Setting goals for energy
When we set out to build our home we had two objectives with regard to energy:
- Minimise environmental impact by reducing reliance on energy sources that produce carbon dioxide and other greenhouse gases, and
- Minimise the cost of running the home, in particular to protect us from rising energy costs in the future.
These two objectives led us to adopt the Zero Energy goal, which requires us to generate as much energy onsite via renewables as we will consume in the home.
We're still connected to the grid like any other home, but at certain times of the day generate more energy than we need and sell this via the grid to an electricity retailer. At other times we generate less than we need and buy electricity from the same retailer. To achieve Zero Energy, over the course of a year these must balance out so that we generate as much as we use.
Zero Energy achieved
We achieved Zero Energy in our first year, generating twice as much energy as we used.
We're pleased to announce in the first year of operation the house achieved the Zero Energy goal. In fact, we ended up generating more than double the energy we used in the home.
Zero Energy is measured over a calendar year and in 2013 the house-wide monitoring system recorded all of the energy generated and used. The results are:
- Energy used: 2,361 kWh.
- Energy generated: 5,387 kWh.
The difference between the two is quite substantial, in part because we designed and built the house to be zero energy when we had a family. For most of 2013 it was just the two of us living here, so our energy use was lower than it will be in future.
When the house is full our models show energy consumption will rise by about 40% to 3,217 kWh. That's still less than what the photovoltaic (PV) system will generate, as we wanted to ensure we had surplus capacity. One way we may use that extra energy in future is to charge an electric car.
Minimising energy consumption is key to achieving zero energy, and is heavily influenced during design.
Our design process followed the True Green approach used by eCubed, where Jo is an associate.
True Green is a results-driven framework focused on building performance and spans all stages of a building's lifecycle.
Energy consumption results
Minimising energy consumption is key to achieving zero energy and in a solar-powered home like ours it reduces the system size required. Our consumption of 2,361 kWh in 2013 was around a quarter of the energy used in a typical Auckland home1 and this was achieved in three key areas:
- Eliminating heating. The typical Auckland home uses around 30% of its energy to heat the home. Through passive design and an efficient building envelope we eliminated the need for heating, cutting consumption by 30%.
- Solar water heating. Water heating takes up about another 30% of the typical Auckland home's energy consumption. By using solar water heating for most of our hot water needs we cut our energy consumption by another 25%.
- Energy & water efficient fittings. The remaining 20% of energy savings came from fittings used in the house. These included LED lighting, energy-efficient appliances, and water efficient tapware and showerheads that reduced hot water use and therefore the energy needed to heat water.
The chart below compares our energy profile (using data from September to December2) against that of a typical Auckland home (using data from the BRANZ Household Energy-use Project3).
Yesterday's energy use
Monitoring system ... kWh
SHW control & pump ... kWh
Fridge ... kWh
Lighting ... kWh
Fans & towel rails ... kWh
Plug outlets ... kWh
Rain & greywater pumps ... kWh
Oven & hobs ... kWh
Hot water cylinder
Total4 ... kWh
Some key points about the circuits shown in the chart:
- Monitoring. This is the house-wide monitoring and control system. It records energy and water use, along with thermal performance. In addition, it allows control via a web interface and smartphone app of many areas of the house, e.g., heated towel rails, garage door, lighting.
- SHW control & pump. This is the energy used by the solar water heating system controller (controls when the pump runs) and pump (circulates a heat exchange fluid to the solar collectors on the roof where it is heated before returning it to heat the cylinder).
- Lighting. The low use here compared with a typical home is due to all-LED lighting.
- Pumps. One pump moves water from the rainwater tank into the home. This water is used for washing clothes, flushing toilets, and outside taps. Another pump is used in the greywater system, which we are not yet allowed to use.
- Hot water cylinder & heating. These two items indicate the biggest areas of saving. We use hardly any electricity to power the hot water cylinder as we rely on solar water heating. And we don't need to heat the home.
Energy generation results
The PV system is from SolarCity and plays a key role in the performance of the house.
It has allowed us to generate twice as much energy as we used, contributing to our achieving the Zero Energy goal.
SolarCity also supplied the solar water heating system, which provides most of our water heating needs.
As mentioned above, in 2013 we generated 5,387 kWh via the photovoltaic panels installed on the roof. This was roughly inline with the predicted system performance.
Monthly patterns of energy generation are shown in the chart below, compared with monthly consumption levels. The four data series displayed are:
- Generation. The yellow line indicates energy generated by the PV system each month. The peak month for generation was January with 660 kWh; the lowest was June with 269 kWh.
- Consumption. The blue line indicates monthly consumption of energy (both generated onsite and imported via the grid), with peak demand in June of 250 kWh and lowest demand in January of 129 kWh. The shaded blue area represents energy consumed throughout the year; the yellow area represents surplus energy generated and exported to the grid.
- Generation (predicted). The green line indicates the modelled performance of the PV system prior to installation. In general, it performed slightly better than expected.
- Consumption (predicted). The red line shows our modelled energy consumption. As mentioned above, this is based on when the house will be full in the future. While every month in 2013 saw us generate more than we used, this modelled line shows in future we can expect an energy deficit during winter months.
Grid connection results
Live electricity performance
Consumption ... W
Generation5 ... W
Exporting ... W
Importing6 ... W
Powered by solar ... %
Solar radiation7 ... W/m2
The Zero Energy House is connected to the electricity grid like any other home. At certain times (during times of high demand or inadequate solar radiation, e.g., cloudy days, nighttime) the solar system cannot meet all our electricity needs and we buy some from a retailer like any other household. However, during most days we generate more than we need and use our grid connection to export this surplus and sell it to the same electricity retailer. The grid connection results from 2013 are:
- Generation. As mentioned, the PV system generated 5,387 kWh.
- Consumption. Again, as mentioned, we used 2,361 kWh.
- Export. Of the energy we generated, we exported 4,341 kWh - roughly 80%. We were paid 17.285c for each kWh exported, earning $750.34.
- Import. We imported 1,348 kWh throughout the year, paying 21.866c per kWh and a fixed daily charge of 25.997c. In total, we paid $389.65 for electricity.
- Net grid result. The net result (export less import) was 2,993 kWh exported. The net financial result (export income less import expense) was a profit for the year of $360.69.
The feeds under 'Live Electricity Performance' indicate the real-time grid-connection performance of the house, along with the percentage of current electricity consumption powered directly from the PV system.
PV economic performance
The economic performance of PV in any given year can be determined by comparing the value the system generates to its cost. 2013 economic performance is detailed below; the 'Live PV economic feeds' (NOTE: currently offline while we migrate our feeds to a new service) show economic performance starting at January 1 2014. At the end of each year we will publish annual results and then restart the live economic performance counters.
System value is calculated by adding two figures:
- Savings made on electricity bills. We used 2,361 kWh of electricity throughout 2013. If we hadn't had a PV system on the roof this would have cost us $611.16 in electricity bills. But we did have PV, and our actual electricity bills were $389.65. The savings on our electricity bills achieved by using PV were therefore $611.16 - $389.65 = $221.51.
- Income made selling surplus energy. Without PV, we wouldn't have had any surplus energy to export. By using PV, however, we were able to earn income of $750.34.
The overall system value, therefore, was $221.51 + $750.34 = $971.85.
This then needs to be compared against system cost. The system we chose to install was a relatively expensive, roof-integrated solution. We chose this partly for aesthetic reasons, and partly because we wanted to demonstrate what can be achieved with the latest solar solution design.
A standard system with the same rated output (4.16kWp), however, would achieve the same performance and therefore deliver the same value.
Such systems are available today for around $12,500. Adding that to a 25-year mortgage along with the rest of the housebuild cost would result in additional mortgage payments of $962 a year.
With the first year value and cost being basically equal, this shows a PV system this size on a house with this energy profile can pay for itself from the first year of operation. Then, in subsequent years, as energy prices increase (and assuming feed-in tariffs remain) the annual cost of the system stays the same but the value it delivers (by avoiding increasingly expensive imported electricity) will increase - the system then starts to return a profit.
- Building Research Association New Zealand (BRANZ). (2006). Energy use in New Zealand households: Report on the year 10 analysis for the household energy end-use project (HEEP). p17.
- While we have total house-level energy consumption recorded for all of 2013, some of the circuit-level monitoring devices were not activated until September, hence circuit-level data is only available from then until December.
- BRANZ (2006).
- A minor measurement error on the sub-circuits results in the sum of sub-circuits adding up to ~8% more than the total shown. The total shown is taken directly from a meter and is more accurate than sub-circuit measurements.
- If you're looking at this at night-time, there's a good chance the generation figure will be negative - somewhere between 1-2 watts. This is because, as well as outputting generated power, the PV system draws a small amount during operation. At night, when outputted power is zero it continues to draw this small amount.
- We are having some trouble getting the import feed to update correctly online. When generation is greater than consumption the actual import will always be zero. If it is not showing as zero then this is an error, please ignore it. We are currently working to fix this.
- Radiant heat loss - which is particularly significant on clear nights - cools the dome of the pyranometer. This creates a measurement error that that results in negative readings at night when there is no solar radiation (~3-4 W/m2). If you'd like to find out more, you can read about it on page 39 of this manual.
- As noted in the main text, this is based on a standard system cost. We chose a system that cost more than this for both aesthetic reasons and because we wanted to demonstrate a leading-edge solution. A standard system of the same size would achieve the same performance and therefore generate the same value.