How to Size a Home Battery in Scotland (2026)

Quick answer

Size a home battery from three numbers, not one: the electricity you use after 4pm, the solar surplus your panels can actually store, and the cheap-rate window your tariff gives you. For a typical 3,500 kWh-a-year home in Scotland on a flat tariff that points to about 4–5 kWh of usable storage; on Octopus Flux or Intelligent Octopus Go it rises towards 8–10 kWh, because the grid can fill the battery when the panels can't. If you want the answer without the working, our solar battery size calculator runs the same method in seconds.

Why your annual total is the wrong starting point

Two homes using 3,500 kWh a year can need very different batteries. One is empty all day and uses most of its power after 4pm; the other runs a home office and uses most of it while the panels are generating. A battery only earns money on energy it moves from one time of day to another, so the shape of your day matters more than the size of your year.

Step 1: Get your half-hourly usage

If you have a smart meter, your usage is recorded in half-hour slots, and many supplier apps and online accounts show it directly. Pull a normal week in winter and a normal week in summer if you can — the shape changes with the seasons, especially with electric heating.

What to look for

• After-4pm total: add every half-hour from 16:00 to midnight on a normal weekday. That is your evening need.
• Overnight base load: the fridge, router and standby devices between midnight and 06:00. A battery covers this too if it isn't already empty.
• Big spikes: an oven, an electric shower or an EV charging session. A spike can exceed a battery's power output even when there is plenty of energy stored.

No smart-meter data yet? Start with 40% of your daily use as the after-4pm share and refine it once you have a few weeks of readings.

Step 2: Check what your panels can refill, month by month

A battery sized to your evening need is only useful if something fills it. In summer your panels will. In a Scottish winter they mostly won't. These are PVGIS averages for a south-facing 4 kWp array at 35° near Livingston (PVGIS v5.3, SARAH3 data 2005–2023, 14% system losses, retrieved 16 September 2026):

Now subtract what the house uses in daylight. A home using 9.6 kWh a day, 40% of it after 4pm, uses 5.8 kWh in daylight — so in May about 9.3 kWh is left over for the battery, and in December nothing is. That gap is the most important fact about battery sizing in Scotland: from November to February the battery has to earn its keep from the tariff.

Step 3: Let your tariff set the ceiling

Flat tariff. Size to your evening need and no bigger than your May surplus. Each kWh stored and used, rather than exported and bought back, saves roughly 14p in South Scotland: Flexible Octopus import is 25.85p/kWh on Direct Debit to 30 September 2026, against 12p/kWh from Outgoing Octopus.

Octopus Flux. The battery refills from the grid at 02:00–05:00 and exports at 16:00–19:00. Octopus's published South Scotland rates for 16 September 2026 were 14.98p/kWh import off-peak, and 34.94p/kWh import with 27.19p/kWh export in the peak. Capacity above your evening need can be sold in the peak, so a few extra kWh earn their place — up to what your inverter can move in three hours.

Intelligent Octopus Go. 8p/kWh from 23:30 to 05:30 for homes with a compatible EV or charger. A battery filled overnight can run most of the next day, so size it towards the use that falls outside that window — about 80% of household use, with the car charged directly.

Cosy Octopus. For heat pump homes: cheap windows at 04:00–07:00, 13:00–16:00 and 22:00–00:00, with a 16:00–19:00 peak. A battery that charges in the afternoon window and carries the house through the peak should be sized to your evening use.

Step 4: Turn kWh into a product

• Compare usable capacity, not nominal. The usable figure is what you can actually draw.
• Check the power rating (kW). It limits how fast the battery charges and discharges, which matters most on tariffs with three-hour windows.
• Prefer modular if you're unsure. SigenStor batteries stack in 5.2 kWh and 8.0 kWh modules up to roughly 48 kWh per stack, so you can fit the size the numbers support and add a module later. Our SigenStor installer page has the full specification.

Illustrative example: a heat pump home in West Lothian

This is a worked illustration, not a customer install. A detached home using 5,500 kWh a year, with an air source heat pump and a 6 kWp array, uses 15.1 kWh a day — 6.0 kWh of it after 4pm and 9.0 kWh in daylight. On an average May day the array makes 22.6 kWh, leaving 13.5 kWh for the battery. In December it makes 3.7 kWh, well short of daytime use, so winter storage has to come from the tariff. On Cosy Octopus that points to about 6 kWh of usable storage, which rounds up to one 8.0 kWh SigenStor module.

The three most common sizing mistakes

• Sizing from the annual bill. It hides when you use the power, which is the only thing a battery cares about.
• Ignoring winter. A battery sized to summer solar sits empty from November to February unless the tariff fills it.
• Buying for export on a flat tariff. Past your evening need, stored solar can only be exported, and at 12p a unit that rarely repays the extra capacity.

Before you buy

Batteries installed in homes are zero-rated for VAT until 31 March 2027, including batteries added to existing solar or fitted on their own (HMRC VAT Notice 708/6). If the battery is part of a wider solar project, our guide to the cost of solar panels in Scotland sets out current installed prices. And when you're ready to size from your own numbers, run them through the solar battery size calculator and send us the result — we'll check it against your meter data at survey.