Start with your devices, not a battery label. The illustrative outage plan needs 1,593 Wh of daily device energy and about 2,343 Wh nameplate after the stated allowances. That is a worked example, not a standard size for every home. The same method also covers travel loads. For deciding which household needs come first, read the separate power-outage guide.
A load list beats a guess.
Choose a starting point, replace the typical device values, and see energy, continuous output and surge as separate checks.
Size the job before the battery
The right station is the one that supports your priority devices for your intended time, with a realistic route to recharge. Start by writing down what must stay on and what can wait. Keeping communications, light and cooling available is a different job from recreating every household convenience. A short list also makes the result easier to verify against device labels and the exact station manual.
For each device, record quantity, running watts, starting watts where relevant, daily use and duty cycle. Add the planned outage or trip duration. Separate essential background loads from equipment you can schedule. A laptop charger can often wait while another flexible load runs; a refrigerator may restart without warning. The calculator’s scheduling toggle only works if you can actually manage the schedule you enter. It does not reduce the total energy needed for those daily hours.
The capacity formula, with the assumptions exposed
For device energy, multiply quantity by running W, hours and the active share of that time, then add the rows. The full calculation is shown on our methodology page. Our default 85% inverter efficiency and 20% unused reserve are editable planning choices, not facts about every battery. The efficiency allowance and reserve serve different purposes, so both appear explicitly.
Reserve is capacity you intend to leave unused. Dividing by the remaining usable fraction is different from merely adding the same percentage to the load. Battery-side inverter idle energy is added after converting the AC device load, avoiding another conversion allowance on that already battery-side quantity. If you do not know idle draw, the default omits it and the calculator says so. Do not interpret that omission as evidence that your chosen inverter consumes no energy.
Worked plans: outage, CPAP and camping
These are transparent arithmetic examples built from published typical device watts and explicitly chosen schedules. The schedules are not promises about how long your appliances should run. Each result applies the methodology defaults, omits idle draw and rounds upward for shopping capacity. Open the calculator to replace every assumption with your own plan.
Read the CPAP power guide for heat-on/off examples, DC options and backup-planning limits. The CPAP example is an energy illustration, not a medical-device backup recommendation. Heated humidifiers and tubing can change consumption, and the required power adapter may affect the complete system. Confirm a reliable backup arrangement with your equipment provider and clinician. A station that works as an occasional travel power source is not automatically an approved substitute for the battery system your therapy equipment requires.
Size classes are a navigation aid
We use the following editorial classes to make calculator results easier to scan. The boundaries are our own interface labels, not industry standards or guarantees about the appliances a station can run. In particular, a compact battery with an unusually strong inverter is still limited by its stored energy. An extended-capacity battery with a weak inverter can still reject a demanding startup.
| Our planning class | Nameplate range | Look closely at |
|---|---|---|
| Compact | Up to 500 Wh | Ports and the energy needed overnight |
| Everyday essentials | Above 500 to 1,500 Wh | Real load duration and startup demand |
| Extended backup | Above 1,500 to 3,000 Wh | Carrying weight and recharge access |
| Large backup | Above 3,000 Wh | Installation plans and expansion limits |
Use the class to narrow a search after doing the load calculation. Do not work backwards from an attractive class label to an assumed runtime. The same advertised capacity can represent a generous trip plan for lights and device charging, or a brief bridge for continuous high-power use. Keeping the load list visible makes that difference concrete.
Usable energy is smaller than the headline
Energy stored in the cells is not identical to energy delivered through an AC socket. OutdoorGearLab reports 60–93% usable AC energy in its tested set. This is attributed research rather than a MySunSpark measurement. Its results support treating nameplate and outlet energy separately, but they do not establish a universal conversion efficiency for your load or station.
Very light loads, battery condition, temperature and automatic shutdown behaviour can make simple estimates less representative. Keep a reserve, document the assumptions and update the load list after observing your own equipment. Check that the station does not disable the relevant outlet when a cycling device rests. A correct daily energy total is not enough if an energy-saving mode turns off the socket before the appliance needs to restart.
Wh, running W and startup W answer different questions
Watt-hours describe energy over time. Running watts describe the load while equipment is operating. Startup watts describe a brief demand as a motor starts. A battery can have enough stored energy for the schedule yet fail to start a compressor. Conversely, a strong inverter can run an appliance but exhaust a small battery before your planned session ends.
The fridge preset uses 150 W running, 35% duty and 1,200 W startup. Duty reduces the energy calculation because the compressor cycles; it does not shrink the inverter rating needed while the compressor runs. Preserve both values when you replace the preset with information for your appliance. A daily energy label can help with runtime, but does not establish startup demand.
Our startup calculation considers a starting device alongside the other concurrent loads, not every motor starting together. A resistive boost or lifting mode is not automatically a motor-start rating. Cards with an unverified surge remain unverified rather than gaining a reassuring tick.
The checked US Jackery example uses 120 V AC; the checked UK C1000 manual specifies 230 V AC. Match the local voltage, frequency, plugs and adapter requirements to your own equipment. Our calculator compares energy and watts, not voltage compatibility.
LiFePO4 and NMC: start with the exact model
The US Department of Energy identifies LFP (lithium iron phosphate) and NMC (nickel manganese cobalt oxide) as different lithium-ion cathode chemistries. LiFePO4 is commonly shortened to LFP. The chemistry name alone does not establish a station’s runtime, service life or safety. Compare the complete product’s operating limits and warranty; avoid moving claims between similarly named models.
Plan the next charge as carefully as the first
A full battery is only the beginning of a longer trip or outage. Solar recharge is battery Wh divided by effective panel W. Effective panel output depends on your selected derate; solar days then depend on the peak-sun-hours input. Both are assumptions in our tool. The result does not represent uninterrupted wall-clock sunshine and does not include loads running during the refill.
Match the panel’s open-circuit voltage and current to the station’s specified input window, including any cold-weather allowance required by the manual. Check connector type and polarity rather than assuming that a physically fitting adapter makes a pairing safe. For a balcony installation, a grid-connected microinverter is a different system from portable DC panels. Use the balcony guide before deciding which kind of solar equipment belongs in your plan.
Look at models by calculated size class
Use the size-class groups below to explore the current shortlist after calculating your load. The comparison page lets you filter published capacity, output, verified surge, weight and solar input. Cards show their checked region and link to the maker, so an omitted specification stays visible instead of being filled from a different regional version.
A class is a browsing aid, not an approval. The nameplate may still fall below the exact result within that class. Open the calculator’s model checks to compare your numbers and inspect shortfalls. We have not hands-on tested these products, and prices or availability can change at the store.
Questions, answered
How do I find my battery capacity target?
Add daily device energy, allow for inverter losses and idle draw, multiply by the backup duration, then leave the chosen reserve unused. Use the calculator with your own inputs; its result is a planning target, not guaranteed runtime.
Should I enter average or running watts?
Use running watts and a duty cycle when both are known. Average watts can describe energy, but keep the real running and startup requirements for inverter checks. Do not reduce startup demand just because a motor rests between cycles.
Can I add another battery later?
Only when the exact station supports a documented expansion system. More storage does not automatically increase inverter output. Check the manufacturer’s compatible battery list and the regional manual before treating expansion as part of your plan.
Can solar let me choose a smaller station?
Solar may supply some daytime energy, but an overnight load or poor weather still needs a backup plan. Our sizing result does not subtract uncertain future solar harvest from the battery requirement. Recharge is shown separately.
Is a small station suitable for a CPAP night?
Capacity alone cannot establish medical-device suitability. Humidifiers, heated tubing, adapters and interruption tolerance matter. Agree a dependable backup arrangement with your equipment provider and clinician; the CPAP scenario here illustrates energy arithmetic only.
Can I use the same station in another country?
Confirm the exact regional AC voltage, frequency, socket and equipment requirements. Similar product names do not guarantee the same electrical specification. A plug adapter is not evidence that a station and appliance are compatible.
Treat the result as an editable plan, then confirm the equipment’s regional operating requirements. Our comparisons use published specifications. How we calculate · Affiliate disclosure