Forecasts vs estimated actuals

Understanding the difference between our forecast and estimated actuals data

The fields of solar radiation, solar forecasting and meteorology are teeming with terminology that can make things hard to understand for those on the outside. Solcast’s mission is to enable everyone, not just experts, to build a solar future.

So, here’s an attempt to make things clearer for all our customers.

Our Historical, Recent and Live Data are ‘Estimated Actuals’

Actuals are bits of information based on measured data – data that was actually measured and obtained from the real world (e.g. solar irradiance detected by a pyranometer in a field). When it isn’t possible to actually measure something, an estimate can be used to obtain a value as close as possible to the actual value. ‘Estimated actuals’ are estimates of actual data, based primarily on data from satellites.

Many folks in the solar industry will refer to estimated actuals as measurements, but weather satellites cannot directly measure the amount of solar radiation arriving at the Earth’s surface. Satellites can only measure clouds or the radiation reflecting off Earth and back into space. So, any solar irradiation value at the Earth’s surface is an estimate of the actual solar radiation at that location.

Solcast’s estimated actuals use images of clouds taken by weather satellites and a series of modelling steps to produce estimates of the available solar radiation at any given location. These values are updated by Solcast with each new satellite scan every 10 to 15 minutes. Solcast continually calibrates and improves estimated actuals against surface measurements.

Solcast has three different timescale products for estimated actuals data:

  • Live is from 7 days ago until the present
  • Recent is from 4 months ago to 2 days ago
  • Historical is from 10-12 years ago to 2 days ago.

solar-irradiance-GHI-validation.png Above: Validation of Solcast’s estimated actuals (global horizontal irradiance) compared to a pyranometer (actual measurement device) at a solar farm.

Register now and evaluate our estimated actuals for yourself!


A forecast is a prediction (e.g. the air temperature in your city tomorrow). Solar forecasts contain uncertainty due to the challenging and somewhat chaotic nature of the atmosphere. Solcast’s solar forecasts incorporate and communicate this uncertainty. For most Solcast forecasts, we provide P10 (lower), P50 (middle) and P90 (upper) forecast values.

forecasts-estimated-actuals-demo.png Above: A Solcast demo chart (available to all users). The dotted line is the current time, and to the right of this are forecasts of total power output. The shaded areas are the probability bounds on the forecast, or in other words, the inherent uncertainty in the forecast.

Solcast’s forecasts reach out to 7 days ahead. The next four hours from any given time are continually improved using proprietary satellite-based forecasting methods with each new satellite scan every 10 to 15 minutes. Five weather satellites around the world and 18 model predictions during each update cycle means that Solcast calculates over 600 million forecasts per hour!

We generate so much forecast data that it’s not feasible for us to allow users to access all prior forecasts via our API (though we do keep many of the forecasts for our internal improvement efforts). However, to allow customers to check forecast accuracy at any location Solcast retains the 1-hour-ahead forecast in the API for at least the past 7 days, to allow you to track our accuracy for yourself.

Utility-scale solar farm tuningAn example of live forecast accuracy tracking available via the Solcast API.

Why not get started now and create a forecast for your location?