Battery STC tier

Written and checked by the Energy Merchants desk · Reviewed 3 October 2026

Battery STC tiers are the stepped shares of the STC factor applied to different slices of a battery’s usable capacity. The first slice earns the most, and each further slice earns less. They were introduced under the Cheaper Home Batteries Program and have applied since 1 May 2026.

The bands

• First 14 kWh of usable capacity: 100% of the factor
• 14 to 28 kWh: 60% of the factor
• 28 to 50 kWh: 15% of the factor
• Above 50 kWh: nothing extra, since only up to 50 kWh is eligible

The factor is 6.8 at the time of writing, stepping down to 5.7 on 1 January 2027 and 5.2 on 1 July 2027.

Worked example

A 20 kWh usable battery installed at a factor of 6.8:

• First 14 kWh: 14 x 6.8 = 95.2
• Next 6 kWh at 60%: 6 x 6.8 x 0.6 = 24.5
• Total: about 119.7, so around 119 STCs after rounding down

A 10 kWh battery gets 10 x 6.8 = 68, all at the top tier. A 30 kWh battery gets about 154. The marginal value of the extra capacity is clearly lower, which is the intention.

In practice

Tiers change how you advise on size. For a home, the first 14 kWh is the best value in certificates. A customer considering 30 kWh should know that the extra 16 kWh earns far less. Quote the STC count from the battery STC calculator rather than doing the sum by hand, and keep the usable capacity, not the nominal, as the input.

Common confusion

Tiers apply to the capacity slices, not the whole battery. A 20 kWh battery does not earn 60% across all 20 kWh. Each slice gets its own share. See usable capacity and STC factor.

See battery STCs and the glossary.