Ukraine's economy as the rest of Europe's is heavily relies on the natural gas supplied from Russia. In addition to that Ukraine is also dependent on the Donbass coal mines. But it doesn't have to be. Ukraine has the higher solar irradiation potential than Germany - the country with the highest density of the solar generated power in the world.
Ukraine could have much bigger energy potential with solar and other alternative technologies properly configured and installed. They could have completely replace the need for natural gas and coal in the country.
The best defense for Ukraine and the best "weapons" to be sent there would be renewable technology - to make it energy independent of fossil fuels and of Russia's economic pressure. Same is true for the rest of Europe.