Solar PV Sizing Calculator

Solar PV System Design Guide for AS/NZS 4777.2 and 5033

Sizing a grid-connected solar PV system involves balancing the customer's energy consumption, available roof space, inverter capacity, and local network export limits. A correctly sized system maximises self-consumption (using solar power on-site rather than exporting it at a lower feed-in tariff), minimises payback period, and complies with AS/NZS 4777.2:2020 for inverter energy systems and AS/NZS 5033:2021 for the DC array installation. This calculator takes the site's daily energy usage, location, roof orientation, and panel specifications to determine the optimal array size, inverter capacity, expected annual yield, and estimated payback period. It also checks DC string voltage limits and inverter oversizing ratios to flag compliance issues before installation begins.

Key concepts

• Peak sun hours (PSH) and location. PSH is the number of hours per day that solar irradiance averages 1 kW per square metre. It varies significantly across Australia: southern cities like Melbourne and Adelaide average 3.5 to 4.5 hours, while Darwin and Cairns average 5.0 to 6.0 hours. The calculator uses PSH to convert the array's rated capacity (kWp) into expected daily energy output (kWh). Choosing the wrong PSH value is one of the most common sizing errors.
• DC to AC ratio (oversizing). AS/NZS 4777.2:2020 allows the DC array to be up to 1.33 times the inverter's rated AC output. This is called oversizing or clipping. Oversizing improves energy harvest in the morning and evening when irradiance is below peak, at the cost of minor clipping losses during the midday peak. It is standard practice in Australian residential systems because panels rarely reach their Standard Test Condition rating in real-world conditions.
• String voltage and temperature effects. Panel open-circuit voltage (Voc) increases as ambient temperature drops. AS/NZS 5033:2021 requires that the maximum system voltage be calculated at the coldest expected temperature, not at STC (25 degrees Celsius). In southern Australia where winter mornings can reach 0 to 5 degrees Celsius, a string of panels that is within the inverter's MPPT range at 25 degrees may exceed the maximum input voltage at 0 degrees. The calculator applies the panel's temperature coefficient to compute worst-case Voc.
• Export limits and DNSP rules. Most Australian DNSPs cap single-phase export at 5 kW and three-phase export at 10 kW at the point of connection. Systems larger than the export limit can still be installed but must use export limiting (zero export or dynamic export control) configured at the inverter. The calculator includes export limit configuration so you can size a larger array for self-consumption without violating network connection conditions.

Common scenarios

• Sizing a residential system for a family in Adelaide. A household using 25 kWh per day in Adelaide (PSH approximately 4.2) needs a minimum array of about 6 kW to offset their consumption. After applying a 30% oversize allowance for system losses (soiling, temperature derating, inverter efficiency, cable losses), the recommended array is approximately 7.8 kWp. The electrician enters the consumption, location, and roof details, and the calculator determines the panel count, inverter size, and expected payback period based on current consumption and feed-in tariff rates.
• Checking string voltage for a commercial rooftop in Melbourne. A commercial installation uses 15 panels per string, each with a Voc of 41.2 V and a temperature coefficient of -0.27% per degree Celsius. At STC the string Voc is 618 V, but at a winter morning temperature of 2 degrees Celsius the Voc rises to approximately 644 V. The calculator checks this against the inverter's maximum input voltage (typically 600 V for residential or 1000 V for commercial inverters) and flags a compliance issue if the string exceeds the limit, prompting the installer to reduce the string length.
• Evaluating payback for a system with low self-consumption. A homeowner who works during the day and uses most electricity in the evening will have a self-consumption ratio of only 20 to 30% without a battery. Most of the solar generation is exported at 5 to 8 cents per kWh rather than offsetting grid consumption at 30 to 35 cents per kWh. The calculator shows how this low self-consumption ratio extends the payback period compared to a household with daytime loads, helping the customer make an informed decision about system size and whether to add battery storage.

System sizing components

• Daily energy consumption (kWh): All on-site loads to be offset by solar generation.
• Peak sun hours (PSH): Australia typically 3.5 to 6.0 hours per day depending on location and season.
• Roof area and orientation: North-facing is optimal in the southern hemisphere; east and west orientations reduce output by 10 to 20%.
• Roof tilt: Optimal angle is approximately equal to the site latitude (for example, 35 degrees for Melbourne).
• System losses: Soiling 2 to 3%, cabling 2 to 3%, temperature 5 to 7%, inverter 4 to 5%.

Key compliance requirements

• Export limiting: Single-phase typically 5 kW, three-phase 10 kW (AS/NZS 4777.2 Cl. 4.2).
• Inverter sizing: Maximum 1.33 times DC array size.
• DC string voltage: Typically 200 to 600 V residential (AS/NZS 5033 Cl. 4.2).
• Anti-islanding: All grid-connected inverters require automatic disconnection on grid failure.

Performance metrics

• Specific yield: 1,000 to 1,400 kWh/kWp/year typical in Australia.
• Self-consumption: Percentage of generation used on-site. Higher self-consumption delivers better return on investment without batteries.
• Payback period: Estimated using approximately $0.30/kWh consumption tariff and approximately $0.05/kWh feed-in tariff (typical current Australian tariffs).