Electricity rates in Japan have been rising significantly due to soaring fuel prices and unfavorable currency exchange rates. Meanwhile, with the widespread penetration of rooftop solar power, feed-in tariffs have been declining gradually, making the shift from selling surplus power to self-consumption more economically advantageous. Additionally, as natural disasters such as torrential rainfalls and massive earthquakes become more severe and frequent, it is increasingly important for homes to maintain a stable power supply both in daily life and during emergencies. While household battery storage is gaining attention as a promising solution to these challenges, standard and simplified operating methods lack weather forecasting capabilities; consequently, these operating methods cannot adequately prepare for reduced solar output during periods of poor weather, such as continuous rainfall, making them insufficient.

To address these issues, this research constructs an ultra-small-scale quasi-microgrid consisting of a group of several households and proposes a battery charge-discharge control scheme utilizing weather forecasting for the next three days, aiming to reduce electricity rates and balance power supply and demand within the group. Furthermore, a coordinated battery control scheme is proposed to enable energy sharing among several households. When consecutive rainy days are forecasted, the proposed scheme increases the battery state of charge (soc) upper limit from 80% to 90%, and the battery is charged using relatively inexpensive purchased utility power during the night before the significant reduction in rooftop solar power output caused by rainy weather. This approach reduces reliance on the grid during rainy days, thereby enhancing energy independence and lowering overall electricity rates.

To validate the proposed scheme, simulations were conducted. A model was constructed consisting of three households, each with a rooftop solar power unit, a residential load, and a battery storage unit, applying actual measurement data at one-hour intervals. The daily energy consumption for each household was set to 12 kWh, with a maximum output of 5 kW for each battery and solar power unit. The simulation compared an independent control scheme, where each battery is operated to balance the supply and demand of its respective household, with the proposed coordinated control scheme, where the three batteries are treated as a single unit to balance the total supply and demand of the three-household cluster. These schemes were evaluated based on the utilization rate of solar power, electricity rate reduction, and the capital cost of the batteries. The simulation results confirmed that, compared to the independent control, the proposed coordinated scheme improved power supply-demand balancing, reduced electricity rates, and lowered the required capital cost.

This research demonstrates that the coordinated battery control scheme based on weather forecasting and supply-demand balancing is effective, providing valuable insights into the optimization of battery storage operation for clusters of households with rooftop solar power units. The full paper will discuss quantitative details through seasonal evaluations and parameter studies, resilience evaluation based on the available stand-alone operation time in the event of power outages, economic assessments including running costs, and energy sharing between multiple microgrid clusters.