Battery sizing for self-consumption
Solar Pv · Lesson 17 · Advanced
Purpose
Size storage from measured load/PV profiles, operating objective, usable energy, power and lifecycle constraints.
Learning objectives
- Explain the architecture
- Match electrical limits
- Identify safety/protection boundaries
- Use exact product evidence
Battery sizing is temporal: daily energy totals hide when PV surplus and demand occur. State whether the goal is self-consumption, tariff shifting, export control, backup or several priorities.
Core theory
Use interval data where possible. Estimate shiftable energy after direct PV self-consumption, then account for usable state-of-charge window and round-trip efficiency on a clearly stated energy side.
Check charge/discharge power, surge, phase allocation, inverter limits, reserve state of charge, seasonal variation, outages, curtailment, degradation and warranty throughput.
A larger battery can spend more time empty or full and may not increase useful cycling. Report scenario ranges and economic/control assumptions separately from electrical safety.
| Term | Meaning | Symbol | Unit |
|---|---|---|---|
| Usable energy | Energy available within permitted state-of-charge window | Not applicable | kWh |
| Round-trip efficiency | Delivered energy divided by charging energy over a cycle | ηrt | Not applicable |
| State of charge | Stored energy relative to defined capacity | SoC | % |
Enominal ≥ Edelivered / (usable fraction × ηpath)Define where Edelivered and efficiency are measured; avoid applying losses twice.
kWhAssumptions: All numerical data are supplied fictional design inputs.
- Product: 0.90×0.94=0.846.
- Capacity: 6.0/0.846=7.09 kWh nominal.
- Select: Use the next suitable product only after power, profiles, aging and safety checks.
Minimum illustrative nominal energy is 7.09 kWh on the supplied basis.
Reasonableness check: The result is checked against the stated architecture and limits, not treated as universal product approval.
- Selecting from power rating alone
- Assuming grid loss makes ordinary outputs backed up
- Mixing nominal, usable and delivered battery energy
Where this appears in practice
PV and storage design coordinates conversion equipment, protection, controls, network requirements, fire safety and user operating modes.
Knowledge check
Why is one daily kWh total insufficient for sizing?
It does not show when surplus and demand occur. Interval timing and power constraints determine usable shifting.
Answer: It does not show when surplus and demand occur. Interval timing and power constraints determine usable shifting.
Practical exercise
Size low/central/high scenarios from supplied half-hourly profiles and control priorities.
Summary
- Architecture changes fault and energy paths
- Every port has independent limits
- Backup behavior must be deliberately designed
Sources and review
- IEC 62109-3:2020: Safety of PV power converters: IEC; 2020; International
- IEC 62619:2022: Safety requirements for secondary lithium cells and batteries: IEC; 2022; International
- Amendment 4:2026 impact on BS 7671: IET; 2026; United Kingdom
- ENA generator type-test register: Energy Networks Association; Current status must be checked in Connect Direct; Great Britain / Northern Ireland
Editorial review date: 2026-08-22. Professional electrical review is pending.