Solar panels and battery storage: designing both at once
Fitting panels and a battery together lets the two be designed as one system. That is an advantage, as long as the battery is sized for the way the house uses energy rather than for the size of the roof.
In short
When panels and a battery go in together, a single hybrid inverter can run both, and the design can match battery size to your evening and night use. Size the panels from the roof and your yearly use, size the battery from the evening, check the seasonal surplus, and expect a G99 application before installation.
Two separate sizing questions
It is tempting to treat a solar-and-storage system as one size: "a 4 kWp system with a 10 kWh battery". In practice the two answer different questions. The panel array's size is limited by the roof and set by how much energy you want to generate over a year. The battery's useful size is set by how much of your use falls when the panels are idle, and by how much surplus there will be to store.
| Panels (kWp) | Battery (usable kWh) | |
|---|---|---|
| Main limit | Roof area, orientation and shading | Use after dark and in the cheap or sunny gaps |
| Main driver | Yearly generation wanted | Evening and overnight use |
| Seasonal effect | Output several times higher in June than December | Fills often in summer, rarely in winter |
| Planning and safety rules | Roof and planning rules for panels | PAS 63100 locations and storage caps |
PVGIS, the European Commission's solar estimator, shows how much the place matters: a south-facing array at 35 degrees makes about 814 kWh per kWp a year in Bangor and about 1042 kWh in Cardiff, with Liverpool at about 965. Your installer's quote should give an annual generation figure for your own roof.
One inverter or two?
A new system can be built around a single hybrid inverter that manages the panels and the battery together. Solar can then charge the battery as direct current, without first being converted to AC and back. Fewer boxes on the wall can also make the location question easier.
Some designs still use separate units, for example to keep the option of a different battery later, or because the battery is going in a different place from the inverter. Neither choice is wrong. What matters is that the quote says which design it uses, and why. The page on existing panels has a side-by-side sketch of the two connection types.
The inverter's power limits the whole system
A hybrid inverter has a maximum power, in kW, that it can deliver to the house and a maximum it can push into the battery. That caps how quickly midday surplus can be stored and how much the battery can supply at once in the evening. When comparing designs, look at the inverter's kW ratings alongside the battery's kWh; the kW and kWh guide explains why.
Estimating self-use before anything is fitted
With new panels there is no export history to go on, so the installer's estimate is the starting point. Installers registered with MCS estimate the share of solar a home will use itself, without and with a battery, using a lookup-table method. A secondary source notes that method does not cover annual generation above 6,000 kWh, use outside 1,500 to 6,000 kWh a year, or batteries above 15.1 kWh, so at those edges ask how the estimate was produced.
Put the quote's two shares into the explorer on our home page. It applies two limits we always use: no more than 75% of generation used at home without a battery, or 90% with one, and never more than the home uses in total. If the quote's battery share sits well above the limit, ask what it assumes.
Then compare the battery's usable size with your evening use. Ofgem's medium typical use for a single-rate meter is 2,500 kWh a year, roughly 6.8 kWh a day; how much of that falls after dark depends on the household. The sizing helper takes your own figures.
Paperwork for a combined system
The network operator sees panels and battery as one connection. MCS says a combined system will often exceed the 16 A per phase limit for G98, so the installer applies under G99 before installation, using either the standard form or an integrated fast-track form. The work should wait for the operator's reply, which can come with conditions. G98 and G99 for a home battery covers both routes.
The battery also needs its own place to go. PAS 63100's location rules apply however new the rest of the system is: outdoors or a separate building preferred; no lofts, bedrooms or escape routes; fire separation, ventilation and an alarm indoors where needed. Plan the battery's spot at the same time as the panels' layout, not afterwards. Where to put a home battery has a plan view.
Certification to check
Solar PV and battery storage are separate MCS technologies, each with its own standard; battery storage uses MIS 3012. Check the installer's entry on the MCS register lists both. Exporting under the Smart Export Guarantee needs an MCS (or equivalent) installation and a meter that records half-hourly exports; exporting from a battery explains how storage fits.
Leaving room to change your mind
A combined design is a good moment to think a few years ahead. If you might add an electric car, a heat pump or more panels later, your evening use and your daytime surplus will both change, and a battery sized for today may be too small or too large. Ask whether the system could take another battery module later, whether the inverter's power ratings would allow it, and whether the network application could be made for the larger system now to avoid a second one. None of this needs to be decided on day one, but a design that leaves the door open costs little thought at the start.
Equally, there is nothing wrong with fitting panels now and storage later. The panels' export history then gives you the best possible evidence for sizing the battery, as storage for existing panels explains.
A design checklist for a combined quote
- Annual generation estimate for your roof, and the self-use shares with and without the battery.
- The battery's usable kWh and continuous kW, set against your evening use.
- Inverter type and kW ratings, for panels and for battery charging.
- Where the battery will go, and how that meets PAS 63100.
- Backup: whether it is included, and for which circuits.
- The network route (G98 or G99) and who applies.
What this page is based on
- PVGIS, the EU Joint Research Centre solar estimator (annual and monthly yields) (read 1 Nov 2026)
- Ofgem decision on typical domestic consumption values (May 2026, in use from 1 July 2026), PDF (read 3 Nov 2026)
- Easy PV help article on the limits of the MCS self-consumption method (secondary) (read 3 Nov 2026)
- MCS guidance for installers on telling the network operator (G98 and G99) (read 3 Nov 2026)
- PAS 63100:2024, fire protection for battery energy storage in dwellings (BSI, sponsored by DESNZ), PDF (read 3 Nov 2026)
- MCS Installer Operating Requirements (2026), technologies and use of the MCS mark, PDF (read 2 Nov 2026)
- MCS public register of certified installers (read 3 Nov 2026)
- GOV.UK guide to the Smart Export Guarantee (read 3 Nov 2026)
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