Your article suggests that a 4 kilowatt (kW) solar panel system can save its owners £980 a year at 28p per kilowatt hour (kWh) and £1,575 at the 45p per kWh that is coming soon (Solar panels: how to fix your energy bills while the sun shines, 20 August). This estimate overlooks the important fact that most electricity is produced in summer, and all of it in daylight, while most consumption is in winter and at night.
Our 4kW system has produced about 4,000kWh a year for the last 10 years. Although that is equivalent to our annual consumption we only consume about 30% of what we produce, despite having a household of three stay-at-home pensioners and electric cooking and water heating.
An average day in summer produces 15-20kWh. In winter it is a fraction of that: the total for the whole of January is about 100kWh. This means that interseasonal storage or huge overcapacity would be required to go off grid; batteries at this scale will not be either environmentally or economically appropriate to achieving self-sufficiency.
I have a log of production and consumption in five-minute intervals for one year, and using that data I calculate that with a battery system I could consume an additional 30% of our production. A larger system (8kWh storage) would not save proportionately more than a smaller (3kWh) system, because a large battery will not be fully charged on most days in the winter.
At foreseeable electricity prices a 4kW system with or without a small battery may pay for itself within its lifetime, but not within the five to seven years suggested – although higher feed-in tariffs that reflect the value of the electricity exported would substantially reduce payback times.
Harry Noyes
Alfold, Surrey
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