Drives are the deposit. Power is the mortgage.
Purchase price is the number everyone compares. Over five years, electricity and replacements routinely add another half of it — and at high power prices they add more than the drives cost. This works out the real cost per usable terabyte per year.
Electricity is drives × watts × PUE × 24 × 365 × years ÷ 1000 × your rate. PUE covers the PSU inefficiency and the cooling needed to remove the heat — 1.3 is a reasonable figure for a home rack, 1.1–1.2 for a well-run datacenter. Set it to 1.0 to count drive draw only.
Replacements are drives × AFR × years, charged at today's price. Backblaze's published fleet data puts modern enterprise drives around 1–1.5% annualised; refurbished drives with unknown power-on hours run materially higher, so 3% is the sane default there.
Not modelled: chassis, HBAs, switch ports, rack space, or your time. This is the drive-level cost, which is the part that scales with capacity.
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