Prime Outgoing

Philip Steele

Philip Steele

— 2 min read
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Prime Outgoing

At the Energy Tech Summit yesterday I published a quick video about our new export tariff, Prime Outgoing.

I mentioned it's going to make sense if you've got a large enough battery to time-shift enough solar to export during Prime Time so let's try and figure out some numbers here.

I'm going to compare our flat rate Outgoing Octopus (currently 12p) to Prime Outgoing which is currently 9p except from 16:00 to 19:00 when it’s 16p.

Let's take a typical UK solar generation profile and a 5kW peak generation and calculate the battery sized in kWh needed to break even by being on Prime instead of Outgoing.  

To determine the break-even battery capacity, we need to model a typical UK solar generation profile for a 5kWp system (5kW peak output; about a dozen panels) and calculate the energy distribution across the day throughout the year. This should be capable of generating around 4,500kWh per year.

Here is the breakdown of the two export tariffs based on this annual generation:

  • Flat Outgoing at 12p/kWh: £540.
  • Prime Outgoing at 9p & 16p/kWh: £442 without a battery. £98 lower.

Only about 14% of a typical UK solar array's annual generation naturally occurs within the 16:00 to 19:00 window so hence without a battery Outgoing makes more sense.

What minimum size battery is needed?

Adding a battery let's you store and export during Prime Time and by my calculations, allowing for 10% efficiency loss, you need a battery size of at least 5.18 kWh to break even.

How the Seasonal Maths Works

The maths behind this depends on seasonal sunlight:

  • In the Summer (May - August): A 5kWp system generates an average of 15kWh to 17kWh of before 16:00 every day. A 5.18kWh battery will easily fill up completely. By shifting that 5.18kWh to the peak window, you turn a net profit of £0.16 x 90% - £0.09 = £0.054 per kWh, gaining roughly £0.28 per day. You are also exporting a significant surplus of summer midday solar at the £0.09 rate too.
  • In the Winter (November – January): Total daily generation drops to around 3kWh to 5kWh per day, and effectively 0kWh is generated during the 16:00 to 19:00 window because the sun sets early. However, a 5.18kWh battery is perfectly sized to capture 100% of your winter daytime solar. Instead of exporting it immediately for £0.09, you hold it and dump it at 16:00 for £0.16/kWh. This makes Prime significantly out-perform Outgoing in the winter, compensating for the summer midday losses and balancing out to a perfect net break-even across the full 12 months.

Going Beyond Break-Even

Hence in the video I said a decent size battery. There's no point in being on Prime with a 5kWh battery. Running the maths again:

  • A 10kWh battery gives an annual export revenue of £603 (£63 greater than Outgoing).
  • A 13.5kWh battery gives an annual export revenue of £637 (£97 increase on Outgoing).

A few notes. I've used a 5kW peak install as the example, not accounted for your consumption profile and not considered the combined impact of a time of use import tariff that you might also use to charge the battery. There's a lot of variables to consider but hopefully this gives a reasonable guide.

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