Here's the honest version — with real numbers from real installations.
The problem solar panels have without a battery
A solar system without battery storage has a structural weakness: it generates electricity when you might not need it, and stops generating when you need it most.
A typical UK household's electricity demand peaks in the morning (7–9am) and evening (5–10pm). Solar generation peaks at midday. Without a battery, you're exporting a large share of your generation to the grid at 4–15p/kWh and then buying it back in the evening at 24–28p/kWh. That's a poor trade.
Homes without batteries typically self-consume only 30–40% of what their panels generate. The other 60–70% is exported. A battery flips this ratio — with a well-sized battery, self-consumption rises to 60–80%.
What adding a battery actually saves
| Scenario | Self-consumption | Annual saving* | System cost |
|---|---|---|---|
| Solar only (no battery) | 30–40% | £350–£500 | ~£4,000–£5,000 |
| Solar + 5.76 kWh battery | 60–75% | £650–£850 | ~£6,500–£7,500 |
| Solar + 11 kWh battery | 70–85% | £850–£1,100 | ~£8,500–£10,000 |
| Solar + battery + smart tariff | 75–90% | £900–£1,200 | ~£7,000–£10,000 |
Note
The smart tariff multiplier
A battery alone is valuable. A battery paired with a smart overnight tariff like Octopus Intelligent is considerably more valuable.
Here's why: even a well-sized battery will empty overnight or on cloudy days. Rather than refilling it at 24p/kWh peak rate, a smart tariff automatically charges it at 7–10p/kWh during off-peak overnight hours. The Levelise Hub that comes with every Alliant system handles this automatically — monitoring tariff rates, forecasting solar generation, and deciding when to store, use, or sell your electricity.
Real example
What size battery do you need?
Battery sizing should match your evening/overnight electricity demand. A rough guide:
- 1–2 person household: 5–8 kWh battery is usually sufficient
- 3–4 person household: 5.76–10 kWh
- Large home or EV owner: 10–15 kWh+
With a smart tariff, battery size matters less because overnight grid charging fills gaps at low rates. A smaller battery with a smart tariff often outperforms a larger battery without one.
The cost of adding a battery
All of our packages already include a battery — we spec them in because the economics are better. If you're adding a battery to existing solar, the cost depends on the battery brand and capacity:
- 5.76 kWh battery (retrofit to existing solar): approximately £2,500–£3,500
- 10 kWh battery: approximately £3,500–£5,000
- Tesla Powerwall 2 (13.5 kWh): approximately £8,000–£10,000 installed
If your existing solar has a standard string inverter (not a hybrid inverter), retrofitting a battery may require an additional AC-coupled inverter, which adds cost. We'll confirm this during a survey.
Get a quote for solar + battery
See exactly what a properly-sized battery does to your payback period.
When battery might not be worth it
There are a few scenarios where the battery addition is less compelling:
- If you're home all day and already self-consuming 60%+ of generation, a battery adds less marginal value
- If your existing solar system is nearing end of life and the battery would outlast it, timing the investment makes more sense
- If your electricity usage is very low and you've already optimised your tariff, the marginal gain is smaller
Even in these cases, a battery still makes financial sense for most people — it's more a question of priority than viability.
Frequently asked questions
Can I add a battery later to an existing solar system?
Yes, in most cases. If your existing inverter is a hybrid or has battery-ready connections, a battery can be added relatively straightforwardly. Older standard string inverters require an additional AC coupler. We'll assess compatibility and give you a clear retrofit quote.
How long does a solar battery last?
Most batteries carry a 10-year or 6,000-cycle warranty with a capacity guarantee (typically 80% of original capacity at end of warranty). In real-world use, batteries typically retain 90%+ capacity at 10 years. The battery will likely outlast the warranty period with normal use.
Which battery brand is best for UK homes?
For most UK homes, GivEnergy and Fox ESS offer the best combination of value, reliability and smart tariff compatibility. Tesla Powerwall 2 remains the premium choice with excellent backup capability. We can install any of these and will recommend based on your system requirements.
