From Path of the Vampire Wiki
There is a limit to the amount of items each character can reasonably carry in their Inventory. The character's strength as well as any relevant relics determines their personal limit. Exceeding this limit increases the energy cost for movement on the grid.
All characters can carry a minimum of 250 Items. Each point of strength increases their item limit by 10 (ex. at 20 strength, the character could carry 450 items). This limit is displayed at the top of the inventory screen. All items in the inventory, with the exception of Ammunition will count as 1 item.
When a character's inventory exceeds their weight limit, they can still move and function as normal. However, their energy costs for movement will increase at the rate of an extra 0.1 per 50 items over the limit; this is capped at 0.5 energy. When over the limit, each time you move, you will receive an alert in red above the grid indicating you are over your limit and how much extra energy it is costing you to move.
Selling or transferring items until you're below the limit will restore your character to normal energy use levels. If you have excess items you wish to store, you may purchase a lockbox; lockboxes will store up to 200 items once placed in a privately owned location. They are available by karma purchase in any shop.
Human thralls and Doppelgangers have a standardized weight limit of 100 items with much harsher penalties for being over the limit. When a thrall is over their limit, they use 20 energy per movement nor can they receive items from other characters. It should be noted that the Titan's Bracelet does not increase a thrall's weight carry capacity.
Certain items - such as the Titan's Bracelet - allow for the storage of additional items beyond the normal limit set by their weight class without assuming a penalty.