The meters make it easier for householders to sell power they generate through wind and solar back to the grid.

They have the most sophisticated kit, respected brands and healthy balance-sheets, notes Travis Bradford, President of the Prometheus Institute, a solar advocacy group.

The data used in FIGURE 1 and FIGURE 2 is based on the ten-digit Harmonized Tariff Schedule (HTS), the most precise measure of solar cell and module imports available (specifically 8541.40.6020&6030).