Why sizing matters more than you think
A battery only earns money when energy flows through it: charged with surplus solar by day, discharged to power your home by night. A battery that is too big for your household never fills up and never empties, so much of its expensive capacity sits idle — you have paid for storage you cannot use. A battery that is too small leaves you buying grid power in the evening you could have covered.
Right-sizing is about matching three things: how much surplus solar you have to store, how much energy you use after the sun goes down, and how much you are willing to spend for diminishing returns. Get those in balance and the battery cycles fully most days, which is exactly what makes it pay off.
The three numbers you need
- Daily surplus solar (kWh)
- How much solar you generate but do not use during the day — the energy currently being exported. Your battery can only store what is left over after daytime use.
- Evening & overnight usage (kWh)
- How much electricity you use from sunset to sunrise — the load a battery would cover. Your bill or smart-meter data shows this.
- Usable capacity (kWh)
- The energy a battery can actually deliver, which is slightly less than its nameplate size because batteries hold back a reserve to protect their lifespan.
- Depth of discharge
- How much of the battery you can safely use each cycle. Most modern batteries allow deep discharge, but check the spec.
A simple sizing method
- Find your evening and overnight usage in kWh — pull it from a recent bill or, better, from smart-meter interval data. This is the load a battery would ideally cover.
- Find your average daily surplus solar (your exports). Your battery cannot store more than you have left over each day.
- Take the smaller of those two numbers — there is no point storing more solar than you use overnight, or sizing for more usage than your solar can fill.
- Add a little headroom for cloudy-day variation and future load (an EV, more electric appliances), but resist the urge to double it.
- Match that target to a real product’s usable capacity, remembering usable is a bit less than the nameplate figure.
Common sizing mistakes
The most expensive mistake is buying to the size of your solar array rather than to your usage and surplus. A big system does not mean you need a big battery — if you use most of your solar during the day, there may be little surplus left to store, and a large battery will rarely fill.
The opposite mistake is sizing only for a perfect sunny day. Cloudy stretches mean less surplus to charge with, so a battery sized for your best day will sit part-empty much of the year. And do not forget growth: if an EV or electric hot water is on your horizon, your overnight load will rise, which can justify slightly more capacity — but base that on a real plan, not a maybe.
Let the numbers do the work
Sizing by hand gets you a sensible ballpark, but your real usage varies hour to hour and season to season, and that is where the accuracy lives. EnergySorted’s battery-size calculator uses your actual usage and export pattern to suggest a capacity that will cycle well rather than sit idle — and because the platform is independent and values your feed-in on every plan, you can pair the sizing with the plan that makes that battery pay off fastest. It is a far better guide than a salesperson’s round number.