Battery specifications describe different aspects of operation, so compare each measure on its own terms. This guide is for homeowners reading quotes, datasheets or app reports. It explains the units and concepts without recommending capacities or quoting efficiency percentages. Ask the installer which definitions and test conditions apply to the complete system proposed for your home.
For the broader overview, read our guide: Energy Storage Solutions.
Independent guide. We are not a lender or a government body. Always check current scheme rules with the official provider.
1. Separate stored energy from delivery power
Capacity expressed in kWh describes an amount of energy, while power expressed in kW describes a rate of delivery or charging. Total capacity and usable capacity are not necessarily the same value. The approved operating range can leave part of the storage unavailable for ordinary use. Ask which capacity figure appears in the quotation and app. Keep power limits alongside storage when assessing whether the system can serve the household loads you intend to use.
A storage figure alone cannot tell you how many appliances can operate together or how long every household pattern will last. Demand changes with the equipment in use, while charging and discharge limits also matter. Ask the installer to connect the specification with your actual records rather than a generic scenario. Do not compare two models using total capacity for one and usable capacity for the other. Request consistent definitions before interpreting a higher value as a better fit.
2. Read efficiency and discharge terms in context
Round-trip efficiency describes the relationship between energy put into storage and energy later delivered under stated conditions. Conversion and operation involve losses, but a headline figure needs its measurement boundary explained. Ask whether it concerns the battery alone or includes associated conversion equipment. Published test conditions are not a guarantee of your household outcome. Compare like-for-like definitions and avoid turning an efficiency claim into a promised bill saving without a documented household assessment.
Depth of discharge describes how much of the relevant capacity is used during discharge. Throughput measures energy passing through the system according to the stated definition. Both can appear in operating specifications and warranty conditions, but they serve different purposes. Ask how permitted limits are enforced and which readings owners can view. Do not change protected controls to recreate a datasheet condition. Keep these concepts separate from a prediction of fixed service life or unrestricted daily use.
3. Interpret monitoring and state of health carefully
State of health estimates battery condition compared with a reference condition using the manufacturer’s method. It differs from state of charge, which concerns the energy currently available within the operating range. App readings can also reflect reserves, temperatures and schedules. Ask how the manufacturer assesses retained capacity and how monitoring records should be compared over time. An unfamiliar reading needs explanation rather than an immediate conclusion that the equipment has permanently lost the displayed amount of storage.
Use the complete product documents when comparing performance claims and suspected faults. Keep definitions, relevant conditions and commissioning records together so the installer can interpret changes. Ask whether any authorised update affects the display or calculation method. If performance remains unexplained, request professional assessment under the relevant support or warranty process. The useful comparison is a documented system operating within approved limits, not an isolated metric treated as a universal ranking of battery technology.
Checklist
- I separate kWh capacity from kW power.
- I compare usable capacity consistently.
- I check efficiency boundaries and test conditions.
- I understand discharge and throughput definitions.
- I interpret health readings through manufacturer guidance.
Frequently asked questions
Does my capacity figure tell me the discharge power?
No. Capacity describes stored energy; power describes its delivery rate. Check both against the household uses included in the design.
Can I compare my total capacity with another usable figure?
Not fairly without clarification. Ask for consistent definitions and the approved operating range of each product.
Does my efficiency rating guarantee lower bills?
No. Check measurement boundaries, operating conditions and household use. An equipment metric is not a financial promise.
Is my state of health the same as state of charge?
No. Health estimates condition against a reference, while charge estimates current availability. Ask how the manufacturer calculates and displays each measure.
So you can compare meaningful performance measures without turning technical specifications into unsupported household promises.
WHLS Editorial Team
Our editorial team writes independent, plain-English guides on home energy finance. We review guides when scheme rules change.
References
- Energy Saving Trust – Energy storage (opens in a new tab)
- GOV.UK – Domestic battery energy storage systems safety review (opens in a new tab)
- Energy Saving Trust – Off-grid (opens in a new tab)
- GOV.UK – Notice of approved grant categories and values for the Boiler Upgrade Scheme (opens in a new tab)
- Energy Saving Trust – Boiler Upgrade Scheme (opens in a new tab)
- MoneySavingExpert – Warm Homes Loan Scheme news (October 2026) (opens in a new tab)
- Financial Conduct Authority – Financial Services Register (opens in a new tab)
- MCS – Find an installer (opens in a new tab)
- Energy Saving Trust – Warm Homes: Local Grant guide (opens in a new tab)
- Home Energy Scotland – Grants and loans (opens in a new tab)
- MCS – Home Energy Scotland Grant and Loan (opens in a new tab)
- Energy Saving Trust – Welsh Government Nest scheme (opens in a new tab)
- Energy Saving Trust – Consumer protection (opens in a new tab)
- RECC – Renewable Energy Consumer Code (opens in a new tab)
- HIES – Home Insulation and Energy Systems (opens in a new tab)
