Fixed! Thank you all.
Here's the correct version:
<?xml version="1.0" encoding="utf-8" ?>
<Patch>
<Operation Class="PatchOperationReplace">
<xpath>*/ThingDef[@Name="StandingLampBase"]/comps/li[@Class="CompProperties_Power"]/basePowerConsumption</xpath>
<value><basePowerConsumption>5</basePowerConsumption></value>
</Operation>
</Patch>
Here's the correct version:
<?xml version="1.0" encoding="utf-8" ?>
<Patch>
<Operation Class="PatchOperationReplace">
<xpath>*/ThingDef[@Name="StandingLampBase"]/comps/li[@Class="CompProperties_Power"]/basePowerConsumption</xpath>
<value><basePowerConsumption>5</basePowerConsumption></value>
</Operation>
</Patch>