Solar Power Calculator

This tool will attempt to calculate the performance of a fixed solar array in specified conditions at any location on a given date. See below for some considerations.

Latitude:

Longitude:

Date YYYY-MM-DD:

Panel Tilt°:

Panel Bearing°:

Panel Capacity kW:

Air Mass Model: None (uniform radiance) Kasten & Young Ineichen & Perez

Cloud Coverage: Clear Sky Mostly Clear Partly Cloudy Mostly Cloudy Overcast

 

Calculate

What can you do with a domestic solar power installation?

• Sell it back to the grid
• Store it in batteries and use it overnight
• Charge an EV
• Some combination of the above!

Facts and figures

Various factors must be considered when planning a solar power installation.

• A typical 200W solar panel measures approximately 1.5m × 0.75m, depending on the brand, a rooftop installation will be made up of some number of these.
• The average domestic solar power installation in the UK is 3.5kW peak, roof mounted, so at a fixed tilt and bearing. This means it will only reach its peak performance for a few hours of the day, and even that might not be ideal for its location.
• The electricity consumption of an average house in the UK is 8-12kWh/day
• Between 50-80% of generated solar power will end up as usable power once it has been through various conversions. Charging a storage battery then running an inverter (to convert from 12-48V DC in the battery to 240V AC that most appliances run on) from it for example may incur significant loss.
• A Tesla Model 3 has a 57.5 kWh battery, it would take an average installation several days to fully recharge it, especially if solar was being used for the house as well.

It can clearly be seen that a solar installation that provides sufficient power on a long, clear day in the spring or summer will be underpowered on a short, overcast day in the autumn or winter, therefore an installation for year-round usage will need to be much bigger.

Air Mass Model

The air mass models the amount of solar radiation arriving at the surface depending on the thickness of the atmosphere it must pass through. The basic setting simply assumes a constant solar radiation of 1kW/m². Two other models are available.The Kasten & Young Model is based on the empirical relation between the clear sky index and the relative optical air mass. It is typically used for the estimation of the air mass and assumes a standard atmosphere. Conversely, the Ineichen & Perez Model is a broad-band model derived from the radiative transfer equation.It calculates Global Horizontal Irradiance (GHI) considering sun's zenith angle and clearness index. It provides estimates for various components of irradiance and uses several atmospheric parameters to refine its estimates.

Comparison:

Criteria	Kasten & Young	Ineichen & Perez
Scope	Focuses on air mass estimation.	Estimates different components of solar irradiance under clear sky conditions.
Complexity	Primarily focuses on air mass.	Takes into account a wider range of atmospheric parameters.
Accuracy	Empirical, based on observational data. Varies based on conditions.	Empirical, based on observational data. Varies based on conditions.
Application	More appropriate for air mass estimation.	Suitable for a detailed breakdown of solar irradiance components.

K&Y seems to be the most pessimistic, I&P the most optimistic, and the basic model somewhere in between.

Cloud Coverage

Cloud coverage is measured in Oktas. It is a very rough guide as it does not account for the type of cloud, different kinds of clouds at different altitudes can block solar radiation very differently. Some factors include:

Cloud Type

Different types of clouds (e.g., cumulus vs. cirrus) have different effects on solar radiation. Some clouds reflect more sunlight, while others might scatter or even enhance the radiation through focusing effects.

Cloud Thickness and Altitude

A thin cloud will have a different effect compared to a thick, dense cloud. The altitude of the cloud layer also matters.

Sun Angle

The angle at which sunlight strikes the cloud layer can influence the amount of light that is reflected, absorbed, or transmitted.

Albedo of the Cloud

This refers to the reflectivity of the cloud. Some clouds have a high albedo and reflect most of the sunlight, while others let a significant portion through.

Atmospheric Conditions

Other atmospheric conditions, such as humidity, pollutants, and other particulates, can also influence solar radiance.

The UK Met Office has a cloud cover map.