The price of renewable energy is dropping rapidly. Energy storage will be needed to take full advantage of abundant but intermittent energy sources. Even with economies of scale, the price is prohibitively high for a lithium-ion battery pack capable of storing tens of kilowatts of energy for many consumers. A more abundant and less expensive material is necessary. All-iron chemistry presents a transformative opportunity for stationary energy storage: it is simple, cheap, abundant, and safe. All-iron batteries can store energy by reducing iron (II) to metallic iron at the anode and oxidizing iron (II) to iron (III) at the cathode. The total cell is highly stable, efficient, non-toxic, and safe. The total cost of materials is $0.1 per watt-hour of capacity at wholesale prices. This battery may be a useful component of open source hardware projects that require a safe and ecologically friendly battery. This is also one of the few battery chemistries that can be built safely in a DIY setting.