Tag: net zero

Net Zero mission accomplished

One of my major goals for our home renovations has been achieving Net Zero, meaning our house produces as much energy as it consumes. As one might expect, producing as much energy as we consume yields energy bills of less than $0.  Today I consulted my energy meter and extracted the following data points for calendar year 2017:

  • Energy Produced: 9,288 kWh
  • Energy Consumed: 12,396 kWh
  • Energy Consumed by Auto: 3,216 kWh
  • Energy Consumed by Building: 9,180 kWh

For the purposes of measuring building consumption, I subtracted the auto consumption (measured by our chargepoint home charger) from total consumption. Since our building produced 108 kWh more than it consumed, I can claim the Net Zero goal as accomplished.

Our home and EV together consumed 1,500 kWh less in 2017 than 2016. Looking forward, I anticipate a similar reduction in 2018 because we were using space heaters for part of the house in Jan-Apr of 2017. The heat pump for that area is now installed. Also, the concrete basement and downstairs walls aren’t insulated yet. I have the insulation (XPS & poly-iso) standing by and it will be installed before the next heating season.

I don’t expect an substantial efficiency improvements after 2018. All our energy systems are super efficient, our insulation levels are super, and our energy surpluses (say hello to Energy Plus) will be consumed by our current and future EVs. Any further efficiency improvements would have no economic justification.

Reference points

  • Average home energy consumption: 23,000 kWh
  • Average auto energy consumption: 15,000 kWh
  • A super-insulated (aka: ultra efficient) home typically needs 1/5 the energy for HVAC.
  • On average, EVs consume  1/3 the energy of an ICE vehicle.

Solar ROI update

I now have a full year of electric production and consumption measured. I also have the SCL rate updates for 2017 and 2018 so I have updated my solar ROI estimates. The significant change is that the Net Metering benefit has substantially increased due to:

  1. SCL electric rates are higher in Shoreline than Seattle.
  2. The 2017 and 2018 rate increases are 5.6% (estimated at 4%)
  3. An added RSA surcharge of 1.5%
  4. The coldest winter in 32 years
  5. More electricity use than I predicted.
    • I was still insulating deep into the heating season.
    • I guesstimated the kWh it would require to heat a 1955 house with heat pumps.
    • I installed a fast (level 2) charger for our Leaf. We were able to use it more, offsetting gasoline with electricity.
  6. The increased usage is all at the higher 0.14¢ price tier.

Reasons 1-4 weren’t known during my initial estimates. Reasons 5 and 6 were planned but their scale was unknown. I knew I’d be removing all natural gas appliances (furnace, water heater, fireplace) but I hadn’t yet decided whether to install tankless electric or a heat pump water heater. I hadn’t chosen the heat pumps for house heat yet so I didn’t know their HSPF. I also didn’t know how much more we’d be able to use the Leaf.

The net result is that I now estimate a 100% return on the solar array in the 6th year instead of the 8th year.

Notes

  • I did not include the cost of the heat pumps or the heat pump water heater. Those were efficiency upgrades that I’d have done anyway. If I were keeping natural gas, I’d have replaced the old 80% furnace with a 97% modulating furnace and the “well past its expected lifespan” gas water heater with a gas tankless. In both cases the costs are comparable and just like replacing the fridge, the efficiency increases have their own ROI schedule.

Solar 11-month update

On a typical “it rained every single day in April” month, we still managed to skate across the finish line at nearly at Net Zero:

April 2017 household energy
April 2017 household + auto energy budget

Last year I removed all the natural gas appliances and converted everything to electric heat pumps. I sized the 10kW array aiming for Net Zero during the calendar year. That would mean producing enough surplus during the summer to carry us through the winter. It looks like we’re going to miss this year:

2016-2017 household energy budget

Even though we’ll be banking surpluses in May, it won’t close that 7MWh deficit. Our household usage includes over 3MWh of car charging and this last winter was Seattle’s coldest in 32 years. The heat pumps were working overtime to keep the house warm.