Rogers Andrews — Energy Matters — May 11, 2015
Blowout Week 70 featured Tesla’s new 7 kWh and 10 kWh lithium-ion battery storage units. Will they allow households with rooftop solar PV systems to store enough surplus solar power to fill domestic demand throughout the year without the need to import grid power when the sun isn’t shining? It all depends on how much storage is needed and how much it costs, and in this post I present ball-park estimates of storage requirements and costs for domestic rooftop solar installations calculated using the following simplifying assumptions:
- Household consumption is 5,000 kWh/year. (About right for Western Europe, low for the US, high for most of the rest of the world.)
- Household demand is constant through the year at 13.7 kWh/day, or 0.57 kWh/hour.
- The impacts of changes in cloud cover are ignored.
- The storage requirements generated by diurnal fluctuations in solar output are insignificant relative to the storage requirements generated by seasonal fluctuations.
- The storage batteries are 100% efficient, with no conversion losses and no charge/discharge restrictions.
- Solar panels are inclined south at the optimum angle for maximum annual generation.