Quite possibly, and it's one of the most common over-specs in 2026 quotes. There's been a clear drift toward large batteries (16 kWh and 20 kWh are now routinely quoted, often as 2× 10 kWh), and for a lot of homes that's more storage than the array can fill or the household can cycle. An oversized battery in a modest-usage home just sits partly empty — you've paid for capacity you never charge or discharge, which lengthens the payback rather than improving it.
The size question is really a usage question. A 20 kWh battery can genuinely make sense for a high-consumption, all-electric home — an EV (or two) charging at home, a heat pump, electric cooking, daily usage up in the 25–30 kWh range — paired with a big enough array and ideally a dynamic or cheap-night tariff to fill it. For an average home with modest evening load and no EV/heat pump, something in the 5–10 kWh range usually does the job, and the money saved is often better spent on more panels (which help year-round, especially in winter).
Two technical checks before you accept a big battery: can your array and/or a cheap-rate charging window actually refill it each day (a 5 kW battery that only charges at ~2.5 kW won't fill from a short overnight window)? And does the continuous discharge rate match the kWh — stacking capacity doesn't help if the discharge rate caps how much you can pull at once. Size against your own half-hourly usage data, not a neighbour's setup or a round number on a quote.