(edited version of article first published in Renew No 148)
It is common practice for sellers of solar systems to ask clients for a copy of their electricity bill. With this information, a solar installer calculates the size of a PV array and inverter that brings your bill close to a net annual zero cost. For most customers this is their goal; recent articles in Renew have featured households whose solar systems have not only provided their home’s power requirements, but also powered a plug-in electric vehicle. This is undoubtedly a trend to be encouraged, but it does lead one to think: just how much bigger does a solar system need to be in order to provide for all of one person’s energy usage and emissions?
Our CO2 footprint goes way beyond our home and our personal transport. It includes everything that you can imagine: roads, hospitals, logistics, food production, goods manufacturing and all federal, state and local government infrastructure, are just a few things that give each of us our current high quality of life. Australia is embarrassingly close to the top of the list for highest CO2 emissions per person on the planet. Several sources suggest average per capita emissions of around 20 tonnes per person per year. As this is an average, many will be well below this, but many are unfortunately well above it.
The amount of CO2 released during the generation of one megawatt-hour (MWh) of energy in Victoria is close to one tonne. This accounts for 21% renewables as part of the energy mix, with 79% coal- and gas-fired generation…In Melbourne, a 1 kW PV array may produce up to 1.3 MWh of electricity in one year (depending on orientation, angle and shading). To offset one person’s 20 tonnes of CO2 by producing 20 MWh of clean electricity in a year, each person needs to install 15 kW of PV. An average home with four people would need 60 kW of PV to offset the carbon footprint that sustains their lifestyle as they enjoy the pleasures of living in Australia.
Rather than using our home electrical energy consumption to match it with a PV array, should we be looking beyond our energy-consuming lives by investing in larger PV systems? Minimising consumption is clearly the best option, but there is an awful lot of consumption on your behalf that you are not getting a say in!
It is clear that a privately owned PV system should be as big as possible, so that your meter’s primary function is for handling energy export. Many private dwellings with a 5 kW export limit also have a 5 kW inverter with only 5 kW of panels or less. In order to export as much energy as possible, an oversized PV array is the way to go. With a 10 kW array on a 5 kW inverter, no more than 12.5% of generation is ‘wasted’ annually due to the 5 kW export limit. This potentially wasted power can be reduced by shifting household loads to when the energy is being generated.
Almost all inverters will function with an oversized array. The inverter typically takes the power that it needs to produce its maximum output. A premium system sizing arrangement for a 5 kW export-limited site is for an 8.2 kW inverter with 10.8 kW of panels. This then allows for solar electricity to be used in the home while the full 5 kW is being exported, with enough energy to be diverted into a home battery or electric vehicle in the future. Such a system would use minimal grid energy.
Customers are often told that an inverter will only function correctly with 133% oversizing of the array, but this may not be a requirement of the inverter. The rebate on solar panels (which helps reduce the cost of new system installs) is capped at 133% of the inverter size. You are able to go well beyond the 133%, for good economic and environmental reasons—but check your inverter warranty as it may be tied to an upper limit. Your best interest is a liveable planet. To achieve this, your energy debt needs to be accounted for, so you need to generate a whole lot more than just your house or electric vehicle uses. Your response to a climate emergency needs to be reduced consumption and increased renewable generation.