A battery storage system captures surplus solar production and makes it available later. This chapter covers what batteries actually do, the key technical parameters, and how to size a system for common residential objectives.
Battery storage does not make you energy independent over weeks or months — the physics and economics don’t work for that at residential scale. What it does do:
For a typical household, a well-sized battery increases SSR from ~50–55% (solar only) to 70–80%, by capturing the midday solar surplus that would otherwise be exported and covering the evening load.
The total energy the battery can theoretically store. Not the usable amount.
The capacity actually available, accounting for depth of discharge limits:
Usable capacity = Nominal capacity × Maximum DoD
LFP batteries typically allow 90–100% DoD (high usable fraction). NMC batteries typically allow 80–90% DoD.
The fraction of nominal capacity you cycle through. Cycling to 100% DoD every day degrades the battery faster than cycling to 80% DoD. Most systems limit maximum DoD in firmware to protect cycle life.
Energy in vs energy out:
RTE = Energy out (discharge) / Energy in (charge)
Typical LFP RTE: 93–97%. NMC: 91–95%.
This means to store 10 kWh and retrieve it, you need to put in ~10.5 kWh. Over a year with 300 cycles at 10 kWh, the efficiency loss is 300 × 0.5 = 150 kWh/year wasted.
The number of full charge-discharge cycles before capacity degrades to 80% of original (the standard end-of-life definition).
| Chemistry | Cycle life | Calendar life |
|---|---|---|
| LFP (LiFePO4) | 3,000–6,000 | 15–20 years |
| NMC (Li-NiMnCo) | 1,000–3,000 | 10–15 years |
| Lead-acid (VRLA) | 300–800 | 5–10 years |
LFP has become the dominant choice for residential storage because of its superior cycle life, thermal stability (no thermal runaway risk), and declining cost.
The rate of charge or discharge relative to capacity:
Residential batteries typically operate at 0.5C–1C. A 10 kWh battery at 1C = 10 kW peak discharge.
| Parameter | LFP | NMC |
|---|---|---|
| Energy density | ~150 Wh/kg | ~200–250 Wh/kg |
| Cycle life | 3,000–6,000 | 1,000–2,500 |
| Calendar life | 15–20 yr | 10–15 yr |
| Round-trip efficiency | 95–97% | 92–95% |
| Temperature sensitivity | Low | Moderate |
| Thermal runaway risk | Very low | Higher |
| Cost per kWh | ~€350–500 | ~€300–450 |
| Typical brands | BYD, CATL, Tesla LFP | Early Tesla Powerwall, Sonnen |
Recommendation: For residential use, LFP is almost always the better choice due to cycle life. The extra size/weight doesn’t matter when the battery is installed in a garage or basement.
The battery needs to store the midday solar surplus and supply the evening load until midnight (or until solar resumes the next morning).
Required usable capacity = Evening load (kWh) − Evening solar (kWh)
From Chapter 3’s profile (4-person house, no heating):
Apply DoD and RTE:
Nominal capacity = 4.5 kWh / (0.90 DoD × 0.95 RTE) = 5.3 kWh
Round up to the next commercial size: 7 kWh or 10 kWh (standard modules are 5, 10, 15, 20 kWh).
For a household consuming 5,000 kWh/year with a 4 kWp solar system:
| Battery size | Additional SSR vs. solar-only | Self-consumption improvement |
|---|---|---|
| 5 kWh | +10–13% | Significant |
| 10 kWh | +15–18% | Good |
| 15 kWh | +18–20% | Modest |
| 20 kWh | +19–21% | Marginal |
Beyond ~12–15 kWh, the additional benefit is small because the battery is rarely fully cycled — oversizing hurts economics.
Some households want battery storage primarily for grid outage resilience (increasingly relevant given weather events). The sizing question becomes: how many hours/days of autonomy do I want?
Required usable capacity = Critical load (W) × Autonomy hours (h)
Critical load = subset of loads to keep running during outage (lights, fridge, router, phone charging, medical devices). Typically 300–800 W average.
| Autonomy target | Critical load | Required usable capacity |
|---|---|---|
| 12 hours | 500 W | 6 kWh |
| 24 hours | 500 W | 12 kWh |
| 48 hours | 500 W | 24 kWh |
| 24 hours | 1,500 W (incl. heating) | 36 kWh |
Backup sizing often drives larger battery systems than self-consumption optimization alone.
If you have a time-of-use tariff with a large peak/off-peak differential (e.g., 3:1 ratio), it can be worth charging the battery from cheap off-peak grid power and discharging during peak hours — even without solar.
Arbitrage value per cycle:
Value = Capacity × DoD × (Peak price − Off-peak price − RTE loss)
Example: 10 kWh battery, 80% DoD, peak €0.30/kWh, off-peak €0.10/kWh, 95% RTE:
Value = 10 × 0.80 × (0.30 − 0.10/0.95) = 8 × (0.30 − 0.105) = 8 × 0.195 = €1.56/cycle
At 300 cycles/year: €468/year.
Compare to a 10 kWh LFP battery at €4,500–5,000 installed: payback ~10 years from arbitrage alone. Pure arbitrage rarely justifies storage economics — self-consumption savings are typically 2–3× more valuable.
Household: 4 persons, Lyon, France. 5,200 kWh/year. 4 kWp solar (4,685 kWh/yr produced). 10 kWh LFP battery, 90% DoD, 95% RTE.
| Metric | Solar only | Solar + 10 kWh battery |
|---|---|---|
| Annual solar production | 4,685 kWh | 4,685 kWh |
| Direct self-consumption | 2,600 kWh | 2,600 kWh |
| Battery stored + discharged | — | 1,400 kWh |
| Grid export | 2,085 kWh | 685 kWh |
| Grid import | 2,600 kWh | 1,200 kWh |
| Self-sufficiency ratio | 50% | 77% |
| Self-consumption ratio | 55% | 85% |
| Annual grid savings (€0.22/kWh) | €572 | €880 |
The battery adds €308/year in savings. At a battery cost of €4,000–5,000 installed (10 kWh LFP), payback is 13–16 years — within the 15–20 year calendar life of LFP.
Battery economics are most favorable where: (1) grid electricity prices are high, (2) export compensation is low, and (3) the daily solar surplus to store is large.
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