We recorded a net profit of € 241 million in 2013, an increase of € 8 million (3%) compared with 2012. Revenue was almost unchanged from the previous year at € 5,251 million (2012: € 5,256 million). Gross margin on the supply and transmission of gas, electricity and heating and revenues from related services rose by € 180 million (10%) to € 1,974 million, due mainly to an increase in regulated transmission tariffs, an expansion of our sustainable production capacity and higher sales of gas and heating during the cold first six months of the year.
Operating profit before depreciation and amortisation (EBITDA) was € 877 million, up € 101 million (13%) on the previous year (2012: € 776 million). Depreciation and amortisation was € 41 million higher, mainly because of expansion of our solar, wind, biomass and network activities. In respect of market prices, it is expected that the current poor relationship between gas and electricity prices (‘spark spread’) at gas-fired power stations and the low prices for CO2 rights will continue for some time. Consequently, we have recognised impairment of € 68 million to electricity-related assets in the Netherlands and Belgium. Net profit rose by 3% from € 233 million to € 241 million.
Our operating expenses were 8% up on the previous year at € 1,579 million (2012: € 1,459 million) as a result of further growth in our solar, wind, biomass and network activities, the development of new products such as our smart thermostat Toon, marketing to gain customers in the Netherlands and Belgium and higher maintenance charges for our networks.
We continue to focus on structural cost control so that we can continue to invest in new sustainable energy production and networks in the future. One example of this is a project started in 2012 which focuses on structuring joint procurement and streamlining ICT costs. One element of this is outsourcing certain indirect activities, such as some internal ICT services. This led to non-recurring additional expenses in 2013.
Everything we do focuses on making the energy supply more sustainable and so we continue to invest substantial sums in our own renewables generation, energy saving and a reliable, modern network. In 2013 we invested € 854 million (2012: € 712 million) and were able to use the tax incentives available in the second half of the year. We invested in the development of solar and wind farms (€ 266 million) in the Netherlands, Belgium and the United Kingdom and in the Golden Raand biomass power station in Delfzijl (€ 33 million in 2013, total € 175 million). We invested a further € 83 million in district heating networks, half of which was in the Leiding over de Noord pipeline, which will transport heat from Botlek to 45,000 customers in Rotterdam and where construction has now commenced. We also invested considerably more in improving and expanding the gas and electricity networks (€ 422 million compared with € 357 million in 2012).
Although the final quarter was mild, the temperature was below normal in every month of the first half of the year, and so our customers’ consumption of gas and heating was higher across the year. The force of the wind remained below the long-term average in 2013. In addition, trends in market prices for electricity, gas and CO2 are still unfavourable for generating electricity in gas-fired power stations and competition in the market is fierce. Expansion of our production, trading and supply activities created an increased margin but also higher expenses. The net effect was positive: profit (EBITDA) rose by 3% to € 290 million (2012: € 282 million).
Revenue from network activities rose this year but there was an increase in costs, in particular for maintenance and investment in the networks. The length of interruptions in energy supply was markedly lower than in 2012 with the average power cut resulting from breakdowns in our electricity network falling by 40% from 35.6 minutes per customer in 2012 to 21.3 minutes this year.
We implemented various efficiency measures in our engineering activities in 2012 and they led to positive results in 2013. In difficult market conditions, revenue rose and the utilisation rate improved. We entered into major contracts with TenneT for the construction of a new high-voltage connection to Goeree-Overflakkee (€ 40 million) and with Total Antwerp for the construction of two switching stations (€ 25 million). Work on both projects started in 2013. EBITDA on network and engineering activities rose by 18% to € 617 million (2012: € 521 million).
We have confidence in the further development of Eneco Group. External factors such as lower regulated transmission tariffs and the continuing negative spark spread will affect the financial results for 2014. Although we will persevere with our sustainability strategy and strict cost controls, we do not believe it will be easy to maintain the net profit for 2014 at the same level as in 2013.