The Future of Residential Renewable Energy

By SESSA member, Dr Anthony Keen of Cape Town. Gazing into my crystal ball, I see a simple black box. It is a photovoltaic (PV) inverter and the device that I have been waiting for all my renewable energy life.


Into one side of this black box is plugged the power cable connecting me to the grid. The PV and battery connect through the other side and the houseload into the back.


The beauty of this black box is that it automatically, effortlessly and smoothly manages the energy supply to the house, taking power from PV, battery and mains (or smooth blends thereof) in that order of preference while exporting any surplus energy generated by the PV system back into the grid.


My friend, who lives in an off-the-grid situation, replaces his mains input with a standby generator set; it too controlled by the inverter.


Both our ‘batteries’ are lithium batteries for short timescales and hydrogen storage devices with hydrolyser/fuelcell components for longer (seasonal) timescales.


This inverter device with intelligent application could simplify and revolutionise the renewable energy landscape for residential installations because:

  • It would ensure your household used as little total energy as possible
  • It would generate as much renewable energy as you can at home
  • It would run your home as completely as possible from your own power sources and batteries
  • It would not limit production but export your surplus to the grid or your neighbours
  • It would remain connected to both grid and houseload at all times
  • It would pull in energy from the grid only if really needed by the house and/or battery (off peak if possible)
  • You would get well rewarded for all PV generated, less well rewarded for all energy exported, and would pay pro rata at time-of-use rates
  • It would allow you to consider exporting stored energy during grid peak load times
  • It would fill your car with PV and/or export credit electricity during off peak hours
  • All of the above would be automated according to rules you programmed


How far am I from getting my hands on one of these inverters?


The locally developed and manufactured inverter from MLT Drives is sophisticated and already permits many of the above functions, and as such is way ahead of its time. However, some of its processes still need to be set manually.


For example, to cope with variable weather or perhaps even anticipate it, it would need to have the ability to switch over from sunny operation to cloudy operation, as dictated by PV, battery and load status. In ‘sunny’ (off grid) operation, the load may be carried by PV or battery with grid only on standby. For long cloudy operation ‘net-metering’ mode, (PV if available or grid, but now with battery in standby) may be more suitable. The MLT inverter with its energy management capabilities can already do most of this but requires manual changeover from ‘off-grid’ to ‘net metering’ modes.


However, in South Africa at the moment (2011), there is no incentive to invest in back-to-grid technology, and every disincentive because electricity distributors simply won’t allow it. The City of Cape Town is at last initiating a net metering pilot project to explore grid connection. Since 2009, I have been doing my best to follow the philosophy of the black box dream, though.1


Voltwerk in Berlin, Germany, recently built an inverter that does automatically switch from ‘off-grid’ to ‘net metering’ modes as part of a European research project (Sol-ion Project) integrating PV, battery and grid in a home power management system. The goal is to optimise household power requirements in respect of grid operation and PV/battery with minimal homeowner intervention.2


It is to be expected that major inverter manufacturers will follow suit, encouraged by the informed German regulations offering financial incentives for self consumption. Another German firm, SMA, is promoting the concept of self consumption . See:


Furthermore, this company is developing a Home Energy Management system (‘Sunny Home Manager’) for release in 2012 that will fit in with its existing products. This is getting closer and closer to my simple black box, and I anticipated SMA would copy MLT and build a single neat package to contain all the functionality in its inverters. See:


With Germany recently extending self consumption incentives to larger commercial systems, SMA is promoting this as well. See:


Perhaps my dream inverter will be a reality far sooner than my crystal ball says!



Dr Keen’s steps to independence from the grid:


Dr Keen has won the Energy Savings in Households category at the eta Awards twice – once in 2008 for reducing his family’s consumption of electricity by 71% and again in 2011 for his exploration into how best to manage energy from three sources – photovoltaics (solar), battery and grid – to satisfy the very variable demands of a home while living mostly by sunpower.


A three-step process brought the Keen’s the average annual monthly consumption down from 1263 to 350 kWh, an overall 71%.


  • Step 1: Utilise solar water heating and wood burning fireplace. The Keen family switched off all power to their geysers for nine months of the year cutting electricity consumption by 44% averaged over a year.
  • Step 2: Install energy efficient lighting including compact fluorescent lighting; improve ceiling, geyser and hot water pipe insulation; draught-proof doors and windows; install a geyser time and adopt a more energy conscious lifestyle as well as replaced appliances with energy efficient versions. This reduced consumption by a further 20%.
  • Step 3: Install a larger swimming pool pump, filter and chlorinating system cutting the running time to only 4 hours a day and cover the pool for winter (seven months) to that the pump could be turned off without compromising water quality or clarity. These steps saved a further 7%.




1) Keen G A Lloyd PJ 2011 Conference Proceedings: Domestic Use of Energy 2011, Cape Peninsula University of Technology, Cape Town, April 2011 “Domestic Application of a PV System with Self Consumption”


2) Schmiegel A U et al 2010 Proceedings of the 25th European Photovoltaic Solar Energy Conference 6-10 Sept 2010, Valencia, Spain, pp3803-3805. The Sol-Ion System, an integrated PV system with Lithium Batteries – System Performance.




Late last year, Dr Keen was named – for the second time – as the winner of the Energy Savings in Households category at the 2011 eta Awards. The awards, run by Eskom and the Department of Energy for the past 21 years, take their name from (eta), the Greek symbol for efficiency. Their purpose is to reward exceptional effort in the more efficient use of energy by individuals, students, companies or other institutions.


A medical doctor, Dr Keen and his family have strived, since 1984, to bring down their monthly electricity consumption. Their success rate is a phenomenal 71%, from 1 263kWh to 350kWh, and has been achieved using a system designed and mostly implemented by Dr Keen himself. They’ve also pioneered the exploration in this country of how to best manage energy from three sources – PV, battery and grid – to satisfy the very variable demands of a home while living mostly by sunpower.


The Sustainable Energy Society of Southern Africa (SESSA) is dedicated to the use of renewable energy and energy efficiency technologies including solar-based energies (such as photovoltaics, thermal heating and cooling), wind, bioenergy and hydro.


Its current objectives include:

  • Promoting and increasing the use of renewable and energy efficient technologies with informal education, demonstration and information dissemination to end-users and decision makers of all levels;
  • Establishing the society as the main regional information centre in close co-operation with similar initiatives;
  • Facilitating the creation and maintenance of appropriate standards for products, systems, methods and training;
  • Supporting initiatives that promote the creation of local ‘green-collar’ jobs and highlight green enterprise opportunities;
  • Lobbying government, big business, civil society and parastatal organisations towards adopting a low carbon energy future.

, ,