Most frequently, community solar offerings will be to buy energy that’s produced by the solar array. This is measured in kilowatt-hours (kWh) and is the same thing you buy from your current energy company. You then get a credit for those kilowatt-hours applied to your utility bill.
Much less commonly, a project may offer you the option to buy a portion of the solar array upfront. In that case you are buying kilowatts (kW) or the capacity to produce energy. Depending on how many kilowatts you buy you will get a certain number of kilowatt-hours produced each month and applied to your energy company bill as a credit.