – Aug 2012). Some of the gadgets consuming electricity are the same — laptop computers, stereo, hair dryer, coffee grinder, etc.  The occupant behavior is somewhat similar too.  The difference in occupant behavior is that I spend far less time at Internet cafes now.  Back in 2012 when I was uncomfortable at the condo (I thought it wasn’t warm enough) I packed up my computer and hung out at the local coffee shops.  Now I don’t do that.

 The gas usage at our condo has an interesting story.  In the winter of 2011-2012 we did an experiment of setting the thermostat for our wall furnace really low and wore layers of sweaters to keep warm.  In our mild climate in Santa Cruz if you set your thermostat at 55F in the winter it hardly turns on.  I thought our gas usage would be pretty flat to track with the summer usage pattern.  But it didn’t.  It was lower than similar homes and the curve rather bumpy. The winter usage went up as if we were turning on the wall furnace but we did not.  Our gas usage (blue line) was even higher than efficient similar homes.  What’s going on?  The likely culprit is the water heater.  The gas water heater at the condo was located in a cabinet next to the refrigerator in the kitchen.  It’s in the conditioned space so it had no insulation around the water heater.  This is OK for most times when the kitchen temperature is between 68F and 72F, but not when the kitchen temperature is at 58F.  So the water heater was using more gas in the heater to keep the water warm.  Interesting, isn’t?

 The similar homes for the condo are all apartments or condos with natural gas heat located within 0.9 miles.  Our condo shared 2 walls with our neighbors so 50% of the walls are well-insulated.  But it is definitely not airtight and I was often cold in the winter.  People in cooler climate may laugh when I complain about it being cold in the winter here but I grew up in Hawaii where it’s nice and warm.

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.

Induction Cooktop

January 12, 2011
Back in September we took a kitchen design class at the local community college.  One of the things the instructor, Yvonne, mentioned was to be aware of the “power-sharing” and “boost” capabilities of induction cooktops because we can easily be misled about the true maximum power of a given unit from reading promotional literature.  Apparently the underlying assumption is that an user would never use all of the burners on the cooktop at the maximum capacity all at once.  OK, so we need to be wary of the marketing material and ask the right questions to get the facts straight.

The few times we browsed around the big box stores and asked the staff about the limitations of power sharing and power boost on induction cooktops we felt that we didn’t get any satisfactory answer.  Maybe the retail clerk at big box stores lacked the knowledge or were coached to answer in a certain way.  Maybe we didn’t ask the right questions.  Maybe we were confused.  The reality may be a little bit of all of that.

Well, I think I understand induction cooktop a little better today thanks to the wealth of information at  This site was recommended by Yvonne in the kitchen design class but I haven’t looked at it closely until now.  I happened to come across it recently while researching and collecting data on various kitchen appliances.  Here is one example of confusing induction cooktop specification I found –
  • The cooktop has 4 burners of different sizes (diameter) and power output 
  • The entire unit has power rating of 7.4KW at 240V
  • The 4 burners are specified as 
  1. 10″ @ 2400W/3200W
  2. 8″ @ 2200W/3000W
  3. 7″ @ 1800W/2300W
  4. 6″ @ 1200W/1500W
So I’m interpreting this to mean a 10″ diameter burner has 2400W power output that can be boosted to 3200W and so on.  Then, if you add up the first set of numbers (2400W + 2200W + 1800W + 1200W) it sums up to 7600W or 7.6KW, which is slightly more than the 7.4KW rating for the entire unit.  If you add up the higher power boost numbers (3200W + 3000W + 2300W + 1500W) then it sums up to 10,000W or 10KW and that is 2600W more than the unit is rated for.  It just doesn’t add up.  The manufacturer must have assumed that you will never use all of the burners at the maximum power output at the same time.  Incidentally, one of the reviews on Consumer Report for this particular cooktop mentioned that there was a burner cycling on/off problem and recommended readers to purchase extended warranty. Maybe this person pushed all the 4 burners to the max?
You might be wondering, “So, what do these numbers mean for the Midori Haus project?”   Well, I learned that depending on the power delivery requirements of the cooktop the kitchen may need to be rewired with wires that can carry higher loads and the house panel will need to be upgraded.  To get a really good understanding of this I encourage you to visit Induction Cooking: Kitchen Electricity 101 page at  They explain the nuts and bolts in detail I really like the water flow and water pressure analogy they use in describing basic electricity principles.
You know, I’ve been cooking on gas range for the last 20 years and like cooking with gas.  We know that induction cooktop is efficient and is better for indoor air quality.  So a happy solution is to have an induction cooktop in the kitchen and have a propane gas grill on the deck for the food we want charred.  By the way, induction cooktop delivers way more power than the gas stove.  The conversion factor for a gas burner (measured in BTU/hour) to induction burner (measured in kW) is BTU/hour = kWx 7185.  Using this conversion factor we can see that the l0″ burner in our example above that delivers 2400W or 2.4kW is equivalent to a gas burner that outputs 17,244 BTU, which is way more than what’s required in wok cooking (the most power hungry cooking) of 12,000 BTU.  For more details on the conversion factor please read the section “How Much Power Is What” in
Right now we are leaning towards the 30″ 4 burner induction cooktop from Miele.

Appliances and Kitchen Design

December 6, 2010

One of the things we learned on Friday was that appliances consume the largest portion of the electrical usage in large homes.  The pie chart based on “PG&E Survey 2009” showed that appliances consume 28%, pool 24%, HVAC (heating and cooling) 16%, lighting 15%, electronics 9% and miscellaneous 8%.  Well, we don’t have a large home, we don’t have a pool and our passive house design will greatly reduce heating needs.  Still, getting energy efficient appliances is a priority for us since we do need new appliances and the energy efficient ones have various rebates and tax credits. 

Another thing that I learned (that Kurt already knew) was that 2/3 of electricity generation in U.S. is wasted.  Of the 40.67 quadrillion BTU energy consumed to generate electricity only 13.21 quadrillion BTU, or 32%, is delivered for end use.  Again, this means 68% is wasted.  Isn’t it amazing?   This source is from U.S. Electricity Generation 2008 chart from Energy Information Administration’s Annual Energy Review 2008.  These charts were part of the workbook material used in the “Integrating Energy Efficiency and Renewables in Home Retrofits” class offered by PG&E.  Another free class offered to the public by PG&E.  To search through their class offering please visit here.

So combining these 2 pieces of information we conclude that paying attention to the energy performance of our home appliances will not only save us money on our utility bill it also can lower our carbon footprint by reducing the demand on energy generation.  Saving money and saving the planet is a win-win formula.   

OK, so we will be replacing old appliances with energy efficient ones.  But which brand and model?

We started our appliance research by visiting the Miele Gallery in San Francisco.  Why Miele?  Because that’s what Flora recommended.  Our friend, Flora, is an artist that happens to be a terrific cook and has a beautiful kitchen.  She shared with us her experience of various home remodels (about 6 previous residences) she had done and one of the things she mentioned was to go to the kitchen gallery at the design center in San Francisco and take their classes.   

On Saturday we went to the kitchen design class at the Miele Gallery.  A nice light breakfast was served before the class and Kurt got to ask questions to his heart’s content as 3 presenters covered different topics.  First, Ruth did a nice job of covering the highlights of kitchen design.  Then Rebecca covered the details of Miele appliances.  (She opened our eyes to the steam oven!)  We will be taking the master chef class at Miele in a week to learn more about this and other appliances.  Then Maureen showed examples of tiles and countertops and went into details of the different countertop materials.  Again, this class was free!  

Later we went to an appliance store in San Francisco and learned about a showroom in Brisbane that displays and hold classes for other brands such as Sub-Zero, Wolf, Thermador and Bosch.  The search for appliances will continue….