How to size a solar and battery system, properly

Sizing runs in one direction

Four things get decided in every solar and battery quote: how much electricity you get through, how much usable storage you buy, what your grid connection will permit, and how many panels end up on the roof. Those four are not independent and they are not interchangeable. Taken in that order, each one narrows the next until only a small range of sensible answers is left.

Most quotes open at the roof, because the roof is the thing a salesperson can point at. As an input it tells you almost nothing about what the house needs.

Step one: the number is already on your bill

Find annual electricity consumption on your last statement or in your smart meter app, and divide it by 365. That single figure is the input everything else hangs off. It sets the storage, the storage sets the inverter, the inverter sets which connection route applies, and the connection route sets the ceiling on the array.

Averages are a bracket, not a target. If you have no statement to hand, use the Ofgem bands to sanity check yourself and replace them with your real figure as soon as you have it. Our guide to battery sizing works through where different households land.

Step two: storage is sized to the dark hours

Daylight loads that run while the panels are generating never pass through a battery at all. So storage is not sized to your day. It is sized to the part of the day the panels cannot reach: the evening peak, the overnight standby load, and the early morning before output picks up.

The number being sized is usable capacity, which is not the headline capacity printed on the brochure. If a quote does not state usable capacity, ask for it before comparing anything. How much load you can shift into a cheap overnight window moves the answer considerably further than roof area does.

Step three: the connection is the real ceiling

Your Distribution Network Operator owns the cables in your street and decides what may be connected to them. The dividing line is 3.68kW per phase, which is 16A. At or below it, G98 applies and the installer commissions the system then notifies the operator within 28 days. Above it, G99 applies, which means an application, a decision and a wait before anyone is on the roof.

Two details catch people out. First, the assessment is on inverter AC rating, so a generously sized array on a modest inverter can sit comfortably inside G98. Second, inverters aggregate: a 3.5kW solar inverter alongside a 3.5kW battery inverter puts the property over the line even though neither is over it alone. That is why storage has to be decided before anybody promises you a timeline.

A three phase supply lifts the same threshold to 11.04kW, which is a different world. Standard G99 determinations run roughly four to eight weeks, fast track routes around two, and constrained cases have stretched to twelve. What a G99 application involves covers the process, and what a DNO actually does covers finding yours.

Step four: the array, against what is left

By now the roof is the last variable rather than the first. UK practice deliberately loads the array above the inverter, at a DC to AC ratio between 1.10 and 1.30 and most often between 1.15 and 1.25, which is why roughly 4kWp of panels on a 3.68kW inverter is the standard single phase configuration. Our guide to inverter sizing explains why that is not the waste it looks like.

One cost point does shift the answer at the margin: around 45% of an installation is hardware and the remaining 55% is labour and logistics that barely move with system size, so panels added to a design that is already going ahead are the cheapest capacity in the quote.

Treat that as a tiebreaker rather than a strategy. Once generation exceeds what you can use or store, its value drops to whatever the export market pays, which is 5p to 15p on widely available tariffs against a capped import rate of 26.11p per kWh. A unit you use yourself is worth several times a unit you export, and that gap is the entire reason storage is decided before panels.

Where the order breaks down

1. Roof first. The array is drawn to the available slate, then storage is chosen from whatever the installer stocks. The result is either a battery full by mid afternoon with nowhere to put the surplus, or one empty by nine with the house back on the grid at 26.11p.
2. Capacity that never cycles. A battery too large to fill and empty most days is capital standing still. It stretches payback without moving a single bill.
3. Inverter left until last. Because the network operator assesses the inverter, leaving it to the end is how a quote quietly acquires a G99 application and several weeks nobody planned for.
4. Export treated as the plan. Export is what happens to the units you could not use or store. Income, not a design target.

Every county page in our coverage area carries a calculator running the same dispatch model as this guidance, which tests whether filling the battery from the roof or from an off peak rate wins for a given configuration and reports the better of the two. Off peak tariffs and battery charging covers that second route in detail.

Clearline gives independent guidance and a free remote design call. Installations are carried out by an MCS-certified installation team for your area.