Know what’s behind the number.
DeskReserve is a planning and comparison tool for small, relatively steady home-office loads. We use manufacturer specifications, not hands-on product tests. Results are scenarios rather than promises of battery runtime.
The calculation
Runtime (hours) = capacity (Wh) × AC efficiency × (1 − reserve) ÷ (device watts + idle watts)
For a 1,024 Wh battery running a 96 W setup, using 85% AC efficiency, 10% reserve and 10 W idle draw, the model estimates about 7.4 hours. Required capacity reverses that formula for your target runtime. The output check adds 20% headroom to the entered continuous load.
What we assume
- AC conversion efficiency: 85%. This is an adjustable conversion assumption, not a manufacturer’s tested usable-capacity figure. We model idle draw separately. If you use a measured all-loss efficiency, set additional idle loss to zero to avoid counting it twice.
- Reserve: 10%. We leave a portion of rated capacity unused to provide a planning cushion.
- Idle draw: 10 W by default. This is an assumption, not a measurement. Where a manufacturer publishes a standby figure, that figure overrides the default for that model. Upper-bound figures are used conservatively.
- Range: a sensitivity scenario using 75–90% conversion efficiency and, for unknown models, 5–15 W idle draw. It is not a statistical confidence interval. Your settings extend this range when more extreme.
- Device watts: presets are examples. Measure your real draw, particularly for desktop computers, monitors and variable laptop charging.
Runtime is different from UPS protection
Rated capacity tells you how much energy a battery stores. Transfer time describes the switch from wall power to battery in supported modes. A 10–20 ms interruption can still reboot equipment. We never infer UPS compatibility from capacity. Check your exact model’s manual and your devices’ requirements.
Scope and limits
Motor starts, heaters, cooking appliances, refrigeration, medical equipment, solar input, battery aging and cold-weather performance are outside this calculator. It assumes individual devices plugged into AC outlets. It does not provide home-wiring or transfer-switch instructions. A battery cannot restore a downed internet-provider network.
Prices, stock and ranking
Prices are for the specified standalone US variant, before tax, shipping or optional accessories. We do not substitute the cheapest bundle, member price or a different color. A price older than seven days is hidden from price sorting and marked for checking. “Orderable” does not guarantee stock or immediate shipment. Results sort by modeled runtime/output fit, then known price. Unknown prices sort last within each fit group; unavailable products are excluded from recommendations, while preorders require opt-in.
Routine price monitoring can update an exact verified variant when its price changes by no more than 15% and its availability category stays the same. New prices and larger changes go to owner review. Specifications and affiliate destinations are never changed by the price agent.