REAL RUNTIME STARTS WITH WATT-HOURS, LOADS AND LOSSES—NOT A BOX LABEL.
A battery pack is labelled 1,024Wh. Your router uses 12W, a light uses 8W and a laptop draws much more while charging. How long could they operate if the electricity went off? A reliable estimate needs more than dividing the number on the battery by the wattage on the appliance label. You need to consider the battery's starting charge, its protected reserve, output losses and the actual load.
Use the working emergency power calculator below to explore those variables, then read the worked examples and safety limits before relying on a result. The tool is a planning model, not a claim that a particular battery will achieve the displayed runtime. It deliberately treats charging from solar panels as a separate problem because the sun does not supply a constant amount of power.
This article is for readers who want practical options for temporary power during an outage. It does not assume a blackout is coming or tell everyone to buy equipment. If your purpose is to compare portable solar stations, see our solar generator runtime guide. If the equipment is built into the house, read the installed home battery guide.
Emergency power calculator: estimate hours remaining
Enter the numbers from a battery's specification and the actual devices you plan to power. The displayed hours assume a steady load. Motor-driven equipment, fridges, variable laptop use and cold batteries can behave differently. Never use a result from this tool to set a safety deadline for medical equipment.
This estimate assumes a constant measured load. Check inverter continuous and starting-surge ratings separately.
How the calculator works
It starts with the battery's rated capacity and uses the difference between starting state of charge and minimum reserve. It then applies an output-efficiency assumption and divides the estimated energy delivered to connected appliances by the combined average appliance load and idle draw.
Estimated runtime in hours = rated Wh × (starting % − reserve %) ÷ 100 × efficiency ÷ 100 ÷ (average load W + idle draw W).
For the default example, 1,024Wh at 100% charge with a 15% reserve leaves 870.4Wh before conversion. An illustrative 85% output efficiency makes 739.84Wh available. A 45W device plus 8W inverter idle draw represents 53W, giving approximately 13.96 hours. That is an arithmetic illustration—not a manufacturer test result. At very low loads, idle consumption can substantially reduce runtime.
Do not also subtract the same losses elsewhere. Some manufacturers already state a battery's usable energy or AC-delivered energy rather than nominal cell capacity. Read the specification to avoid applying the reserve or conversion penalty twice. Electrical Safety First's appliance-rating guidance helps distinguish appliance power ratings and actual use.
What to enter for a router, phone or laptop
Where possible, measure typical energy consumption with a compatible power meter while normal electricity is available. Appliance labels can show maximum rated input rather than steady usage. A laptop that sometimes uses 65W may average much less while idle. A router may have a low continuous draw but cannot provide internet access if the external provider network is down.
| Appliance | Illustrative input | Important qualification |
|---|---|---|
| LED light | 8W average for five hours | 40Wh if the light really stays near 8W |
| Router | 12W average continuously | 288Wh over 24h, but depends on network availability |
| Laptop | 45W average during a task | Load changes while charging or running demanding applications |
| Phone charging | 15W over two hours | 30Wh at that assumed steady charging input |
| Fridge | Measure daily Wh | Compressor cycles; peak surge must be checked separately |
These are example values, not verified figures for your own products. If you want to estimate several appliances together, add their simultaneous average watts to enter one combined load, then separately check whether the total instantaneous load fits the station's maximum output.
Why starting surge is not included in runtime
Inverter output ratings specify continuous power and often a separate short-duration surge. A fridge compressor or pump may need a high starting wattage even though its daily average consumption is modest. A battery could have enough energy for a long runtime but still fail to start that appliance.
Check the appliance documentation or a suitable professional measurement for startup power. If the power station's surge rating is insufficient, the calculator cannot make it compatible. Never bypass fuses or protection circuits to force a device to start. When safety or expensive equipment is at stake, consult the supplier.
Calculating energy for devices used intermittently
If a device runs for only part of the day, calculate its watt-hours separately: watts × operating hours = watt-hours. For example, a 40W laptop used for three hours requires about 120Wh; a 10W light for six hours needs 60Wh. Their combined demand is 180Wh before output losses.
You can then estimate the battery energy required to deliver that demand: load Wh ÷ assumed delivery efficiency. With an illustrative 85% efficiency, 180Wh of output would draw approximately 212Wh from battery energy, plus any inverter idle demand during the time the output remains on. This approach can be more accurate than pretending devices run constantly all day.
If devices operate at different hours, build a simple schedule and calculate the energy in each window. The calculator's single-runtime output is best understood as a steady-load snapshot, while a schedule handles mixed duty cycles.
Battery capacity is not the same as the inverter output rating
A 512Wh station might support a high-wattage device briefly or a low-wattage load for much longer, provided the inverter permits that demand. Conversely, an appliance drawing 1,200W may exceed a station's continuous 600W output regardless of the battery's remaining watt-hours.
Check both numbers on the manufacturer's technical sheet and distinguish nominal from usable capacity. For permanently wired household batteries, also identify which circuits are backed up and whether a qualified installer has provided correct grid-isolation protection. Our home battery backup article covers those differences.
What changes when the battery is partly charged?
A unit reading 60% has less energy available than one at 100%, and an owner may deliberately reserve part of that energy rather than discharging it completely. For a battery rated 1,000Wh, at 60% with a planned 20% reserve, the portion between those points is 400Wh before output losses. At 85% efficiency the arithmetic gives 340Wh available to the load.
If an inverter uses 5W while it is on, include that consumption alongside the devices; the calculator has a separate idle-draw field. Some stations turn off automatically at very low demand, and performance varies with temperature and battery condition. The displayed estimate cannot model all device behaviours without specific measured data.
How solar charging changes the equation
Solar input can extend operating time, but it is variable. A panel rated 200W doesn't deliver that continuously under normal conditions. Cloud, shade, orientation, panel temperature and station input limits affect how many watt-hours are generated and retained in the battery. Don't add the printed panel rating directly to the steady-load field.
For a separate energy budget, add the measured Wh charged during the day and subtract the Wh used by devices. If the station receives 250Wh after charging losses but devices consume 400Wh, stored energy falls by approximately 150Wh before any additional omitted consumption. US Department of Energy guidance distinguishes standard panel ratings from actual operating performance.
For realistic solar assumptions rather than a fixed daylight multiplier, see solar generator charging and runtime.
A paper worksheet for a complete evening
| Step | Example input | Result |
|---|---|---|
| Battery rated capacity | 768Wh | 768Wh |
| Start and reserve | 90% start, 20% reserve | 70% of nominal, or 537.6Wh |
| Output efficiency | 85% illustrative | 456.96Wh to loads |
| Four-hour device use | 30W average for four hours | 120Wh |
| Idle output consumption | 6W for the same four hours | 24Wh |
| Estimated budget remaining | 456.96Wh minus 144Wh | 312.96Wh output-equivalent |
This estimate assumes the efficiency percentage applies to the energy delivered and the device load is measured independently of the inverter's idle draw. It leaves a buffer only if the assumptions prove reasonable. A real station's battery management system may limit output earlier.
A safe test that improves your estimate
- Use an approved charger to set the station to a known state of charge.
- Connect a non-critical compatible device with a measured load, such as a low-watt LED light.
- Record the initial time, reported percentage and the load indicated by the station.
- Run the device for a reasonable interval without approaching the battery's protective cutoff.
- Record the remaining charge and energy used if available; compare with the calculation.
- Note ambient temperature, whether the inverter remained on and whether other devices were connected.
- Refine the assumptions; do not generalise one result to every device or season.
Never run down a battery connected to life-support equipment as a test. Do not use damaged plugs, charge near flammable materials or connect a power station to home wiring through a wall socket. The Electrical Safety First guidance explains why overloaded and improvised connections are hazardous.
What the calculator cannot decide for you
It cannot verify a battery's health, calculate a fridge's varying compressor surge, ensure a solar panel is wired correctly, determine medical-device compatibility or predict how long an outage will last. It also does not model ageing, severe cold, changing input demand or battery-maker-specific shutdown behaviour.
If the concern is simply preserving phone calls and basic light, a modest independent charging option may provide more useful flexibility than an unnecessarily large system. For an outage affecting essential medical equipment, use a professional contingency plan. For the broader first-hour choices without electricity, see our blackout first-hour guide.
How to use an uncertainty margin without fooling yourself
The numbers from a calculator are conditional: they answer what would happen if the input assumptions held. An output of 13.96 hours is not a precise forecast. Instead, perform a second estimate using less favourable conditions, such as a higher load, lower starting charge or lower delivery efficiency. If a modest change in assumptions cuts runtime dramatically, the plan is sensitive to measurement error.
For instance, suppose you thought a router and accessories used 20W, but a meter later shows 30W. With 400Wh available at the output and 5W idle draw, the first assumption gives 400 ÷ 25 = 16 hours; the updated load gives 400 ÷ 35 ≈ 11.43 hours. The six-hour difference is not an equipment failure; it is the effect of a wrong input. This is why measuring representative use is more valuable than adding extra decimal places to an answer.
You can compare a conservative and a favourable case in the calculator, then write down the range instead of promising a single hour of shutdown. If any connected device has health or safety significance, independent backups and a professional plan should not depend on either estimate. The calculation can guide a non-critical trial, but it cannot replace verified operating procedures.
Frequently asked questions
How do I calculate battery runtime from watt-hours?
Estimate the usable energy remaining after reserve and conversion losses, then divide by the combined average appliance and inverter-idle wattage. Verify the battery's power limits separately.
Does 1,000Wh mean a 1,000W appliance runs for one hour?
Only in ideal arithmetic with no reserve or conversion losses, and only if the inverter can actually support that continuous appliance load. Real operating time will usually differ.
Why does a 20W device sometimes flatten a battery faster than expected?
The inverter and other internal electronics may draw power even when the connected device uses little. Battery protection reserves, ageing, temperature and automatic shutdown settings can also affect results.
Can I rely on this calculator for a medical device?
No. Critical devices require a supplier- and clinician-approved backup arrangement, verified compatibility, monitoring and a plan for what happens if the supply fails unexpectedly.