How Our Calculator Picks Your Recommendation

Sizing backup power is not complicated, but it is annoyingly easy to get wrong by hand. A missed efficiency loss, a forgotten surge, or an optimistic guess on runtime can push you into the wrong battery class very quickly.

The calculator exists to make that process explicit and repeatable. It is not a black box with mystery recommendations. The logic is simple enough to inspect.

Quick Answer

• Start with total running watts
• Multiply by the hours you need
• Divide by 0.85 twice for battery and inverter losses
• Add a 30% surge buffer for motors and compressors
• Add a final 10% safety margin

This usually means a setup needs more battery than the raw watts × hours number suggests. For the battery-loss side of that problem, see You Only Get Half the Battery.

The Formula

(watts × hours) / 0.85 / 0.85 × surge_multiplier × 1.1

Part	What it does
watts	Total simultaneous device load
hours	Target runtime
/ 0.85	Battery discharge loss correction
/ 0.85	Inverter conversion loss correction
surge multiplier	1.3 for motor/compressor loads, otherwise 1.0
× 1.1	Final safety margin

What That Means in Practice

Setup	Raw need	Adjusted recommendation
Laptop + monitor + router + phone for 4 hours	440Wh	~670Wh
Router + modem for 8 hours	200Wh	~305Wh
Small fridge for 8 hours	1200Wh raw label-style thinking is misleading	Runtime depends on average draw plus surge headroom

Darker cells mean a smaller requirement. Real appliance behaviour can change the runtime side of the estimate.

For fridge-specific maths, see Your Fridge Uses Less Power Than You Think and What Is Startup Surge?.

Try It in the Calculator

Setup	Scenario	Open
Laptop + monitor + router + phone	4 hours	Calculate
Small fridge	8 hours	Calculate
Router + modem	8 hours	Calculate

What People Miss

Raw watt-hours are not the final answer. Losses and safety margin move the number materially.

Surge changes appliance sizing. Fridges and similar loads need different treatment from routers and laptops.

Context matters after the maths. The same capacity target can point to a UPS, a power station, or a different portability tier depending on use case.

The calculator is conservative on purpose. Running out early is worse than a little extra headroom.

Bottom Line

The calculator takes a messy set of small corrections and turns them into one clear capacity target. The core logic is simple: total the load, correct for losses, add surge where needed, add a safety margin, then match real products against that result.

If you want the number without doing every step manually, try this in the calculator. If you want to audit the logic, the formula above is the logic.