Walmart continues to offer the Stanley FatMax 100-watt / 500-watt Power Inverter, model no. PC509, for $27. Opt for in-store pickup to dodge the $6.99 shipping charge. That's the lowest price we could find by $7. It plugs into your car's DC power outlet to provide 100 watts or you car's battery to provide 500 watts and features two 120-volt AC outlets, one USB outlet, and 4-position mounting grommets.