Capacity is not about how much energy a home uses in a day. It is about how much it uses after the sun stops producing. That gap, measured in kilowatt-hours, is the number the battery has to cover.

Start with the evening-to-morning window. A typical European household draws 4–7 kWh between sunset and sunrise, split across the fridge, lighting, a heat pump or air conditioner, and the always-on electronics. A 5 kWh battery covers the low end of that range and nothing more. A 10 kWh battery covers the full overnight draw for most homes. A 15 kWh battery only earns its keep when the home runs a heat pump, an EV charger, or a pool pump through the night.

Now look at the solar side. A 6 kWp array in southern Europe produces 20–30 kWh on a good summer day and 4–8 kWh on a grey winter day. The battery can only store what the array produces in surplus, so a 15 kWh battery paired with a 3 kWp array is a waste of money for half the year. Match the storage to the smaller of the two numbers: the overnight load or the daily surplus.

The third variable is the tariff. Under a flat feed-in tariff, the battery’s job is self-consumption, and the payback comes from every kilowatt-hour you don’t buy from the grid. Under time-of-use pricing, the battery can also arbitrage – charge cheap at night, discharge at the evening peak – and that changes the ideal size upward, because you now want enough headroom to shift load, not just cover it.

A drop-shipper sells you a 10 kWh box and moves on. A manufacturer helps you size the box so your installers stop fielding “why is my battery empty by 2 a.m.” calls.

A genuine OEM/ODM partner will give you the discharge curve and the usable-capacity figure, not just the nominal number. A 10 kWh pack with a 90% depth-of-discharge window delivers 9 kWh of usable energy, and that distinction changes the sizing math by a full tier. They will also offer the same enclosure and BMS platform across 5, 10 and 15 kWh, so your installers learn one wiring scheme and one commissioning routine instead of three.

The customisation that matters here is firmware. A partner who can tune the SOC window, the charge current and the grid-export threshold to your market’s tariff structure turns a generic battery into a product that actually fits the local grid code. They will quote MOQs down to 100 units with shared-container options, and they will tell you honestly when a 15 kWh stack is overkill for a market that mostly installs 5 kWh units.

Here is how the three common capacities line up against real household profiles, assuming a 48V LFP wall-mount platform with a 90% usable depth of discharge.

The payback signal column is the one to watch. A 5 kWh unit pays back fastest in a market with a flat tariff and a small evening load, because every stored kilowatt-hour directly offsets a grid purchase. A 15 kWh unit only justifies its cost when the tariff structure or the load profile gives it a second job to do.

• One – Sizing to the roof, not the load.

  The most common error is matching the battery to the array size. A 10 kWp roof does not need a 15 kWh battery if the household only draws 5 kWh overnight. Size to the evening-to-morning gap first, then check the array can actually fill the battery.

• Two – Ignoring usable capacity.

  Nominal and usable are not the same number. A pack with a 90% depth-of-discharge window delivers 10% less than the label suggests, and a pack with a conservative 80% window delivers 20% less. Always size on the usable figure, and ask the supplier for the discharge curve that proves it.

• Three – Forgetting the winter derating.

  LFP cells lose usable capacity in the cold, and a battery installed in an unheated garage in northern Europe will deliver less in January than in July. If the market has real winters, add 10–15% to the capacity or specify a pack with internal heating.

• Four – No room to grow.

  A customer who buys a 5 kWh unit today and adds an EV charger next year will be back asking for a second unit. Check whether the platform supports parallel expansion before you quote, so the first sale doesn’t become a dead end.

The right residential capacity is the number that matches the household’s overnight draw, fits the array’s daily surplus, and earns its keep under the local tariff. Five, ten or fifteen kilowatt-hours is not a preference – it is the answer to a calculation, and the calculation starts with the load curve.