What it is and why it is essential to your return on investment

What Is the Simultaneity Factor?

The simultaneity factor (the self-consumption ratio) is a key concept for maximising the financial return on grid-connected PV systems with on-site self-consumption. Broadly speaking, it represents the share of the energy generated by the system that is consumed at the very moment it is produced, without needing to be exported to the distribution utility’s grid.

In practical terms, a simultaneity factor of 100% means that all the energy generated by the PV system is self-consumed the instant it is produced, removing any need to export surplus energy to the grid — in other words, without it ever passing through the meter.

A low simultaneity factor, by contrast, means that a significant amount of surplus energy is exported to the grid, converted into energy credits and offset against future electricity bills.

However, since January 2023, when the Distributed Generation Regulatory Framework (Lei 14.300/22) came into force, these credits are offset net of the “Fio B” tariff — the distribution network usage charge levied on the energy exported to the grid — which reduces the savings your solar PV system delivers.

Why the Simultaneity Factor Matters for the Return on Your Solar PV System

The “Fio B” tariff, which applies to the surplus energy exported to the grid and later offset, rises year on year on a stepped basis under what is known as the “Transition Rule” (Regra de Transição).

This means that, from January 2023 through 2029, the more surplus energy your system exports to the grid (that is, the lower the simultaneity factor), the greater the impact of the “Fio B” tariff component and the longer the payback period of your investment, as those savings are eroded.

Given its importance and its direct bearing on the savings a system can deliver, analysing the simultaneity factor has become a critical step when sizing the system to be installed.

How to Optimise the Simultaneity Factor in PV Systems

There are several strategies to raise the simultaneity factor, optimise on-site self-consumption and reduce grid exports, softening the impact of the “Fio B” charge:

Right-Sizing the Solar PV System

Sizing the generating system at the lowest capacity that still meets your energy needs, without producing surpluses, is a sound way to soften — or even avoid — the impact of the “Fio B” charge.

Shifting Your Consumption Profile

Adjusting your habits so that energy is used during the day, alongside generation, raises the simultaneity factor. Where possible, this can mean running the most energy-hungry appliances and equipment — such as washing machines and water heaters — during the hours of peak solar generation.

Battery Storage

With battery prices falling by around 90% over the past 15 years, energy storage with batteries and hybrid inverters has become a clear trend. Batteries let you store the energy generated during the day — avoiding surplus exports to the grid — and use it at night. This raises simultaneity while adding the other benefits that storage can bring.

How to Measure or Estimate the Simultaneity Factor?

To calculate the simultaneity factor accurately, you need to measure consumption over a given period and compare it with the energy generated in that same period. At residential sites, for example, it can take several months to obtain a reliable average that smooths out seasonal effects. That process, however, can be slow and costly. In the meantime, we await the wider adoption of smart meters and IoT-based telemetry solutions, which will make analysing the simultaneity factor far easier.

Simultaneity Factor Benchmarks by Consumer Type

In a recent study aimed at addressing power-flow issues on distribution networks, ANEEL — the Agência Nacional de Energia Elétrica, Brazil’s national electricity regulator — compiled simultaneity factor data based on load curves from the metering campaigns used in tariff processes, arriving at the following values:

  • Residential: 46%

  • Rural: 53%

  • Commerce and services: 63%

  • Industrial: 69%

Although these figures are averages, they give a fairly reliable indication of simultaneity by the type of activity carried out at the site and can serve as a benchmark when analysing and proposing a new PV system.

How the 3D Watt Solar Simulator Uses the Simultaneity Factor

Recognising the importance of the simultaneity factor, the 3D Watt Solar Simulator draws on market-validated values for each type of consumer site, together with technical and climate data for the installation location. This breadth allows a sound estimate of the energy to be self-consumed in real time and a reliable simulation of energy bills and return on investment.

The simultaneity factor is an essential element in sizing residential PV systems and assessing their economic viability. With regulatory changes gradually raising the cost of using the network in distributed generation, and with recurring grid constraints on connecting new systems, it has become crucial to maximise instantaneous self-consumption and reduce surplus exports to the grid.