Stainless steel hot water cylinders should be specified with solar hot water systems to allow water storage at higher temperatures, storing more solar energy. Daily usage might require 9kW, so 3 hours at 3kW in winter. Twin elements allow for greater flexibility in electrical boosting and therefore greater savings. Get 220v and 48v elements (3kW)
Could use a pre-heat small tank for solar and feed into a standard main tank.

315 L Dux Hot Water Cylinder Twin 3.6kW or 4.8kW elements

• Storage Capacity (L): 321L
• Hot Water Delivery Rating (L): 315L
• Height: 1765mm
• Diameter: 620mm

Features

• Dual handed plumbing connections
• Internal heat trap
• Coated steel finish
• Features durable Y class commercial grade enamel
• Hard-wearing, tough polymer base resists damage and is rust-proof
• Feature 50mm thick, dense foam insulation for less heat loss and lower running costs
• Water connections on both sides of the cylinder
• Screwless cylinder lids offer a cleaner appearance and are safer too
• Low maintenance and long service life
• Full flow pressure to all outlets
• Can be installed indoors or out
• 10 years cylinder warranty, 1 year parts and labour