Two years of operating a solar+ home: Does it work?

This article is a follow-on to Clean Power Research’s 11-part series on Solar+ homes series. Solar+ homes describes how homeowners can combine simple energy efficiency measures, solar PV, electric vehicles (EVs) and appliance electrification to create households that are fully powered by solar electricity—even in existing homes. A Solar+ home provides substantial economic and environmental savings to the homeowner and creates load profiles that are low-cost for utilities to serve. This article will answer the question, does a solar home work?

Testing a prototype solar+ home

Clean Power Research implemented the Solar+ home technologies in a prototype house in order to validate results. The implementation included detailed, circuit-level load monitoring and temperature monitoring. The team completed installation of monitoring equipment by June 1, 2014. Two years have passed since the test began.

Observations after two years

Figure 2 summarizes annual energy consumption and PV production for the first and second years of operating the prototype Solar+ home. The data in Figure 2 is grouped as follows: Other Electricity, Water Heater, Space Heater, EV (Home and Away) and PV. The following sections discuss each of these groupings.

Other electricity

The first bar in the consumption portion of Figure 2 is Other Electricity consumption. Other Electricity consumption was 2,991 kWh and 2,645 kWh in the first and second years, respectively. This translates to an 11% reduction in the second year.¹

The Figure 3a presents the components of Other Electricity consumption and compares results by year.² The figure shows that Baseload, Lights and Misc., and Refrigerator consumption were consistent from year to year. The major change was due to a 30% reduction in Cooking and Laundry consumption. The Figure 3b presents the Cooking and Laundry sub-components. This figure suggests that the reduction is fairly consistent across the Cooking and Laundry sub-components. The reason for the reduction is due to a change in household size.

Water heating

The second bar in the consumption portion of Figure 2 is Water Heater consumption. Water heating consumption was 766 kWh and 592 kWh in the first and second years, respectively. This corresponds to a 23% reduction.

The prior blog series describes how the performance of the heat pump water heater was impressive compared to results predicted by EnergyGuide (1,400 kWh). Part of the second year reduction was due to a change in occupancy. Part of the reduction was due to optimizing water heater operation.

Space heating

The third bar in the consumption portion of Figure 2 is Space Heater consumption. Baseboard electric heating was the source of heat. Consumption was 1,766 kWh and 3,886 kWh in the first and second years, respectively. This corresponds to a 120% increase. This increase is concerning as it represents more than a doubling of energy consumption for space heating. What is the explanation for the extra 2,120 kWh of consumption?

Two conditions that can cause an increase in heating fuel consumption are:

1. A larger difference between average indoor and outdoor temperature.
2. A reduction in the solar radiation.

Figure 4 presents seasonally-averaged, measured indoor and outdoor temperatures from November 1 to March 30. The 2015-2016 season required an average of 4.4˚ F more of heating than the 2014-2015 season (i.e., 4.4˚ F = 16.6˚ F – 12.2˚ F). In addition, the solar resource was 9 Watts/m² lower in the second year.³

Several conclusions can be drawn:

1. The house performed as expected.
2. The change in consumption is due to weather conditions (plus a slightly warmer house).
3. It’s important to pay attention to weather variability when selecting Solar+ home investments. In particular, a heat pump space heater may be more appropriate than electric resistance heating to accommodate weather variability, particularly since the increased fuel consumption occurs during the winter when PV production is at its lowest.

EV

The fourth and fifth bars in the portion of Figure 2 correspond to EV fuel consumption, including charging at home and away from home. Figure 5 presents the details of EV consumption.

The most important thing to notice in Figure 5 is that home charging is reduced by 30% and away charging is reduced by 65%. This is a concerning finding. EV consumption is approximately proportional to the number of miles driven.

The occupant provided an explanation for the reduction. The EPA listed range of the EV (75 miles) was a deterrent to using the vehicle on longer trips. This was true even when EV charging was available. The occupant was willing to encounter charging inconvenience during the first year. This willingness was reduced in the second year even though the car had the ability to use fast charging.

PV production

The PV system produced 8,196 kWh and 7,846 kWh in the first and second years, respectively. Thus, it produced 4% less energy in the second year. The cause of the reduction was investigated by examining historical solar energy data from SolarAnywhere®. It was found that the period of June 1, 2015 to May 31 2016 had 3% less energy than the previous year. Thus, the explanation is that the reduction in PV production was attributed to a reduced solar resource. The conclusion is that the PV system performed as expected.

Key takeaways

• The prototype Solar+ home performed as expected over the two-year period.
• Changing the number of occupants can reduce Other Electricity and Water Heater usage, although the change may be small relative to total consumption.
• Variable weather conditions and changes in thermostat settings can have a major impact on HVAC consumption, particularly when the home is efficient. These issues need to be taken into consideration at the time of equipment selection.
• EV consumption was noticeably reduced in the second year. The primary reason was that the occupants reduced their usage of inconvenient, non-home, charging networks. This suggests that EV range is critical if policy makers have the goal that consumers travel most of their personal vehicle miles using EVs. Extensive non-home, non-work EV charging networks that inconvenience users compared to gas vehicles are not likely to compensate for limited EV ranges.

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¹ The house does not require any air conditioning because all cooling is provided using nighttime ventilation (see previous blog post for more details).
²The split between Baseload and Lights & Misc. is estimated for each year.
³ The average global horizontal irradiance in the first and second years during the heating season was 124 Watts/m² and 115 Watts/m² correspondingly (source www.solaranywhere.com).