Estimate what a contract will cost you per year. Enter your yearly use (your grid company's app shows it; a one-person flat is about 1,500 kWh, a family flat 2,500 to 3,500, an electrically heated house 15,000 to 20,000), then the seller's terms and the transmission price from your grid company's tariff.
How to read the result
Only the seller's part changes when you switch contracts. For a small apartment the monthly fee is often bigger than the margin, so the cheapest contract is usually the one with the lowest fee. For a house, the price per kilowatt hour dominates. The spot average is your guess about the year ahead; recent years have ranged from a few cents to well over ten in expensive winters.