Guides

Backup power fundamentals

Start here before choosing a backup system. These short guides explain the inputs used by Backup Power Map and the checks that a calculation cannot replace.

1. Watts and watt-hours answer different questions

Watts (W) describe power at a particular moment. Watt-hours (Wh) describe energy over time. For a steady load, multiply watts by operating hours. For several devices, add their individual energy use.

Example: a 40 W light used for 5 hours consumes 200 Wh. Two such lights consume 400 Wh and draw 80 W while both are on. These are illustrative inputs, not a typical rating for every light.

A 200 Wh energy requirement does not mean a product advertised as a 200 Wh battery will deliver that energy to your equipment. Battery operating limits and conversion losses are a separate question.

Source: U.S. Energy Information Administration: measuring electricity.

2. Keep total and additional starting watts separate

Some devices have a brief starting demand above their normal running demand. Check the manufacturer’s information for the exact device and starting method. Do not multiply all appliances by a universal startup factor.

If a device runs at 100 W and requires 300 W in total at startup, its additional starting demand is 200 W. If other devices draw 20 W, this startup scenario is 320 W: 120 W of combined running load plus 200 W extra. Adding the full 300 W to the 120 W would count the first device’s running load twice.

Our calculator’s one-device-starting scenario uses total running watts plus the largest additional startup demand. It also shows all devices starting together when all startup entries are known. These are simplified watt scenarios. An inverter must also support the required current, VA where specified, and duration of the surge.

Sources: Generac Wattage Worksheet, PDF page 25; Victron inverter selection guidance.

3. Build a device list from your equipment

1. Identify the devices you need and which may run at the same time.
2. Find electrical input watts in the device manual or rating label. Heating output, cooling capacity and microwave cooking watts are not automatically electrical input watts.
3. Record startup information separately. If you only have amps or VA, do not silently treat them as real watts.
4. Set the operating time during your planned outage. A refrigerator being plugged in for 12 hours does not mean its compressor draws the stated running watts for all 12 hours.
5. For cycling or varying loads, compare your estimate with representative energy measurements. Include standby consumption where relevant.

Do not use a whole-day average wattage as the device’s instantaneous running or startup rating. Energy measurement and output sizing serve different purposes.

Source: Virginia Tech: estimating appliance energy use.

4. Treat battery capacity as a conditional estimate

Rated battery energy, energy available within its permitted discharge range, and energy delivered through an inverter are different quantities. Efficiency depends on the equipment and operating conditions. A peak-efficiency figure alone does not account for the entire discharge period.

Backup Power Map V1.1 leaves battery inputs blank until you have evidence for the specific system. It models load-specific inverter efficiency, usable DC energy, additional idle consumption and an explicit capacity allowance. The method section shows the equation and its conditions. Any reserve or design allowance needs an explicit basis; it is not automatically a universal electrical-code requirement.

Check continuous output, startup capability, voltage, frequency, individual outlets, and battery discharge limits before choosing a product. Household wiring, pumps and other hardwired loads require an appropriate installation assessment. These guides are not an NEC service or branch-circuit calculation.

Source: Victron: continuous output, surge, temperature and battery limits. Sources reviewed September 12, 2026.

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