Quick answer: A power station sizing guide needs two numbers, not one. Watt-hours decide how long, watts decide whether it starts at all. For most homes: 300 Wh keeps phones and a router alive, 1 kWh covers a fridge for a night, 2 kWh covers a fridge and a freezer for a day, and 5 kWh is where a sump pump or a furnace blower becomes realistic.
Almost every power station sizing guide starts from the product. This one starts from your house, because the hard part of choosing a power station is not comparing batteries — it is knowing what your appliances actually draw. We spent sixteen articles establishing that from certified federal datasets and manufacturer data sheets, and the whole of it is condensed into our appliance wattage chart. This guide turns those numbers into a purchase.
Three things decide the answer, and they are independent of each other: the energy your loads consume over the outage, the power the biggest one demands at any instant, and the surge the worst one demands for half a second. A station can pass two of those tests and fail the third, which is how people end up with a box that runs a fridge beautifully and cannot start it.
Step 1: list what you actually want to keep alive
Not what you own. What you want running at three in the morning with the street dark. For most households that list is shorter than expected, and the figures below are the ones this site has verified rather than the ones printed on charts.
| Load | Running | Starting | Where the figure comes from |
|---|---|---|---|
| Wi-Fi router and modem | 8.22 W | none | median of 334 named models |
| CPAP, machine only | 11.2 W | none | ResMed, 0.93 A at 12 V |
| Chest freezer, 9 cu ft | 23 W average | compressor | 201 kWh/yr certified, smallest of six |
| Gas furnace blower, ECM | 27–499 W | low on ECM | Trane published range |
| Refrigerator | 52.6 W average | see below | median of 2,886 certified |
| CPAP, humidifier and heated tube | 59.0 W | none | ResMed, 4.92 A at 12 V |
| Window AC, 10–12k BTU | 750 W | variable speed, no inrush | certified CEER |
| Sump pump, 1/2 HP | 920 VA | 2,645–2,875 VA | Liberty 280, locked rotor |
| Well pump, 1 HP | 1,210 W | 11,201 VA | Franklin AIM Manual |
| Coffee maker | 1,450 W | none | Moccamaster element |
| Space heater, high | 1,500 W | none | resistive, rated = running |
Add the running watts of everything you want on at once. That is your continuous requirement. Then find the single largest starting figure in your list — not the sum of them, because motors do not start in unison. That is your surge requirement, and adding every surge together is the most expensive arithmetic mistake in backup power.
Step 2: the number that stops you is watts, not watt-hours
Capacity is the specification everyone shops on and the one that rarely decides anything. A 2 kWh station with a 1,200 W inverter cannot run a 1,500 W space heater for one second, while a 500 Wh station with a 2,000 W inverter will run that same heater — for sixteen minutes. The two specifications answer different questions and neither substitutes for the other.
Manufacturers publish three output figures and they are not interchangeable. Continuous is what the inverter will hold indefinitely. Surge or peak is what it will tolerate for a moment, usually about double. And some makers publish a boosted mode — EcoFlow calls its version X-Boost and states it handles appliances up to 2,200 W on an 1,800 W inverter — which works by reducing the voltage delivered to a resistive load, and therefore does nothing at all for a motor.
That distinction matters because the loads worth backing up are mostly motors. A refrigerator’s compressor is the clearest case: our own refrigerator article found that no appliance maker publishes a starting figure at all, and the compressor makers who do publish quote 3,146 to 4,626 VA for the instant the motor turns. A 1,000 W station with no surge headroom may simply refuse.
Step 3: rated capacity is not delivered capacity
A station labelled 1 kWh will not hand your appliance 1,000 Wh. Some is lost turning DC into AC, some is held back so the cells are not run flat, and a fixed few watts are burned simply keeping the inverter awake. That last one is invisible on a spec sheet and it dominates the small loads this whole category exists to serve, and you can see it in a manufacturer’s own numbers. EcoFlow publishes a runtime table for the DELTA 2 on its product page: 89 charges of an 11 Wh phone, but only 58 hours of a 10 W light. The first works out at 979 Wh delivered. The second at 580 Wh. Same battery, same table, a factor of 1.69 between them. Spread the 399 Wh difference over those 58 hours and it is just under 7 W of overhead, burned by the inverter before your appliance sees anything.
That is not a criticism of one brand; it is how AC inversion works, and a maker who publishes both numbers is being more transparent than one who publishes neither. But it changes the arithmetic. Run a 10 W load and you may be paying 17 W. Run a 500 W load and the same 7 W is noise. Delivered energy is worst at both ends: small loads lose to idle overhead, large ones lose to conversion.
Two practical consequences. First, use the DC ports where you can — USB and 12 V outputs skip the inverter entirely, which is why the phone figure is so much better than the light. Second, when you plan, assume about 80 per cent of the rated capacity reaches an AC load, and treat that as generous below about 20 W.
The power station sizing guide table: what each tier covers
Every runtime below is the rated capacity multiplied by 0.80, divided by the verified running watts from the table above. Nothing here is a manufacturer claim.
| Tier | Router | Chest freezer | Refrigerator | CPAP, full | What it is really for |
|---|---|---|---|---|---|
| 300 Wh | 29 h | 10 h | 5 h | 4 h | Phones, laptop, router. Work through a short outage |
| 500 Wh | 49 h | 17 h | 8 h | 7 h | One night of CPAP, or a small chest freezer for most of a day |
| 1 kWh | 97 h | 35 h | 15 h | 14 h | A fridge through the night. The common sweet spot |
| 2 kWh | 195 h | 70 h | 30 h | 27 h | Fridge and freezer for a day, with a real inverter |
| 5 kWh | 487 h | 174 h | 76 h | 68 h | Where a sump pump or furnace blower becomes realistic |
The pattern in that table is the whole argument of this power station sizing guide. Going from 300 Wh to 5 kWh multiplies your capacity by about seventeen and your fridge runtime with it — but it never once puts a space heater on the list, because a 1,500 W heater eats a 5 kWh station in under three hours and a 1 kWh station in half an hour. Capacity buys duration for small loads. It does not buy you big ones.
Which tier fits your household
You lose power for a few hours, twice a year
300 to 500 Wh. The honest use case for the smallest units is communication and light, and they are very good at it. A router and modem at 8.22 W is the single best value load in the house: a 300 Wh unit keeps the internet alive for more than a day. Whether the desktop also wants a UPS beside it is a separate question with a published answer. Do not buy a small station expecting it to hold a fridge; at 52.6 W average it manages five hours, and that is before the compressor’s surge is considered.
Someone in the house uses a CPAP
500 Wh minimum, 1 kWh comfortably. This is the clearest sizing case on the site because the load is small, constant and known. The machine alone draws 11.2 W; add the humidifier and heated tube and it is 59.0 W — more than five times as much. A 500 Wh unit gives roughly 7 hours with everything on, which is one night with nothing spare, or over 30 hours with the humidifier off. Most machines run on 12 V DC, so using the DC output instead of the AC socket avoids the inverter overhead entirely and is worth more than a capacity upgrade.
You want the food to survive
1 kWh, with 2,000 W of surge. Energy is the easy half here. A refrigerator averages 52.6 W and a chest freezer 23 W, so 1 kWh carries either through a night with room to spare. The hard half is the compressor: the only published inrush figures come from compressor makers and run to 4,626 VA. Buy surge headroom, not capacity.
And before you buy anything, know that a full, unopened freezer holds safe temperatures for about 48 hours on its own, and about half that if it is only half full. For a short outage the correct amount of backup power for a freezer is often none.
You have a well, a sump or a furnace
2 to 5 kWh, and check the surge before the capacity. These are the loads that break the category. A 1 HP well pump draws 1,210 W running but Franklin’s own manual gives 11,201 VA locked rotor — five times the 2,200 W boosted output of the one station whose full specification we read. A 1/2 HP sump pump is more tractable at 2,645 to 2,875 VA depending on its voltage, and a gas furnace blower with an ECM motor can idle at 27 W. If your list contains a well pump, the answer is probably a generator rather than a battery, and no capacity figure changes that.
Chemistry and cycle life, briefly
Two cell chemistries dominate. NMC is lighter and was standard until recently. LiFePO4, also written LFP, is heavier for the same capacity and lasts far longer — EcoFlow rates its LFP pack at 3,000-plus full charge and drain cycles, where NMC packs are typically rated in the hundreds.
For a unit that sits on a shelf and is used four times a year, cycle life is nearly irrelevant and you are buying the warranty instead. For a unit cycled daily — solar self-consumption, van life, a CPAP every night — it is the specification that decides the cost per year, and LFP wins by a wide margin. Check the warranty term against the cycle claim: a five-year warranty on a 3,000-cycle pack is a manufacturer betting you will not cycle it daily.
The four mistakes that cost the most money
- Adding every surge together. Motors do not start in unison. Size on the largest single starting figure, not the sum, or you will buy several times the station you need. This is explained in full in starting watts versus running watts.
- Buying capacity to solve a power problem. If the inverter is too small the appliance will not run for one second, and a bigger battery behind the same inverter changes nothing.
- Planning around heat. A space heater, a kettle, a coffee maker and an electric hob all sit in the 1,400 to 1,500 W band, and all of them empty any portable station in well under three hours. Resistive heat is the one job batteries are genuinely bad at.
- Trusting a nameplate you have not read. The rating plate gives amps or watts for the appliance in front of you, not the class average. Reading your own labels costs ten minutes and beats every chart, including ours.
How we calculated this
Every load figure in this power station sizing guide comes from one of this site’s sixteen verified appliance articles, and every one of those traces to a certified federal dataset or a manufacturer document rather than to another chart. Refrigerator and freezer figures are medians of ENERGY STAR certified records — 2,886 refrigerators and the certified chest-freezer set — converted from annual kilowatt-hours by dividing by 8,760. The router figure is the median of 334 named models. CPAP, sump pump, well pump and furnace blower figures are read from manufacturer data sheets and stated in the units those sheets use, which is why several appear in volt-amperes rather than watts.
Runtimes assume 80 per cent of rated capacity reaches the load. That single assumption carries the whole table, so here is where it comes from: EcoFlow’s published runtimes for one product imply between 57 and 86 per cent across its constant AC loads, with the worst results at the smallest ones. Eighty per cent sits in the upper half of that range, and it is optimistic below about 20 W. It is our planning figure, not a manufacturer’s, and we would rather you knew that than believed the table to two decimal places.
No product was tested by us and none is recommended here. The only manufacturer page cited is cited for the figures it publishes about itself. The full list of documents, with the date each was opened, is in this article’s verification log, and our method is described at how we test.
Before you rely on any of this
A portable power station is a plug-in appliance. Do not wire one into your home’s circuits, and never connect any generator or battery to a wall socket to feed the house — that is backfeeding, it is prohibited in most jurisdictions and it can kill line workers. A transfer switch is the only correct answer and it is electrician work.
If someone in the house depends on medical equipment, do not size that from an article. Ask the equipment supplier and your electricity network operator — many keep a priority-restoration register for medically dependent customers, and being on it can matter more than any battery.
Measure before you buy. A plug-in energy meter costs less than any station and tells you what your own appliances draw, which no chart can.
Frequently asked questions
What size power station do I need for a refrigerator?
For energy, 1 kWh covers a typical certified refrigerator for about fifteen hours at its 52.6 W average, and rather less if its ice maker is left running. For starting, look at the inverter’s surge rating rather than the capacity: published compressor inrush figures reach 4,626 VA, and a station with no surge headroom may refuse to start a fridge it could easily run afterwards. If a unit lists a continuous rating and no surge figure at all, treat that silence as an answer.
Can a power station run a space heater?
Technically yes, usefully no. A 1,500 W heater needs an inverter of at least that size, and it will drain a 1 kWh station in about half an hour and a 5 kWh station in under three hours. Heat is the worst possible use of stored electricity. In an outage, insulation, layers and closing off unused rooms do more than any battery you can carry.
Is watt-hours or watts the more important number?
Watts decide whether it works; watt-hours decide how long. Check watts first, because a failure there is absolute — the appliance simply will not run. Once every load sits inside the inverter’s continuous and surge ratings, capacity becomes a question of comfort rather than function.
Why is the advertised capacity never what I get?
Three losses stack up: DC-to-AC conversion, a reserve the battery management system holds back to protect the cells, and a fixed idle draw for keeping the inverter powered. The third is the one that surprises people, because it is a constant few watts whether your load is 10 W or 1,000 W. That is why the same station delivers a far higher share of its capacity to a phone on USB than to a lamp plugged into an AC socket — 979 Wh against 580 Wh on one maker’s own figures.
Should I buy LiFePO4 or is NMC fine?
If the unit will be cycled regularly, LiFePO4 is worth the extra weight and price: cycle ratings of 3,000-plus against a few hundred change the cost per year completely. If it will sit charged on a shelf waiting for emergencies, cycle life barely matters and what you are really buying is warranty length and low standby self-discharge.
Do I need a power station or a generator?
Draw the line at your largest motor and your longest realistic outage. Batteries are silent, safe to run indoors and maintenance-free, and they cover small always-on loads superbly for a day or two. Once a well pump, a central air conditioner or a multi-day outage is on the list, the fuel-based answer wins and no amount of capacity closes the gap. Generator sizing is covered separately.
The short version
List the loads you want running at three in the morning. Add their running watts for the continuous figure, and take the single largest starting figure for the surge. Check both against the inverter before you look at the capacity at all. Then multiply the rated watt-hours by 0.8 and divide by your running watts to get honest hours. For most households that lands on 1 kWh with roughly 2,000 W of surge — a fridge, a router and phones through a night, which is what the great majority of outages actually require.
This article is general information, not an installation guide and not a substitute for a licensed electrician. Electrical work on a service panel, a transfer switch or a utility connection is regulated in most jurisdictions and normally requires a permit and an inspection. Fuel-burning equipment produces carbon monoxide and must never be run indoors, in a garage or near an opening into the home.
