When sizing a charge controller for your solar panel, the first thing you need to do is determine the wattage of your panel. A 250 watt solar panel produces about 30 volts and 8.3 amps. The next thing you need to consider is what type of battery you are using. If you are using a lead acid battery, you will need a 12 volt charge controller. If you are using a lithium ion battery, you will need a 24 volt charge controller. Finally, you need to consider the amperage rating of the charge controller. A good rule of thumb is to choose a charge controller with an amperage rating that is double the amperage rating of your solar panel. This means that for a 250 watt solar panel, you would want to choose a 16 amp rated charge controller.