Keep the baseline fixed
Use the same household demand, solar generation and direct-use assumption in both cases. Identify additional battery cost separately. If an installer’s self-consumption percentage includes storage delivery, ask for a direct-use and storage split before adding any battery benefit.
Our example solar-only case generates 3,600 kWh, uses 1,440 kWh directly, exports 2,160 kWh and imports 2,560 kWh against 4,000 kWh demand. Rates of 30p import and 15p export give GBP 756 annual benefit before any additional costs.
Follow the stored energy
Diverting 1,000 kWh into storage at 90% round-trip efficiency supplies 900 kWh to loads and loses 100 kWh. Export falls to 1,160 kWh and imports to 1,660 kWh. Direct use stays at 1,440 kWh. Every generated kWh is accounted for once.
The battery adds GBP 120 a year in this synthetic example. GBP 3,000 additional cost therefore has 25-year simple payback, while the combined GBP 9,000 system has about 10.3-year simple payback. That combined number does not make the additional battery a 10-year investment.
Capacity and backup are separate constraints
Usable kWh limits stored energy; kW limits how quickly it can be charged or delivered. A reserve for backup may reduce ordinary savings. Ask which circuits run in an outage and what happens when the battery is empty. Do not assume ordinary grid-connected solar provides power during an outage.
Annual throughput also depends on seasonal surplus and timing. Capacity multiplied by assumed cycles provides an upper bound, not proof that those cycles occur. Read the storage-sizing guide.
Compare before including storage in the contract
Test higher export value, lower shifted energy and an operating-cost allowance in the battery comparison. Grid charging, dynamic prices and paid grid services are not included in this solar-charging scenario.
An existing owner should use the retrofit guide and check FIT/export arrangements before a financial verdict. No generic brand choice establishes compatibility or preserves an agreement automatically.
Worked scenarios from the shared model
Synthetic examples, not measured installations or current offers. Each assumes 3,600 kWh generation, 4,000 kWh household demand, GBP 6,000 solar cost, 30p import, 15p metered export and zero additional annual operating cost. Direct-use shares vary. No tariff escalation, degradation or replacement is included.
| Scenario | Annual total benefit | Solar / combined simple payback | Additional battery payback |
|---|---|---|---|
| Lower daytime use | GBP 648.00 | 9.3 years | No battery |
| Central solar only | GBP 756.00 | 7.9 years | No battery |
| Higher daytime use | GBP 864.00 | 6.9 years | No battery |
| Central with additional battery | GBP 876.00 | 10.3 years | 25.0 years |
The battery example diverts 1,000 kWh from exports, delivers 900 kWh after 100 kWh losses and costs an additional GBP 3,000. The battery adds GBP 120/year and does not recover its cost within the selected 10-year horizon. High export prices can reduce or reverse this benefit. See equations and limits.
Sources and review dates
Research-based guidance. Source checks do not imply product testing or verification of an individual installer.
- Energy Saving Trust: Battery storage. Storage losses, scope of cost estimates and possibility that savings do not justify battery cost. Scope: UK. Checked .
- Ofgem: FIT generators. Existing FIT arrangements require agreement-specific checks before storage or export changes. Scope: Great Britain. Checked .
Registry sources-2026-11-01. Tariffs have a 30-day internal review interval. Methodology and assumptions.