How much solar energy does a Solar P.V. system generate?

Introduction to Solar PV System Generation Calculations

In this article, we aim to demystify the calculations behind a solar PV system's annual generation for a typical property, outlining the underlying formula and applying it to an example. We believe at the Solar and Battery Company that if you can understand the maths behind the calculation, it will give you more confidence in your system and what it can generate for you.

The Role of MCS Guidelines in System Output Predictions

Solar and Battery Company.con are MCS accredited.

For the sake of clarity, we'll refer to the MCS (Microgeneration Certification Scheme) guidelines, the standard for system output predictions. These guidelines aim to prevent overestimation, especially during the Feed-in Tariff (FIT) era, when homeowners were reimbursed for their generated electricity, regardless of usage. You should always check that the system output predictions are using the MCS approved formulas.

The Challenge of Predicting Annual Sunlight

Predicting annual sunlight in specific areas remains unpredictable, leading to assumptions in these calculations. MCS employs historical data and averages to estimate sunlight, with a noteworthy tendency to underestimate system performance. Thus, there's potential for better-than-predicted generation.

The MCS regions map. Each area has their own data for the amount of sunlight a property could typically receive.

Regional Variations and Their Impact on Predictions

MCS categorises regions in England based on varying annual sunlight. This acknowledges that certain areas receive more sunlight than others; for instance, Cornwall experiences better weather (more sunny days) than Newcastle. The region, roof angle (30 or 45 degrees for example), and the roof's deviation from due south impact the predicted annual kWh per kWp (kilowatt-peak) allocation for that region.

Calculating Your Solar PV System's Potential

For simplicity, this below estimation projects the total kWh’s a one kWp system on your roof in a random area would generate annually. Multiplying this figure by your system's size provides an estimate based on your roof's characteristics.

Example Calculation and Financial Savings

For instance, if your 25-degree, 45-degree-from-south roof predicts 833 kWh per kWp and you install a 4kWp system, the annual estimate would be 3,332 kWh per kWp (833 kWh * 4). Calculating potential savings involves multiplying this with your electricity unit rate, e.g., if your rate is 28p, resulting in a yearly saving of £932.96 (assuming you use 100% generation - which you won’t, but you want to get as close to it as you can).

Further Calculations

The above system example generated 3,332 kWh’s per year. For a more accurate calculation of annual savings, you shouldn’t assume 100% usage, certainly without a battery. If you as a homeowner used 80% of what you generated in your home, that would mean a saving on 2,665 kWh’s. If your rate was .28p from your energy company, your saving would be £746.37. Assuming that there is no battery in this scenario, the remaining 20% (which would be 666.4kWh’s) would be exported back to the grid. This is when the SEG kicks in and you should be paid 5.5p (or whatever the rate your energy company has agreed to pay you) for each of the kW’s. This would be a payment of £36.65, resulting in a total saving (or revenue) of £783 per year.

Even Further Calculations

There are of course further calculations to be performed once a battery is included, but this goes beyond the scope of this article, and we plan to do a separate article on battery calculations in the near future.

The Benefits of Solar and Battery Systems Beyond FIT

During times when your battery is full and there's no immediate need for energy in the house, surplus energy is exported back to the grid, activating the Smart Export Guarantee (SEG) where energy companies reimburse you for exported kWh’s. While maximising self-consumption is advisable, dropping battery costs have made solar and battery setups feasible without previous government incentives like the FIT. When the FIT ended, and battery costs were still high, the UK Solar PV domestic market ground to a halt, without a battery homeowners were exporting as much as 70% of their annual generation. This wasn’t as much of a problem when the FIT was paying homeowners for it at very generous rates, but the moment that stopped understandably there was little attraction to invest in solar for a homeowner. That has all changed significantly now that it makes financial sense all on its own.