Flow batteries represent an intermediate point between regular batteries and synthetic fuels.<p>A regular battery costs in capital expenditure per kWh it can store when fully charged. Once full, the only way to store more is to buy more. But efficiency is usually high, and rate of energy in and, particularly, out are high.<p>Synthetic fuels suffer conversion losses, but more storage is as cheap as tankage. The expensive parts are the synthesis equipment -- electrolyser, catalyzer, compressor, cooler -- and stuff to get the stored energy back out, such as a fuel cell or turbine. Those cost per kW, not per kWh.<p>Flow battery storage capacity can be extended by adding tankage and electrolyte. The expensive part is the bit the electrolyte flows through. So, kW in and kW out cost, per, but round trip efficiency is higher. And you can't generally sell somebody charged-up electrolyte, or buy it.