Prebuilt (BTO) vs Self-Built PC: Cost and Trade-offs
Self-building is often said to be cheaper. Depending on the parts, it is not. Here is where the difference actually comes from.
Where the price difference comes from
The gap is less about part prices and more about which parts a vendor can cut. Power supplies, cases, and cooling are where cost is reduced, because those rarely appear in a spec sheet.
CPU and GPU model numbers do appear, so they cannot be cut without being noticed. The same CPU and GPU in a prebuilt does not mean the same power supply.
Self-building is cheaper when you match quality, not when you match model numbers.
| Part | How it is cut | What it costs later |
|---|---|---|
| 電源ユニット | 容量だけ書き、変換効率や保護回路は書かない | 高負荷で落ちる。他の部品を巻き込むことがある |
| ケース | 見た目は同じで、鋼板が薄く風の通り道が無い | 温度が下がらず、GPUが本来の性能を出さない |
| CPUクーラー | 付属品のまま | 高負荷時にCPUが自分で速度を落とす |
| メモリ | 枚数と容量だけ書き、速度と枚数構成を書かない | 1枚挿しだと帯域が半分になる |
| SSD | 容量だけ書き、DRAM有無や種類を書かない | 書き込みが続くと速度が大きく落ちる |
When self-building is the wrong choice
The assembly is not hard. Diagnosing a machine that will not boot is. Without spare parts you cannot isolate the fault.
Most no-boot cases are not failures: memory in the wrong slots, an unplugged CPU power cable, or a BIOS too old for the CPU. Minutes if you know them, days if you do not.
If you need a shop to hand it to, or cannot afford downtime, a prebuilt is cheaper overall. Time is part of the cost.
Common first-build mistakes
Physical fit and order of operations cause far more failures than part incompatibility. All of these are checkable before buying.
| What to check | Where | If you miss it |
|---|---|---|
| GPUの長さ | ケースの対応カード長 | 蓋が閉まらない。前面ファンと干渉する |
| CPUクーラーの高さ | ケースの対応クーラー高 | 側板が閉まらない |
| メモリの背の高さ | クーラーとの干渉情報 | 空冷の大型クーラーと当たる |
| 電源の奥行き | ケースの電源スペース | ケーブルが曲がらず挿さらない |
| マザーボードの大きさ | ATX / MicroATX / Mini-ITX | ネジ穴が合わない |
| CPUとマザーボードの世代 | 対応ソケットとBIOS版数 | 起動しない。BIOS更新が先に要る |
Frequently asked questions
Which is actually cheaper?
Matching CPU and GPU models, self-building is usually cheaper. Matching the quality of the power supply, case, and cooling narrows the gap. Prebuilts tend to win at the low end, self-builds at the high end.
Is there a warranty on a self-built PC?
Each part has its own warranty; the machine as a whole has none. You must identify the faulty part yourself before contacting that maker.
How long does assembly take?
Two to three hours of assembly. Budget half a day for a first build including OS and drivers, and more if something does not work.