or longer – the amount of electricity produced varies with time of day, moon phase, season, and random factors such as the weather. Thus, renewables in the absence of storage present special challenges to electric utilities. While hooking up many separate wind sources can reduce the overall variability, solar is reliably not available at night, and tidal power shifts with the moon, so slack tides occur four times a day. How much this affects any given utility varies significantly. In a summer peak utility, more solar can generally be absorbed and matched to demand. In winter peak utilities, to a lesser degree, wind correlates to heating demand and can be used to meet that demand. Depending on these factors, beyond about 20–40% of total generation, grid-connected intermittent sources such as solar power and wind power tend to require investment in grid interconnections, grid energy storage or demand-side management. In an electrical grid without energy storage, generation that relies on energy stored within fuels (coal, biomass, natural gas, nuclear) must be scaled up and down to match the rise and fall of electrical production from intermittent sources (see load following power plant). While hydroelectric and natural gas plants can be quickly scaled up or down to follow the wind, coal and nuclear plants take considerable time to respond to load. Utilities with less natural gas or hydroelectric generation are thus more reliant on demand management, grid interconnections or costly pumped storage. The French consulting firm Yole Développement estimates the "stationary storage" market could be a $13.5 billion opportunity by 2023, compared with less than $1 billion in 2015. Demand side management and grid storage The demand side can also store electricity from the grid, for example charging a battery electric vehicle stores energy for a vehicle and storage heaters, district heating storage or ice storage provide thermal storage for buildings. At present this storage serves only to shift consumption to the off-peak time of day, no electricity is returned to the grid. The need for grid storage to provide peak power is reduced by demand side time of use pricing, one of the benefits of smart meters. At the household level, consumers may choose less expensive off-peak times to wash and dry clothes, use dishwashers, take showers and cook. As well, commercial and industrial users will take advantage of cost savings by deferring some processes to off-peak times. Regional impacts from