Fundamentals › Climate data

Every calculation in EPOS-Plan starts with the weather. The climate data set of the site determines the hourly profile of space heating demand, the coefficient of performance of a heat pump, the yield of solar collectors and photovoltaic modules, and the coincidence of generation and demand. Choosing the right climate data set is therefore the single most important boundary condition of a simulation.

Data sources

EPOS-Plan uses hourly test reference years for ambient temperature, humidity, global and diffuse irradiance and wind. For radiation data the program can additionally fall back on PVGIS, the reference database of the European Commission, which provides satellite-derived long-term averages for any location in Europe. Measured data from a nearby weather station can be imported if it is available in hourly resolution.

Temporal resolution

All calculations run in hourly steps over a full calendar year, that is 8,760 time steps. This resolution is fine enough to represent storage management, self-consumption of photovoltaic electricity and the alternation between generators realistically, and coarse enough to complete a full annual simulation within seconds. Shorter time steps add little to the sizing of complete supply concepts while substantially increasing the data required.

Site selection and altitude

The site is defined by place name, postcode or coordinates. EPOS-Plan assigns the nearest reference data set and accounts for altitude. Between a site in a river valley and one at 700 metres there are several kelvin of difference in annual mean temperature, which visibly shifts both the heating demand and the seasonal performance factor of a heat pump. For projects in pronounced valley or ridge positions it is worth checking the actual elevation rather than the centre of the municipality.

Design temperature and heating limit

From the climate data set EPOS-Plan derives the design outdoor temperature used to size the heat load as well as the degree hours. The heating limit temperature separates days with space heating from days on which only domestic hot water or process heat is required. It depends on the insulation standard: well insulated buildings reach the heating limit at 12 to 14 °C, poorly insulated existing buildings only at 16 to 18 °C.

Radiation data

Global and diffuse irradiance are held for the horizontal plane and converted to the tilted, oriented surface. This allows east-west arrays, façade modules and ground-mounted systems to be evaluated realistically, including shading losses entered as monthly or hourly reduction factors.

Reference year, extreme year and climate change

A test reference year describes a typical, not an extreme course of the weather. Peak load sizing therefore requires an additional look at a cold year, and the assessment of cooling and overheating a look at a warm one. Climate projections shift both ends: heating demand falls while cooling demand and summer temperatures rise. Anyone evaluating a plant over twenty years should at least run the comparison between a present-day and a future climate data set as a sensitivity.

Practical notes

The climate data set should be fixed before any other input, otherwise demand figures and sizing results shift retrospectively. If metered consumption data is available, check which year it comes from: a mild year leads to a systematic underestimate of demand when transferred to a reference year without correction. EPOS-Plan offers weather correction based on degree day figures for this purpose.