Friday, July 12, 2013

Running the house on Prius Power


On Sunday morning June 30th, we experienced our first prolonged power outage since moving in.  A tree came down on one of the power lines around the corner and power was out for about 6 hours. It was time to see the inverter that we bought from Converdant Vehicles to turn our Prius into a backup generator in action.

The inverter works by taking energy from the big hybrid battery in the Prius and converting it to a pure sine wave 240/120 AC current, no different from what we typically get from the power grid.  We parked the car in front of the garage and connected it to the inverter with a cable we had professionally installed into the hybrid battery. We then turned on the car, turned on the inverter and flipped the switch on the generator sub-panel we had installed in the mechanical room that contains our critical circuits. Instantly, our well pump and ventilation system were back up running.

When I turned on the car, I noticed that it was almost out of gas, down to the last bar on the display.  Fortunately, this setup is very efficient. The inverter takes as input the DC current from the hybrid battery. As the hybrid battery loses it's charge, the Prius' gas engine turns on to recharge the hybrid battery. If there is only a small appliance load on the inverter, the gas engine turns on infrequently.  We only had a few hundred watts of power being drawn, so we used little gasoline.

Another cool thing is that the only noise this set up makes is the sound of the Prius idling. We could hear our neighbors' loud gas generators from hundreds of feet away, but most of the time we were pulling electricity from the Prius in silence because there was plenty of charge in the hybrid battery without the car even idling.

Perhaps the most impressive aspect of this setup is that the inverter generates 240/120 split phase pure sine wave AC power. With it, we can operate both 240 volt appliances (well pump, HRV) and 120 volt appliances (fridge, lights, computers). Because it's pure sine wave power, we don't have to worry about the generator frying our computers, TVs, etc. I was able to watch recorded TV on an LED flat panel using a signal from our Windows Media Center PC we use as a DVR while the family took hot showers (complements of the sunny day prior to the outage). All this while the Prius either sat silently or idled in the driveway.

Energy Consumption at Mid-Year

We've made it through half of our first calendar year in the house and have some additional energy usage information to share.  First off, on May 6th, we became net producers of electricity for the calendar year.  From January 1st to June 30th, we produced 6580 kWh of electricity and consumed 5220 kWh, a net surplus of 1360 kWh, which at $0.14/kWh is worth $190.  Our HERS rater estimated that we'd produce about $400 surplus of electricity for the year.  We seem to be on track to realizing that.  The chart below illustrates our experience.

In the beginning of the year we consumed more electricity than we produced.  There were two drivers of this.  One, fewer daytime hours means less electricity produced by the PV panels on the roof.  And two, colder outdoor temperatures means that we need to consume more electricity running the heat pumps.  You can see in the chart above that this trend was reversed in March as evidenced by the orange "Net" line in the chart above starting to slope upwards.

In April, May and June we produced significantly more electricity than we consumed.  We see the same two drivers at work again.  One, more daylight hours means more electricity produced by the PV panels on the roof and two, warmer outdoor temperatures means we consume less electricity because we need to run the heat pumps less frequently.

It'll be interesting to see how often we need to run the heat pumps for cooling in July and August.  We have seen the house heat up pretty quickly to 83 or 84 degrees when it's 95 outside.  Fortunately, there have only been a couple of days like that and as a result, we've hardly used the heat pumps for cooling.  They seem to use 10 to 15 kWh per day or $1.40 to $2.10 at $0.14/kWh.  We've had them on the last couple of days more for dehumidifying than cooling.  It's been so wet and humid around here lately that turning on the heat pumps has been a better option than opening the windows at night for cooling the house.

It'll also be interesting to see how our PV panels perform the rest of the year.  So far they are performing very close to the estimates we received from the company we bought them from, Aegis Solar.  You can see from the chart below that we produced more electricity in April than in June.  This probably has something to do with the 34 degree angle to the horizon of the panels on our roof being better suited to the sun being somewhat lower in the sky.

Stay tuned to see what our stats will be in July and August, the hottest months of the year.