Understanding How Solar Power Is Measured: MWp, MWac and MWh

7th June 20

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Solar power systems are cost-effective at all scales, and this versatility is possible thanks to the modular design of solar panels. A residential installation may use 20-30 individual panels, while a large industrial installation will use thousands. However, the basic building block is the same in both cases. Several measurement units are used to describe solar power systems, depending on what aspect of the project is being described.

When discussing large solar power projects, their installed capacity is described in terms of megawatts-peak (MWp) and megawatts of AC power (MWac). When describing the amount of electricity produced over a period of time, the units commonly used are megawatt-hours (MWh) and gigawatt-hours (GWh), where 1 GWh = 1,000 MWh.

Residential and small commercial installations have smaller capacities, and describing them in megawatts is not practical, since the unit is too large. In these cases, the most common units are kilowatts-peak (kWp), kilowatts of AC power (kWac) and kilowatt-hours (kWh).

What Is a Megawatt-Peak?

Since the available sunshine varies throughout the day, the output of solar power systems is constantly changing. The hours around noon are normally the most productive, since the sunshine per square meter reaches its highest value. Megawatts-peak or MWp are used to describe the rated power output of solar power systems, which would be achieved under ideal conditions.

If you add the capacity of all the solar panels in a photovoltaic system, the result is the installed capacity in megawatts-peak (MWp):

  • Assume an industrial facility uses 20,000 panels, each with a rated power of 350 watts.
  • In this case the total power is 7,000,000 watts-peak, or 7 MWp.

This would be the power output if the entire solar array operated under Standard Test Conditions (STC) used by solar panel manufacturers. These conditions assume a photovoltaic cell temperature of 25°C, a solar irradiance of 1,000 watts per square metre, and the incidence angle of sunlight for a latitude of 35° north during summer. These are idealized conditions, but they allow standardized testing for all solar panels.

What Is a Megawatt of AC Power?

As mentioned above, solar panels only operate at rated wattage under Standard Test Conditions. Also consider that solar panels produce direct current, while appliances and equipment in homes and businesses run with alternating current. In solar power systems, the DC to AC conversion is accomplished by inverters. Residential and small business installations normally have a single inverter, but large systems use multiple inverters to handle the total generation.

Since solar panels operate below their rated power most of the time, inverters are normally sized smaller than the solar array. For example, a 12-MWp solar array may use inverters with a total AC power of 10 MW, or 10 MWac. This relationship is normally described with the DC-to-AC ratio or DC load ratio, which is 1.2 in this case.

Using inverters equivalent to the solar array capacity is a waste of capital, since they would operate at partial load nearly all the time. A more economic option is sizing the inverter smaller than the photovoltaic array, and clipping any DC power in excess of the inverter capacity.

The media often uses megawatts alone (MW) to describe solar power systems, without specifying MWp or MWac. However, in most cases this refers to MWp, or the total installed capacity of solar panels according to Standard Test Conditions.

Measuring Energy Output: kWh, MWh and GWh

The units MWp and MWac describe the installed capacity of a solar array and its inverters. However, they don’t describe how much electricity is produced in a specified period of time. This quantity is described in kilowatt-hours, megawatt-hours or gigawatt-hours, depending on the project scale.

  • 1 MWh is 1,000 kWh, and 1 GWh is 1,000 MWh.
  • 1 kWh is the output of a 1 kW system operating steadily during 1 hour.

The capacity in MWp gives an idea of how much a solar power system will cost upfront. On the other hand, the electricity output allows an estimate of the potential savings.

In Australia, large-scale solar power systems now cost less than $1,000,000 per MWp. In sites with good conditions, one MWp of installed capacity can have a yearly output of over 1,500 MWh. If the consumer is currently paying $200/MWh to the local power company, the savings are $300,000/year. This means the payback period can be around three years or less, which is great considering that high-quality solar panels have a service life of over 25 years. Australia offers excellent incentives for solar power systems, which improve their payback period and ROI.

The relationship between installed capacity and annual electricity output is described with a value called specific yield, which normally has units of kilowatt-hours per kilowatt-peak (kWh/kWp) or megawatt-hours per megawatt-peak (MWh/MWp).

Assuming favorable site conditions for solar power, the sunniest places of Australia have a specific yield of over 1700 kWh/kWp.  On the other hand, less sunny places like southeast Victoria and Tasmania have a specific yield of around 1300 kWh/kWp. A higher specific yield shortens the payback period while increasing the return on investment – each dollar invested upfront saves more dollars over time.

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