AC-module inverter(s): OKE4, OK4E, OK4U, OK5 and follow-ups

In 1991 Henk Oldenkamp started the development of the first AC-module inverter: the OKE4. AC-module inverters are also called micro-inverters

The idea is that each PV-module will have its own (micro-)inverter, which is mounted on the rear side of the PV-module, and therefore no DC-wiring is needed anymore.

The first papers about the development of the OK4 were presented in 1993, and by 1999 over 100 000 OK4-inverters were sold and implemented, mainly in the Netherlands.

Why AC-modules?

Traditional photovoltaic (PV) systems require complex DC cabling giving rise to a lot of problems. In particular grid connected PV systems with one central inverter have shown significant problems with respect to:

  • High DC voltage levels.
  • Safety.
  • Cable losses.
  • Risk of DC arcs.
  • Fire hazard and protection

Most of these problems can be overcome by expensive cabling and installation systems, but such solutions increase the costs at system level. Components like inverters, switchgear, cables have to be selected in accordance with system size. This requires an individual design of each system and is an obstacle in expanding the system. Thus, in general the installation of traditional PV systems is rather complex, which is reflected by figure 1.

AC modules, in which the inverter is integrated in the PV module or mounted on the rear side of the PV-module, overcome these problems. The advantages of AC modules compared to traditional PV systems are:

  • Each module works independently: if one fails, the other AC modules will keep on delivering power to the grid.
  • High modularity allowing easy system expansion.
  • Low minimum system size of one AC module, lowering the threshold for individuals to start their own PV plant.
  • Use of standard AC installation material, which reduces costs of installation material and system design.
  • Low conduction losses and cable costs.
  • No mismatch losses at system level as each AC module operates in its own Maximum Power Point (MPP).
  • No need for string diodes.
  • No need for bypass diodes.
  • Low lightning induced surge voltages, because of the compact DC system layout.

The main advantage of the use of AC modules is reflected by figure 2: simplicity of the total system. All these features make AC modules an interesting option for the integration of photovoltaic systems into buildings. Especially the independent operation of AC modules, which prevents that a whole PV system will function poorly due to shading of only one PV module.

Traditional PV-systems with one central inverters

PV-systems with AC-modules


On September 3, 2009, we sold the technology which is implemented in the OK4All, to SMA. For more information download the press release about this acquisition by SMA.

Downloads - in descending chronological order

  • The return of the AC-module inverter
    H. Oldenkamp, I.J. de Jong
    Paper presented at the 24th EU PVSEC, Hamburg, 21-25 September 2009
    Download (word, 598 kB)
  • Next generation of AC-module inverters
    H. Oldenkamp, I.J. de Jong
    Paper presented at the 2nd World Conference and Exhibition on Photovoltaic Solar Energy Conversion, Vienna, 6-10 July 1998
    Download (pdf, 291 kB)
  • AC modules: past, present and future
    H. Oldenkamp, I.J. de Jong
    Paper presented at the workshop Installing the future, 22-23 January 1998, Hatfield, U.K.
    Download (pdf, 490 kB)
  • Nieuwe technologie zorgt voor prijsdoorbraak van OK4-100 mini inverter voor wisselstroompanelen (in Dutch)
    H. Oldenkamp, I.J. de Jong, C.W.A. Baltus, S.A.M. Verhoeven
    Bijdrage aan de Nederlandse Duurzame Energie Conferentie, 17-18 november 1997, Ede
    Download (pdf, 50 kB)
  • Advanced high frequency switching technology of OK4 AC modules inverters break the 1 US$/Watt price barrier
    H. Oldenkamp, I.J. de Jong, C.W.A. Baltus, S.A.M. Verhoeven
    Paper presented at the 14th Photovoltaic Solar Energy Conference, Barcelona, Spain, 30 June - 4 July 1997
    Download (pdf, 105 kB)
  • Reliability and accelerated life tests of the AC module mounted OKE4 inverter
    H. Oldenkamp, I.J. de Jong, C.W.A. Baltus, S.A.M. Verhoeven, S. Elstgeest
    Paper presented at the 25th IEEE Photovoltaic Specialists Conference, Hyatt Regency Crystal City, Washington D.C., May 13-17, 1996
    Download (pdf, 189 kB)
  • Competitive implementation of multi-kilowatts grid connected PV-systems with OKE4 AC modules
    H. Oldenkamp, S.W.H. de Haan, I.J. de Jong, C.W.A. Baltus, C.F.A. Frumau, S.A.M. Verhoeven
    Paper presented at the 13th European Photovoltaic Solar Energy Conference, Nice, France, 23-27 October 1995
    Download (pdf, 126 kB)