Plus: the value of the power (energy) is variable throughout time: solar produces energy synchronously and tends to drive power prices down due to a very low marginal cost (hence negative power prices when there's a lot of wind and solar in Germany e.g.). When there's no wind and no solar, combined with a high demand (e.g. low temperatures in Europe), prices will be much much higher. So the remaining 70% of your capacity factor is worth more than the first 20%.