Policy objectives to increase biomass' contribution to the energy supply in industrialised countries are quite ambitious, but biomass resources are rather limited and expensive in many situations. Therefore, an optimal utilisation of resources producing a maximum of energy at minimal costs is desirable. A wide variety of biomass conversion options with different performance characteristics exists. Also, the economic and energetic performance depends on many variables, such as costs of logistics, scaling effects and degree of heat utilisation to name a few. Therefore, system analysis is needed to identify optimal systems. In this study, different biomass energy systems are analysed regarding their energetic and economic performance related to fossil primary energy savings. The systems studied contain residual woody biomass, logistics, heat distribution and combustion or gasification units producing heat, power or CHP. The performance of systems is expressed as a function of scale. This is done by applying generic functions to describe plants' efficiencies and specific investment costs and by expressing costs and energy use of logistic and heat distribution as a function of conversion unit capacities. Scale effects within biomass energy systems are significant. Up-scaling increases the relative primary energy savings of the studied systems within the scale range of 0-300 MWth-input regarded, while costs per unit of primary energy savings decrease or have an optimum at medium scales. The relative primary energy savings lay between 0.53 and 1.13 GJ(fossil-saved) GJ(biomass)(-1). With costs of 4-20E GJ(fossil-saved)(-1) systems are not profitable under Dutch conditions with residual wood prices of 3.8 E GJ(LHV)(-1) while firing waste wood with zero costs at the plant gate renders profitable operation possible. Favourable in both economic and energy terms are BIG/CC plants.