Solar panels with battery storage: how does it work?

The fundamental problem batteries solve

A solar-only system without a battery exports roughly 60–70% of generation to the grid. Why? Because solar generation peaks at midday (12:00–14:00), when household electricity demand is typically lowest (washing machine isn't running; nobody's cooking; TV's off). Meanwhile, household demand peaks between 16:00–20:00, when solar generation has dropped to 10–20% of its midday peak.

This mismatch is well quantified. Data from the UK government's BEIS (now DESNZ) Smart Meter Electricity Consumption data shows an average UK household uses only 15–20% of total daily electricity between 10:00 and 16:00. The remaining 80% falls outside the solar generation window. A battery shifts that midday surplus into the evening peak.

How the system actually works

In a solar+battery installation, the hybrid inverter (SolarEdge Home Hub, GivEnergy Hybrid, Fox ESS H1, or equivalent) manages power flow according to priorities:

1. Solar electricity powers the house first (direct self-consumption).

2. Any surplus charges the battery until full.

3. Once the battery is full, additional surplus exports to the grid under SEG.

4. When solar generation drops below household demand (typically by late afternoon), the battery discharges to cover the gap.

5. If the battery depletes before the next day's solar generation, the house draws from the grid as normal.

The round-trip efficiency — how much of the stored energy you get back — ranges from 90–96% depending on battery chemistry and inverter quality. Tesla Powerwall 3 claims 97.5% DC round-trip. GivEnergy All-in-One states 93%. Fox ESS EP series: 95%. The difference matters: for every 10 kWh you store, you lose 0.3–1.0 kWh to conversion losses.

The battery brands you should know

**Tesla Powerwall 3** (13.5kWh, £7,500–8,500 installed): The benchmark. LFP chemistry (previous generations used NMC). 97.5% round-trip. 10-year warranty, unlimited cycles. Integrated backup gateway for off-grid capability. Seamless Octopus Tesla Energy Plan integration (12p/kWh export). Best-in-class software. Expensive. AC-coupled (works with any existing solar).

**GivEnergy** (5.2kWh All-in-One £2,500–3,500; 9.5kWh £4,000–5,000): UK-based company dominating the mid-market. LiFePO4 chemistry. 93–95% round-trip. Warranty: 12 years or unlimited cycles (whichever first). Excellent app with granular control. Works with Octopus Agile and Flux. No native backup capability (grid-tied only — needs EPS add-on for backup).

**Fox ESS** (EP5 5.2kWh £2,200–3,200; EP11 10.4kWh £4,000–5,500): Aggressive pricing. LiFePO4. 95% round-trip. 10-year warranty (10,000 cycles). Fast-growing UK market share. Good Octopus integration. Reliable but less polished app experience.

**SolarEdge Home Battery** (10kWh, £4,500–6,500 paired with SolarEdge inverter): LiFePO4. 94.5% round-trip. The tightest integration if you have a SolarEdge inverter — single ecosystem, single app, single warranty. 10-year warranty. Good but more expensive than standalone battery options.

**Enphase IQ Battery 5P** (5kWh modular, £3,000–4,500 per unit): LiFePO4. 96% round-trip. Excellent if you already have Enphase microinverters — integrated monitoring across panels and battery. Scalable: add units as needed. Premium pricing.

What a battery costs per cycle vs grid electricity

If a 5kWh battery costs £3,000 installed and delivers 10,000 cycles over its lifetime, each stored kWh costs £3,000 / (5 × 10,000) = 6p/kWh in capital cost. Add 5–7% round-trip losses: effective cost ~7–8p/kWh. Compare to importing at 24–30p/kWh peak: you save 16–22p per stored kWh. Over 50MWh lifetime throughput (10,000 × 5kWh), that's £8,000–11,000 saved against capital cost of £3,000. The battery pays for itself roughly 2.5–3x over.

Time-of-use tariffs: the second revenue stream

This is where batteries get truly powerful. With a solar+battery+TOU tariff combination, the battery earns its keep even on cloudy winter days:

**Octopus Intelligent Go:** 7p/kWh off-peak import (00:00–05:00 + dynamic slots). Charge battery fully overnight for ~£0.35 (5kWh × 7p). Discharge during the day and evening, avoiding 24–30p/kWh grid electricity. The battery arbitrage alone saves roughly £1–1.50/day, or £365–550/year, even if solar generates absolutely nothing.

**Octopus Flux:** Import: 02:00–05:00 at 15p/kWh (cheap), 16:00–19:00 at 32p/kWh (peak), rest at standard rate. Export: pays 4p–30p+ depending on time band. This tariff was specifically designed for solar+battery households. Export during the peak band (16:00–19:00) earns 28p+/kWh — more than many fixed SEG rates. Charge battery from cheap import or solar, then export everything possible during the premium window.

**E.ON Next Drive:** 6.9p/kWh off-peak (00:00–07:00). Similar arbitrage potential as Octopus Go but with a longer 7-hour cheap window.

Self-consumption: before and after battery

Typical UK solar-only system (4kWp, no battery): ~30–35% self-consumption. 65–70% exported at SEG rates.

Same system with 5kWh battery: ~70–80% self-consumption. The improvement alone is worth £300–450/year in saved import costs.

Same system with 10kWh battery: ~85–95% self-consumption. At this point, you're approaching near-total energy independence during summer months. In winter, TOU charging bridges the gap.

Backup power — what you actually get

Most UK battery installations are grid-tied, meaning they shut off during a power cut (safety requirement to prevent back-feeding the grid). Some systems offer backup capability:

- Tesla Powerwall 3 with Backup Gateway: Seamless whole-home backup. Powers lights, sockets, heat pump, EV — everything. ~£1,500 extra for gateway hardware and installation.

- GivEnergy with EPS (Emergency Power Supply): Single socket output only — enough for fridge/freezer, router, and phone charging during an outage. Add-on cost: ~£200–400.

- Fox ESS EPS: Similar single-output backup. ~£150–300 add-on.

Full backup costs more but, for households in rural areas with frequent power cuts or those running critical medical equipment, the value is obvious.

Is it worth it?

Solar alone is a good investment. Solar+battery is a better one — provided your household has meaningful evening electricity usage and you're willing to engage with a time-of-use tariff. For a retiree at home during the day with £40/month electricity bills, a battery may not be worth it. For a family of four with £140/month bills, an EV, and peak-time cooking/electricity use, the battery adds substantial value. At Sunlit Solutions, we model your specific consumption pattern and tariff before recommending a battery capacity. One size absolutely does not fit all.