Do solar PV systems export more than they should?

By ECD (Electrical+Comms+Data) Staff
Tuesday, 23 September, 2014

The National Electricity Market wasn’t designed with distributed generation in mind. Indeed, one major design flaw acts to penalise solar power, and does so more in Australia than in practically any other country. 

In the USA, net metering means solar earns the same tariff as the retail electricity rate, up until generation exceeds consumption across a billing cycle. 

In Australia, net metering means if you produce more energy than you consume in a half-hourly period, you’ll receive a pittance for the excess power production. Households may pay 25-35 c/kWh for power consumption but receive 0-8 c/kWh for any excess production in any given half-hour. One would almost guess this is an antisolar conspiracy.

Electricity networks have been claiming that solar owners aren’t paying their way for network use. If all solar production was consumed within the house, then their argument might have some merit. What they ignore is that the network operators are charging solar exports full-fare for electricity that is only transported next door, which is one reason export buy-back rates are so low. It’s like charging $50 to drive the entire length of a 50 km toll road in one direction, while charging $50 to drive only 100 m in the other direction. Even as solar is causing less congestion on the network, some operators are saying we won’t take your excess power, which must be the ultimate own-goal from the utility businesses Australians most love to hate.

Australia’s version of net metering is problematic for solar homeowners as most solar production occurs when the home is unoccupied. Homeowners can maximise their daytime self-consumption of solar production by running pool pumps, dishwashers and washing machines during daylight hours. But the full benefit of doing so is only gained with a house consumption monitor (ideally linked to a weather forecast) that intelligently sequences the whitegoods into operation over the day, and there is a limit to the extent to which householders can shift consumption patterns.

Analysis by SunWiz of 300 solar households shows that export levels are higher than most estimates. The median 1.5 kW system exports 37% of its production, but half of 1.5 kW systems export between 28-45% of their power production. This calculation has been extended to larger systems that are more representative of recent installations. Most systems sold these days are 3 or 5 kW, which have median export volumes of 60% and 74% respectively. These exports are far higher than most would guess, and have significant ramifications.

Solar owners need to be careful to size the system appropriately to their load. Analysis of the same extended data set shows that to keep export below 50% of produced solar energy, a 1.5 kW system is a wise choice if the owner consumes 10 kWh/day (excluding off-peak hot water). If the consumption is 15 kWh/day, a 2-2.5 kW system is likely to keep export below 50%. A 3 kW system is suitable if the consumption is 19 kWh/day, and the chances are more than half of the production will be exported from a 5 kW system, unless the user consumes 40 kWh/day.

Export varies considerably depending on a house’s daytime occupancy. Households with people at home during the day will naturally self-consume more of their solar production and thus export less. This means a quicker payback period, or the ability to opt for a larger system with improved reduction in electricity bills. From extensive analysis, SunWiz has identified five major classes of residential consumption patterns, which are integrated into the company’s PVsell software. These are: double hump -  typical of families with school-age children; evening peak - typical of households without children; high day, higher evenings, low overnight - typical of families with infants and pre-school children; day focus - typical of retirees and work-from-home households; night focus - typical of night-shift workers. 

The problem is that very few solar salespeople account for the shape of the load profile when recommending a system. Sunwiz’s analysis shows that a 3 kW system may export as little as 33% of its energy if there is a high level of daytime occupancy, but could export 85% of its production if there is low daytime consumption. This could mean the difference between a five-year payback and a 10-year payback, so it is crucial to obtain hour-by-hour calculation of solar export in order to ensure a favourable return on investment.

PVsell is produced by SunWiz and provides accurate calculations that are tailored to the customer’s needs. The company’s Big PV Wisdom report provides analysis of thousands of results from solar proposals entered into PVsell.


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