Printing a poster at home or at a print shop
A shop hands you one sheet with no seams. At home you assemble several and pay in time instead of money. The decision is almost always about which of those two you have more of.
What you are actually choosing between
Both routes end with a poster on a wall, so the question is not whether a home printer can do it. It can, and it does not need to be a special one. The question is what a print shop sells you that a home printer cannot, and that list is short: a single unbroken sheet, paper heavier than your tray will take, and someone else doing the work.
Everything below is a way of asking what those three are worth on the particular poster you have in mind. For one framed piece that will hang for years, quite a lot. For a wall chart, a map, a mock-up or anything you expect to reprint next month, close to nothing.
What each one costs
At home the cost is paper plus ink, and it scales with the number of sheets, which scales with area. There is no floor and no minimum: a four-sheet A2 costs four sheets, and printing it twice costs eight.
A shop prices the other way round. Large-format work carries a setup cost that dominates small jobs, so the first poster is expensive and the tenth copy of it is cheap. That shape is what makes the honest answer awkward: for a single poster a shop is often far more competitive than people assume, and for anything you expect to revise it is far worse, because every revision is a new job and another trip.
The cost that gets left out of the comparison is the failed print. At home a mistake costs a few sheets and ten minutes. At a shop it costs the whole job, and you usually discover it after you have paid.
Where the difference actually shows
Seams, first. No home method removes them — an overlap leaves a visible ridge, butted sheets leave a visible line — and no amount of care changes that up close. At the distance a poster is actually looked at, two or three metres, a carefully assembled one reads as a single image and nobody notices. At thirty centimetres they always do.
Colour is the quieter difference. A large-format printer lays the whole poster down in one continuous pass; a home printer prints your sheets one at a time, and a cartridge that runs low in the middle of a sixteen-sheet run puts a visible step across the picture. Print the whole set in one go, from the same tray, before you use the machine for anything else.
Heavy ink coverage is the third. A dark, full-bleed image on ordinary 80 gsm (20 lb bond) paper will cockle: the sheet buckles as it dries and the seams stop meeting. Heavier paper fixes it, which is exactly the thing a home tray limits.
The size where a home printer stops making sense
Sheet count grows with area, which means it grows with the square of the size. That is the whole story. A2 is four sheets and a pleasant twenty minutes; A1 is eight; A0 is sixteen at best and closer to twenty once you allow for the overlap. Somewhere in there the assembly stops being a small task at the end and becomes the task.
For most people the line falls just past A1. Below it the seam count is low enough that a careful job looks deliberate. Above it you are aligning twenty sheets on a floor, and the odds that all twenty printed identically are not in your favour.
What a shop can do that you cannot
One continuous sheet at any size. Paper weights and finishes a home tray will not feed: matte, satin, canvas, board. Lamination and mounting. And real colour management, if you supply a profile and care about the print matching a screen you calibrated.
If the poster is a gift, a piece of work you are being paid for, or something that will hang in a room you show people, this is the list that decides it.
What you can do that a shop makes painful
Iterate. Print at two in the morning. Print a size nobody sells — a poster that has to fit an alcove 68 cm wide is a special order at a shop and an ordinary afternoon at home. Print at 1:1 because the dimensions are the point, which is most of what patterns, maps and plans are for. And print the things you would rather not hand to a stranger behind a counter.
A middle path worth knowing
Proof at home, then order. Tile the poster, print it, tape it up and live with it for a day. It is the only reliable way to find out that the crop is wrong, that the type is too small at that size, or that colours which looked deep on a screen are muddy on paper. Then send the corrected file to a shop and pay once.
A few sheets of paper is a cheap way to avoid learning all of that from a print you have already collected.