Battery size needs three separate numbers: usable energy in kWh, power in kW and energy moved over a year. A capacity label alone cannot tell you the annual saving. Begin with evening demand, otherwise-exported solar and the exact import/export payment arrangement, then ask an installer to assess the equipment.
Usable capacity and nominal capacity
Nominal kWh is a pack label; usable kWh describes accessible energy under specified conditions. Reserve, depth-of-discharge settings and operating limits can change how much is available for ordinary household use. Compare the same boundary and battery count in each proposal.
If a quote states only nominal capacity, leave usable capacity unknown in the quote checker. Ask for the data sheet and configured reserve. Do not assume the numbers are interchangeable or that every product exposes its entire nominal capacity.
Power limits which loads can be covered
A 5 kWh usable battery cannot necessarily supply a 5 kW load. kWh is energy; kW is the delivery rate. Ask for continuous charge and discharge power, relevant short-duration ratings and limits imposed by the inverter or installation. Simultaneous kettle, oven, heating and vehicle loads can exceed available power even while stored energy remains.
Backup is another design. Identify the supported circuits, gateway or isolation equipment, reserve and restart behaviour. Do not infer whole-home outage support from a storage capacity. A larger reserve may improve backup preference while reducing energy available for everyday savings.
Throughput needs surplus and demand at the right time
Our annual scenario lets the user choose how much otherwise-exported solar goes into storage. Delivered energy equals that input times AC round-trip efficiency. Delivery cannot exceed remaining household demand, available solar surplus or usable capacity multiplied by assumed equivalent discharge cycles.
For example, 1,000 kWh entering storage at an illustrative 90% efficiency delivers 900 kWh and loses 100 kWh. A 5 kWh usable battery at 250 assumed equivalent cycles has a 1,250 kWh delivery bound, but that bound does not prove the 900 kWh can actually be shifted. Winter surplus, summer demand and daily power limits still matter.
Generation and consumption annual totals cannot establish those timing details. The initial tool does not claim half-hourly simulation or use capacity to manufacture an annual benefit. Compare a transparent battery scenario and label the shifted-energy input as an assumption.
Export payments change what stored energy is worth
Metered exports are income you give up when diverting solar into storage. At example import/export rates of 30p/15p, the 900 kWh delivery avoids GBP 270 of imports while 1,000 kWh diverted gives up GBP 150 of exports. The additional benefit is GBP 120 before further operating costs. At 30p exports, the same operation loses GBP 30 before costs.
Legacy deemed FIT exports or unknown arrangements need agreement-specific information. Our tool withholds a financial verdict for those cases rather than silently applying metered SEG arithmetic. Check your generation meter and payment documents before changing an existing installation. Understand export payment types.
Ask for a useful installed proposal
Provide inverter model, measured generation, import/export statements, desired backup and any space or cable constraints. Ask what equipment stays, what must change, who checks network requirements and whether warranty conditions allow the configuration. A matching brand name does not remotely confirm compatibility.
For existing owners, the original solar cost is sunk. Compare only the additional installed battery work against additional benefit, including required inverter or backup changes. The retrofit journey collects the relevant information and remains useful when installer matching is unavailable.
Research checked 1 November 2026. Primary references: Energy Saving Trust battery guidance and Ofgem FIT generator information. Examples use illustrative assumptions, not specifications for a recommended product.
Your next step
Prepare a relevant installation enquiry
Check current service availability and prepare your property and energy-use details. Installer matching is currently unavailable; any available request is for internal website review.
Free to requestNo obligation to proceed
UK Solar Expert is an information and comparison website, not an installer. Installer matching is currently unavailable. Any available online request goes to the website team for internal review; we ask before forwarding details to an installer.