Capex = nameplate kWh × pack €/kWh × FX + install + EMS (install £20k & EMS £5k are fixed, so they weigh more on small batteries).
Net saving per delivered kWh = day − night/η. Break-even
day price = night/η — below it arbitrage loses money (shown red).
O&M is taken as a % of the yearly saving (default 3%) and subtracted from
every year ⇒ net saving = gross × (1 − O&M%).
Discounted payback (yr) = the year in which the cumulative net saving, each year discounted at the capital interest rate (default 3.5%, the cost of financing the capex), first repays the capex. It is longer than simple payback because it charges interest on the outstanding capital. “—” means not repaid within 60 years.
15yr net gain (today's £) = the 15 years of net (after-O&M) savings, each restated to today's money via the present-value rate, minus the up-front capex — the net cash the build is worth in today's terms over its life (a.k.a. net present value).
A (energy-led): sweep nameplate, pick the size with the largest 15yr net gain. Discharge capped at each day's own day-window load (no export); weekend under-load lowers cycles.
B (peak×1.1): inverter = peak demand×1.1; energy = worst-day day-window load×1.1. Robust peak-shaving spec, but oversized for arbitrage → idle capacity.
C (custom): seeded as a copy of A, but nameplate capacity, inverter and charge power are all editable — enter real product sizes (module counts, off-the-shelf PCS) to cost a build you can actually buy; the rest recomputes live. “↺ copy A” reseeds C from the current optimum.
Charge power: the figure shown for A/B and the “needs … kW” note is the power
required to fully charge the usable energy within the 7-hour night window. If C's charge
power is set below that, the battery under-fills overnight, so daily delivery (and the
saving) is capped at charge kW × 7h × η — flagged in red, with the required
figure still shown.
Day window = 07–23h (17.5p target); night charge window = 00–07h (7h). Pack-level capex — real installed cost is higher; raise “Pack €/kWh” to test. No degradation/O&M.