The Downtime Budget
Every reliability tier is really a downtime budget in hours per year; the burner spends far more of that budget than Tier III allows, which is what redundancy must recover.
Uptime is a budget you spend
It is easier to reason about downtime than about decimals. Tier III's 0.99982 is a budget of about 1.6 hours of outage per year. Tier II is roughly 22 hours; Tier I about 29 hours. The burner's modelled 0.995 spends about 44 hours; its 0.86 low end spends about 1,225 hours — roughly 51 days.
Laid out as hours, the gap is unmistakable and there is no honest way to shrink it with framing. A single burner overspends the Tier III budget by 30 to 100 times depending on where in its modelled range it lands. That overspend is the exact quantity the hybrid architecture exists to recover.
Budgeting also clarifies how to spend the outage wisely. If most of the burner's downtime is planned maintenance, it can be scheduled into low-demand windows and covered by the grid import path, so the disruptive portion of the budget — sudden trips — is what the battery layer must absorb. Separating planned from unplanned downtime is how the site turns a large raw budget into a small felt one.
The discipline is to treat downtime as a resource: measure it, allocate it to planned windows where possible, and back the remainder with independent layers until the felt outage matches the Tier III budget.
- Tiers are downtime budgets: Tier III ~1.6 h/yr
- Burner spends ~44 h (best) to ~1,225 h (worst)
- Redundancy recovers the overspend
- Planned vs unplanned split decides how disruptive the budget feels