What is the difference between a fixed-price and a spot-price contract?

Under a fixed-price contract, the unit price of energy is the same every hour. The supplier builds its risk premium into it, so it is more expensive on average but predictable in return. Under a spot (HUPX-indexed) contract, the price follows the day-ahead market price in 15-minute resolution: it is cheaper on average, but the consumer bears the full volatility. The choice has a direct consequence for how much each technology is worth at the site — and the relationship is inverse. Solar saves more under a fixed-price contract, because every kWh it replaces displaces the expensive, flat unit price; under a spot contract, however, it generates precisely in the cheapest hours, since the HUPX price is typically low in midday sunshine, so it saves less per unit. With battery storage it is exactly the opposite: it is valuable under a spot contract, because the intraday price difference is the source of arbitrage, whereas under a fixed-price contract there is nothing to arbitrage. That is why the combined installation of solar and storage is, overall, strongest for customers with spot contracts.