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 final 10% safety margin
• Separately require enough continuous output for the running load
• For known refrigeration presets, use an explicit starting-watt estimate
• For an unknown motor load, use a conservative 5× running-watt fallback

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

required Wh = (average watts × hours) / 0.85 / 0.85 × 1.1

required output W = running watts, preset starting watts, or up to 5× an unknown motor load plus other loads

Part	What it does
watts	Total simultaneous device load
hours	Target runtime
/ 0.85	Battery discharge loss correction
/ 0.85	Inverter conversion loss correction
× 1.1	Final safety margin
output watts	Checked separately so adequate battery capacity cannot hide an undersized inverter

What That Means in Practice

Setup	Raw need	Minimum estimated requirement
Laptop + monitor + router + phone for 4 hours	640Wh	~975Wh and 160W output
Router + modem for 8 hours	200Wh	~305Wh and 25W output
Small fridge for 8 hours	400Wh using the 50W average runtime model	~610Wh and 800W estimated starting output

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.

Traditional UPS runtime needs a manufacturer curve. Lead-acid UPS runtime changes materially with load, so the current product shortlist covers rated-capacity portable power stations only.

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

Bottom Line

The calculator produces two targets: enough watt-hours for runtime and enough output watts to carry the load. Motor starting demand changes the output target, not the energy formula. Product matches are limited to portable power stations until model-specific UPS runtime curves are available.

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