Battery Payback Calculator

The simulation runs through 13 months of 15-minute interval data. Solar excess charges the battery for free when spot price is below the daily median — when price is high, solar exports directly to avoid efficiency losses. During expensive peak hours (above daily median), the battery self-consumes to offset your usage (saving the full all-in price) and may sell to the grid if the spot price is at least 3× the day's cheapest spot price. On cloudy/winter days, the battery charges from the grid at cheap off-peak rates, but only when the expected peak all-in price × efficiency exceeds the charge cost. Grid selling earns only the electricity spot price (no grid fees), while buying costs the full all-in price (spot + grid + fees + VAT), so grid arbitrage requires large price swings to be profitable.