How do battery warranties and degradation affect business payback?

Read battery warranty duration alongside energy throughput, retained capacity and permitted use. A warranty is a contract to examine, not a prediction of annual output. SAM's battery-life methodology models both calendar and cycle ageing [S1]. In our assumed 100 kWh example, 80% retained usable capacity means 80 kWh before any additional operating reserve. Checked October 2026.

Last checked 16 October 2026

How long is the battery warranty, and what does it cover?

Request the complete warranty before comparing battery prices. Put calendar duration, permitted operating pattern, energy-throughput limit and retained-capacity test on the same comparison sheet. Ask which condition ends cover first. Do not turn a headline number of years into a promise about output.

Treat the warranty as a set of questions to answer for the specific equipment. Does the wording cover commercial tariff shifting? What about flexibility dispatch, grid charging and emergency backup? Is registration required, and who completes it? Request written answers for your proposed use.

Check the repair route too. Ask who diagnoses a fault, how a claim is submitted and whether travel and labour are included. Have your solicitor review unclear liability or remedy terms. This guide explains procurement checks rather than interpreting your contract.

How does battery degradation change useful capacity?

Calendar ageing means capacity can fall with time. Cycle ageing relates to charging and discharging. The SAM methodology distinguishes these mechanisms and provides ways to model available capacity and replacement [S1]. There is no universal annual degradation rate in this guide.

Define the starting point. If a proposal refers to nominal energy while another refers to usable energy, ask both suppliers to restate their numbers on the same basis. Agree temperature, power, reserve and the measurement method for any capacity assessment.

For an assumed 100 kWh starting usable capacity, 80% retention gives 80 kWh and 70% gives 70 kWh. These are arithmetic examples, not warranty thresholds for a particular battery. If your site needs 80 kWh for its operating window, the latter case would leave a 10 kWh shortfall.

See depth of discharge and state of charge to prepare the specification. Ask for a year-by-year energy model using the proposed dispatch pattern.

What do throughput limits mean for business payback?

Throughput is cumulative energy measured at the boundary stated in the contract. Ask whether the limit counts charging, discharging or both, and whether capacity is measured at the battery or system output. Use that definition consistently when testing the limit.

As a purely assumed example, a discharge-throughput allowance of 200,000 kWh divided by 20,000 kWh annual discharge gives ten years. At 30,000 kWh annually it gives about 6.7 years. That does not describe any product warranty; it demonstrates why dispatch matters.

Ask for the monitoring dashboard to show the same quantity as the contract. If it reports different quantities, request a reconciliation. Extra cycling to capture a tariff spread needs a cost for the capacity it uses, even when an individual dispatch appears profitable.

What could your site earn after allowing for wear?

This is an illustration, not a quote. All inputs below are assumptions. The method includes conversion losses because batteries are not fully efficient [S2]. It includes a wear allowance so that maintenance alone is not mistaken for a complete ownership budget.

There are no driver sessions, charger power or driver tariff: this battery offsets site imports. Net annual benefit is delivered kWh × (£0.30 − £0.12 ÷ 0.90) − £600 − £500. Results below are rounded from the full calculation.

For the mid case, £6,000 avoided imports minus £2,667 charging energy, £600 operating costs and £500 wear provision leaves £2,233. The part funded host retains 70%, or £1,563. A fully funded contract is assumed to pay 20%, or £447; actual payment terms need confirmation.

How many years could the investment take to pay back?

Simple ROI means annual host benefit divided by host capital, multiplied by 100. Payback divides host capital by annual benefit. Here, part funded capital is £10,000–£15,000 and self funded capital is £20,000–£30,000.

These constant-output calculations exclude tax, financing and grants. They are not lifetime forecasts. The wear provision is not a replacement quote. If ageing or a throughput limit prevents the assumed output, rerun annual cash flows with lower energy and actual replacement spending. Do not count both a provision and the same replacement expense twice.

What should you require at handover?

Request baseline test results, remote-data access and a maintenance schedule. DESNZ discusses monitoring and maintenance in its grid-scale guidance; use it to frame questions, while your installer identifies the requirements for your system [S3]. Ask who retains records if the installer or software service changes.

Confirm network conditions and cabinet access before installation. Ask the installer and council to resolve applicable siting and planning questions. Compare storage without solar with business battery costs, then check tax assumptions with your accountant.

What should you do next?

Bring the proposed warranty, dispatch schedule and competing quotes. Request a written comparison of capacity cover, throughput and repair obligations before committing capital.

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