Solar Panel & Battery Storage Calculator

Updated: 21 Feb 2023

To assess the impact of adding solar PV panels or battery storage on your energy consumption use our calculator. The calculator helps evaluate the financial benefit of an investment in solar panels and/or battery storage.

The calculator takes your annual electricity use (kWh) and the annual output of your solar system and works out how much of your solar generated electricity will be used in the home or exported to the grid. Add the price you pay for electricity together with the export tariff rate (Solar Export Guarantee or SEG payments) to calculate the annual benefit of:

• Savings on electricity bills
• Export payments under your SEG tariff

If considering adding battery storage then enter the storage capacity to see the impact. If the purchase cost of the solar panels and/or the battery storage are available, then the calculator also works out the payback on the investment.

For more information see Frequently Asked Questions at the bottom of this page.

How are the results calculated?

The calculator uses typical profiles of annual domestic energy usage and solar output to project a likely energy export profile for each 30 minute minute period over the year. It then works out your export payments under the SEG scheme, based on your tariff, to project indicative SEG payments under these conditions. For wholesale price based SEG tariffs these can not be guaranteed, as market based pricing changes every 30 minutes. However, they are a good indication of likely payments.

The Calculator’s default settings are:

• Home energy use – 3,500 kWh per year (typical UK home).
• Electricity price of 45p per kWh (excluding any standing charge which is ignored as you have to pay it in any event).
• Solar electricity generation – 3,400 kWh per year (typical 4kWh solar PV system with average output of 850 kWh per year per kW of panel).
• Car charging rate of 2.3kW (charging using a standard 13 Amp plug)
• No battery storage system connected
• Any battery storage is assumed to be uncharged to start
• A fixed rate SEG payment of 5.5p per kWh
• Solar panel and battery storage costs based on typical prices available if both are installed together.
• A max power output of 5 kW and a max charging capacity of 3.68 kW is assumed for a 13.5 kWh storage battery. Power characteristics of larger sized batteries are adjusted pro rata.

Following helpful feedback from users, the February 2023 update adds options for:

• off peak and car charging demand
• Car charging start time and maximum charging rate
• Off peak tariff and times

We would be delighted to get suggestions for improving this calculator via the comments section below.

Frequently Asked Questions

1. What are Solar Export Guarantee (SEG) payments?

Solar Export Guarantee (SEG) payments are paid by energy companies when you supply your excess electricity to the electricity grid. They have been available from all large energy suppliers from January 2020 and are a replacement for Feed in Tariff (FIT) payments which were stopped for new solar installations from April 2019.

Most suppliers are offering a similar approach to the old FIT export payments, a flat rate for each kWh exported. Typical SEG rates are from 3.5 – 5.5p per kWh so it pays to shop around.

One supplier, Octopus, are offering a rate which varies every half hour, based on day ahead auction prices for each 30-minute time period. The auction takes place once a day and allows the energy supplier to pre-sell the electricity likely to be exported. As a result, prices for each 30-minute period of the next day are known a “day ahead”. To access this variable SEG rate you will need a smart meter that can cope with half hour pricing.

Some SEG tariffs from energy suppliers could also vary by electricity distribution network. This is because the electricity distribution costs of each network do vary. There are fourteen different distribution networks in Great Britain which between them are owned by six different suppliers. You can find out more about this on the Ofgem website (See ofgem.gov.uk/key-term-explained/map-who-operates-electricity-distribution-network).

2. Where can I find my annual electricity usage figure

If you do not already have solar panels fitted then your annual electricity usage information is available on your electricity bill.

If you do have solar panels fitted it can be a little more challenging to calculate your usage figure. Your electricity bill, and the electricity meter in your home, will only show your electricity grid purchases (the electricity you paid for). Your annual usage is a combination of grid purchases PLUS the solar generated electricity that was used in your home rather than being exported to the grid. As a guide, typically around 40% of the solar electricity generated by solar panels is used in the home and 60% exported to the grid. So if your total solar generation was circa 4,000 kWh per year then about 1,600 kWh of this would have been used in the home and 2,400 kWh would have been exported.

Example: Your electricity bill shows your annual purchases from the grid were 2,500 kWh and your solar panel meter also shows that you generate 2,500 kWh of solar power annually. What is your likely energy usage?

Solution: Your usage is likely to be 3,500 kWh.
This is made up of: 2,500 kWh (grid purchases) + 1,000 kWh of self consumed solar power (40% of your 2,500 kWh solar power generation). You would have exported 1,500 kWh solar power generation to the grid.

If you have a smart meter then the actual usage figures may be available.

3. Where can I find my annual solar generation figure

The annual Solar generation figure should be available from your old Feed in Tariff (FIT) payments if you were part of this scheme.

For new installations an annual figure can be estimated based on typical generation of 850 kWh of electricity per year for each kW Peak of solar panels. A 4 kW Peak solar array should generate around 3,400 kWh per year. In an unshaded south facing location with good climate then panels can generate, on an annual basis, upto 1,000 kWh per kW of solar panel fitted.

4. What are the “30 minute prices” shown on the calculator?

Selecting this takes historic 30 minute prices from Octopus Energy. As homes switch to Smart Meters, more suppliers will offer pricing which varies every 30 minutes based on potential energy demand. The Octopus Energy pricing varies every 30 minutes. Pricing for each 30 minute period is available 5-30 hours before it is applied, allowing different battery strategies to be applied.

5. What does the Battery Charge Change figure mean?

It reflects the increase in electricity stored in the domestic storage battery which has not already been used in the home or exported.

6. What does the battery utilisation figure mean?

This measures how charged the domestic storage battery is on average throughout the year. A figure of 50% means that the battery is typically half charged. In reality the figure will be lower in winter months when output is lower and much higher in summer months.

7. What are round trip losses?

When energy is stored in a domestic storage battery and then released there are conversions losses, mainly from the conversion from AC to DC electricity and back again. The calculator assumes a battery round trip efficiency of 90%, meaning that if 1 kWh is stored then 0.9 kWh will be available for use.

8. What data do I enter?

• Enter the annual amount of electricity your home uses in kWh and the price you pay in pence including VAT. See FAQ 2 above on how to calculate your usage figure if you already have solar panels fitted. Where you have an off peak rate enter the off peak usage separately. Also enter any figure you have for electric car battery charging.
• Choose your electricity usage profile. Decide whether occupants are usually home during the daytime (eg older retired people and those who work from home) or whether occupants are out at work (eg those working elsewhere or with children at school). The default is the average profile for all types of home.
• if you have a dedicated car charger for an electric car then enter the maximum charging rate and the time it is programmed to start charging.
• Does the home have a solar pv array? How much energy does it produce each year?
• Is a storage battery connected in the home? If so what is the storage capacity?
• What charging strategy does the battery use?
  • The standard battery strategy just stores solar generated electricity for later use, so if you chose this option and do not have solar panels attached then no energy is stored or exported.
  • The arbitrage battery strategy works even when there are no solar pv panels attached but you will need to have a smart meter to allow access to half hour electricity prices and SEG payments and the battery software must permit energy arbitrage. This means you could sell energy to the grid when prices are high, knowing you can buy it back in the future when prices are lower.
• Choose a fixed rate SEG or one linked to wholesale electricity prices. If fixed enter the price paid per kWh exported.
• How much do the solar PV panel and battery storage systems cost?

9. How do I change figures in the greyed out areas?

To access the battery storage areas select yes to battery available input area.

10. Where does the data come from?

The solar data is from a 2013 study by UK Power Networks and the usage profile is from DECC Household Electricity Study. The day ahead figures are from historic “Outgoing Agile” calculations from Octopus Energy.

11. Are there any other considerations ?

Energy prices can vary significantly from one year to another. The payback calculation assumes that electricity grid purchase and export pricing remains at similar levels throughout. A rise in long term electricity prices would generally mean that payback will be faster than indicated.

Since half hour wholesale pricing for domestic users is a very recent innovation there is little history to compare the gap between grid purchase and export pricing from a supplier which is used for energy arbitrage. It is assumed that this gap will be maintained at similar levels.

It also assumes that the equipment will function throughout. Warranty periods currently on battery storage vary from 2 – 10 years and a working assumption is that 2% battery capacity is lost per year of use. For solar panels, warranty periods vary from 10 – 25 years and again output may degrade over time. Solar inverters typically have a 10 year warranty and, depending on power output, are likely to cost circa £600 to replace.

12. How does the calculator use the standard, off peak and car charging demand?

When fixed or 30 minute rates are chosen then the standard and off peak demand is combined and spread over a 24 hour period.

For an off peak rate then it is assumed that the standard demand is spread over the standard times and the off peak demand spread over the off peak time selected.

Car charging is the same for all options, assuming maximum charge rate from the charging start time.