For free advice call 0118 951 4490 or email

Request free advice / quote 

Homeowner Blog

Spirit Energy’s New Toy: FREE Battery Storage Calculator

Erica Charles 08 Feb 2019

A few weeks ago we launched a Solar PV Calculator and wrote a blog about solar in a post subsidy world.

However, for many people looking at renewables, solar is only one part of the story. Increasingly, solar goes hand-in-hand with battery storage. And many of us are at least wondering whether we should switch to an electric car.

Finally, in 2019 we expect to see a proliferation of ‘time-of-use’ electricity tariffs from the utility companies, all aimed at transforming electricity demand and supply.

To handle all of this, we’ve been busy developing the all-singing-and-dancing calculator which looks at the package of solar, battery storage, electric car usage and choice of tariff. And of course if you are building your dream home, you'll probably be wanting a heat pump...

Soft Launch of Version 1 Today

Call me a nerd but I love a mathematical model…

Version 1 evaluates the benefits of on-site consumption of excess solar energy, as well as the benefits of switching to a ‘time-of-use’ tariff.  You can also add in electric vehicles to ensure you are looking at the ‘whole’ picture.

Note that getting an electric car will almost double the electricity consumption in most households – driving 10,000 miles a year will increase consumption by more than 3000kWh.

Version 2 will allow for the potential for battery systems to earn income from the grid for ‘grid balancing’ and from energy trading in the wholesale electricity market. 

We are aware that several utility companies are launching domestic tariffs that will enable domestic batteries to be used to offer grid services, and we need to be able to evaluate the potential cost / benefit of these tariffs. Watch this space…

Battery Systems

So far we have only added a few systems to the calculator, but over the coming months we expect to add many more.

An Example

We have of course played around with our new toy.

At the moment it confirms what we know already:

  1. Battery storage makes most economic sense with larger properties with annual electricity demand of around 8000kWh, all the better if the system is installed alongside a new or extension solar PV system, thus enabling a VAT rate of 5% instead of 20%.

  2. Tesla Powerwall 2 is currently the best system out there, offering a significant saving in electricity costs over the system life. And in fact savings can be achieved whilst reserving some capacity for back-up;

  3. Battery storage typically increases on site usage of solar from around 30-40% to around 70-80%.


We ran the calculator for a large domestic property with 8000kWh usage, 4000kWh of solar generation and a Nissan Leaf doing 50 miles a day.  We compared a flat rate tariff of 16p per kWh to the TIDE tariff (variable rate 'time-of-use' tariff).

Headline results are here:

Solar Generation Report

The system increases on site usage of solar electricity from 45% to up to 77%:

  Solar PV Only 1 × Tesla Powerwall 2 (13.5 kWh) with Solar PV 1 × Powervault 3 (12.3 kWh) with Solar PV 1 × SoFar / Pylon (14.4 kWh) with Solar PV
Solar Usage Breakdown
Immediate Use On site (kWh) 1,800 1,800 1,800 1,800
% total 44.9% 44.9% 44.9% 44.9%
Charged to battery (kWh) 0 1,300 1,250 1,180
% total 0% 32.4% 31.1% 29.6%
Exported to Grid (kWh) 2,200 908 958 1,020
% total 55.1% 22.7% 23.9% 25.5%
Total Generation 4,000 4,000 4,000 4,000
Percentage of Solar Electricity Used On-Site 44.9% 77.3% 76.1% 74.5%
Including solar electricity charged to battery.        


Installing the system and switching tariff is expected to save around £800 a year:


Battery System 1 × Tesla Powerwall 2 (13.5 kWh) 1 × Powervault 3 (12.3 kWh) 1 × SoFar / Pylon (14.4 kWh)
Available Capacity (kWh) 13.5 12.3 13.0
Approximate Cost (including 20% VAT) £8,500 £10,600 £7,250
Optimal Tariff for System Proposed Tariff (TIDE) Proposed   Tariff (TIDE) Proposed Tariff (TIDE)
Annual Saving with Optimal Tariff and Battery System [Saving when comparing the optimal tariff plus battery system to the existing setup (i.e. existing tariff and solar PV, but without the battery system). Includes electric vehicles.] £817 £814 £809
All-in Cost per kWh Discharged from System 18.3 p/kWh 20.2 p/kWh 21.5 p/kWh
For Comparison: Average Expected Cost of Electricity Over System Life (in Real Terms) * 21.2 p/kWh 22.0 p/kWh 19.4 p/kWh
* Estimated by assuming electricity price inflation will exceed general inflation by 2.1% per annum (as has been the case historically). Base cost used: 16.00 p/kWh.    



Financial Report - Under Proposed Tariff

Obviously the homeowner could switch to the TIDE tariff and not install a battery. The calculator analyses the expected electricity bill under the Proposed Tariff and isolates the savings that would be due to the battery (rather than the tariff switch) - just under £500 per year - and to the combination of battery and solar - around £800 a year.

  No Solar PV or Battery Solar PV Only 1 × Tesla Powerwall 2 (13.5 kWh) with Solar PV 1 × Powervault 3 (12.3 kWh) with Solar PV 1 × SoFar / Pylon (14.4 kWh) with Solar PV
Annual Net Electricity Cost (excluding Electric Car) £1,400 £1,100 £608 £612 £613
per kWh used 17.4 p/kWh 13.7 p/kWh 7.50 p/kWh 7.55 p/kWh 7.44 p/kWh
Annual Cost of Charging Electric Car (assumed to be at night) £425 £425 £425 £425 £425
Annual Net Cost (including Electric Vehicles) £1,820 £1,520 £1,030 £1,040 £1,040
Annual Savings Relative to "No Solar PV or Battery"   £299 £788 £783 £783
Annual Savings Relative to "Solar PV Only"     £489 £484 £484


Try it yourself

You can try the calculator by clicking here:

 Go to Calculator


Of course if your idea of a new toy is 0-60 in 1.9 seconds rather than a mathematical model, we can always do the modelling for you...


Call us on 0118 951 4490 or click here to request a quote:

Request free advice / quote 

Feedback is welcome, as always….






Topics: Solar PV battery storage, Battery storage for solar panels,, Battery calculator

Leave us your thoughts on this story