Yes, a 1W solar panel can charge an 18V battery. It requires proper voltage conversion and a charge controller. The panel should ideally output around 21V. Ensure the battery has enough capacity to store the charge. Use a charge controller to prevent over-charging and protect the battery from damage.
[pdf] Battery Storage System – typically lithium-ion or advanced lead-acid batteries to store excess solar energy. Inverter and Power Electronics – convert DC to AC for practical use and manage system performance.
[pdf] As opposed to independent solar containers that generate electricity alone or independent energy storage containers requiring additional solar components, this technology integrates photovoltaic power generation, energy storage, and smart energy management technologies into a pre-fabricated, weather-tight box.
[pdf] 
