The short version: sizing is arithmetic, not opinion. List the loads you must keep on, add up their watt-hours a day, multiply by the days you need to cover, then check that the unit’s output in watts and its voltage can actually run those appliances. Most households finish that exercise at 2,000 to 2,500Wh a day, which is why the 3,072Wh Jackery HomePower 3000 is the unit most of them end up buying.
Here is the method, step by step, with three worked houses at the end.
The three numbers that matter
Every spec sheet leads with the same three figures, and they answer three different questions.
Capacity, in watt-hours (Wh), is the size of the tank. It tells you how long the unit runs. Divide capacity by your daily watt-hours and you have days of runtime with no sun. A 3,072Wh unit carrying 2,300Wh a day lasts a little over a day.
Output, in watts (W), is the size of the tap. It tells you what you can run at once, and whether a given appliance will start at all. A 3,600W unit runs everything on a normal essentials list simultaneously. A big motor can still trip it on start-up, which is step four.
Voltage, 120V or 240V, is the shape of the plug. Ordinary outlets are 120V. A few large appliances are wired for 240V, and no 120V-only unit will run them however big its battery. This is the one that cannot be worked around, so it is worth checking early.
Capacity without output is a big tank and a thin tap. Output without capacity is a wide tap and an empty tank. You need both to be right, and the voltage to match.
Step one: list what must stay on
Not what would be nice. What you would genuinely regret losing. Write the list on paper before you look at a single product, because the list is what you are buying.
| Load | Running draw | Energy per day | Notes |
|---|---|---|---|
| Refrigerator or fridge-freezer | ~150W | 1,000 to 1,500Wh | Cycles on and off; keep the door shut |
| Chest freezer | Cycles | 600 to 900Wh | Stays cold for a day unopened even with no power |
| Wi-Fi router and modem | ~10W | ~240Wh | Ten watts across a full day |
| LED lights (ten bulbs) | ~50W | 300 to 400Wh | Assumes six to eight hours |
| Phones, tablets, a laptop | Small | 150 to 300Wh | Charging is cheap; screens are not |
| CPAP | 30 to 60W | 300 to 500Wh per night | The load nobody can skip |
| Gas furnace blower | 400 to 600W | 3,000 to 5,000Wh in deep cold | The load winter buyers forget |
| Small window AC | 400 to 600W | 4,000 to 8,000Wh | Depends on how hot the room is |
| Sump pump | ~800W, higher to start | Only when it rains | The surge matters more than the total |
| Well pump | ~1,000W, 2 to 3x to start | Needs 240V | Not optional in a rural house |
| Electric space heater | 1,500W | Drains 3,000Wh in ~2 hours | Do not plan on this |
| Central AC | 3,000 to 4,000W, big surge | Needs 240V and 7,200W | Whole-home units only |
| Electric water heater, electric dryer | Large | Needs 240V | 240V-capable units only |
Step two: add up the watt-hours a day
Add the daily column for the loads on your list. That single number is what sizes the battery.
For most households the essentials total looks like this: fridge 1,200Wh, Wi-Fi 240Wh, lights 350Wh, phones and a laptop 250Wh, and a CPAP 400Wh. That is about 2,400Wh a day, and it is remarkably consistent from Florida to North Dakota, because a fridge is a fridge everywhere.
What varies is the heating and cooling load bolted on top. A gas furnace blower in deep cold adds 3,000 to 5,000Wh a day. A small window AC in a heatwave adds 4,000 to 8,000Wh. Either one roughly triples the answer, which is why climate decides the size of the unit far more than house size does.
If you want to skip the estimating, plug the appliance in on a normal day and watch the unit’s display. Your own fridge is the only fridge whose number is exactly right.
Step three: decide how many days you must cover
Multiply the daily total by the days. The days figure is a judgement call, so use your own outage history rather than a worst case you read online.
- Suburban grid, storms knock it out for a few hours, once or twice a year: cover one day. Anything more is insurance you will not use.
- Rural line, or a coast with hurricanes, or a region with ice storms: cover two to three days. This is where expansion earns its price.
- End of a long line, wildfire shutoffs, or a history of multi-day outages: cover three days or more, and buy a platform you can expand rather than a fixed unit.
- Anyone with a medical device: add a day to whatever you decided, and never plan to run it down to empty.
Essentials at 2,400Wh a day over one day is 2,400Wh, which a 3,072Wh unit covers. The same essentials over three days is 7,200Wh, which needs expansion.
▶ Shop the HomePower 3000, 3,072Wh with panels use code DEAL5
Step four: check the output and the surge
Capacity tells you nothing about whether an appliance will start. Add up the running watts of everything you would have on at the same moment, and check it against the unit’s output rating. The essentials list runs at roughly 400W together, well inside a 3,600W unit.
Then check the surge. Anything with a motor or a compressor, a pump, a fridge, an air conditioner, a furnace blower, draws two to three times its running wattage for a second or two at start-up. A sump pump at ~800W running spikes well above that. A well pump at ~1,000W running wants two to three times that on start. Central AC at 3,000 to 4,000W surges higher again, which is why it needs a 7,200W unit.
Get this wrong and the failure is obvious and total: the unit shuts off when the pump kicks in, with a battery still nearly full.
Step five: check the voltage
This is a yes-or-no gate, not a preference. These appliances are wired for 240V:
- Well pump
- Central air conditioning
- Electric water heater
- Electric dryer
If none of those is on your must-keep list, any 120V unit is fine and you can buy on capacity and output alone. If one of them is, your shortlist is short: the Jackery Solar Generator 5000 Plus at 5,040Wh and 7,200W with 120V and 240V, the Anker SOLIX F3800 Plus kit, or the EcoFlow DELTA Pro Ultra. Our home backup buyer’s guide compares those three.
Step six: does solar recharge change the answer?
Sometimes, and less than people hope. A set of panels might return 1,500 to 2,000Wh on a bright summer day and only a few hundred watt-hours on a grey winter one.
Read that against your own outages. A summer hurricane outage in a sunny climate is genuinely different with panels: 1,500Wh a day back into the battery covers most of an essentials load, so a 3,072Wh unit stops being a countdown. A January ice storm under thick cloud gives you almost nothing, so size the battery to cover the whole outage on its own and treat any solar as a bonus.
Two habits matter more than panel wattage. Top the unit up from the grid whenever a storm is forecast, and put the panels where they get real sun rather than where they are convenient.
Three worked houses
A city apartment
Fridge 1,200Wh, Wi-Fi 240Wh, lights 350Wh, phones and a laptop 250Wh. Daily total about 2,000Wh. No 240V loads, no furnace to power, outages of a few hours to a day. Running load under 400W, no serious surge.
Verdict: a 3,072Wh unit covers a day and a half with margin, and the panels work on a balcony or in a window. The HomePower 3000 review walks through exactly this case.
▶ Get the HomePower 3000, panels included use code DEAL5
A suburban house with a gas furnace
Essentials 2,400Wh including a CPAP, plus the furnace blower at 4,000Wh in deep cold. Daily total about 6,400Wh. Outage history says two days. The blower runs at 400 to 600W and surges on start, which a 3,600W unit handles.
Two days at 6,400Wh is 12,800Wh, so a single 3,000Wh-class battery is half a day, not two days. This house needs expansion.
Verdict: the HomePower 3600 Plus at 3,584Wh, expandable to 21kWh and transfer-switch ready, so the furnace comes back on through the panel instead of an extension cord. Our three-unit comparison covers where the 3000 stops and the 3600 Plus starts.
▶ Shop the HomePower 3600 Plus, expandable to 21kWh
A rural house on a well
Essentials 2,400Wh, a sump pump when it rains, and a well pump. The well pump settles it before the arithmetic does: it needs 240V and draws ~1,000W running with two to three times that on start, so a 120V unit is out at any capacity.
Verdict: the Solar Generator 5000 Plus at 5,040Wh and 7,200W with 120V and 240V, expandable to 60kWh. A house with no running water is not a house you can stay in, which is why the well pump, not the fridge, sizes this purchase.
▶ Shop the Solar Generator 5000 Plus, 7,200W, 120V/240V
Common sizing mistakes
- Sizing for the whole house. Your utility bill is not your backup requirement. You are keeping a handful of loads alive for a few days, not running normal life at normal comfort.
- Forgetting the furnace blower. A gas furnace burns gas, but its blower and ignition run on electricity. Leave it off the list and the unit that looked generous in the shop keeps the fridge cold in a freezing house.
- Planning to run a space heater. At 1,500W it drains a 3,000Wh battery in about two hours. Powering a furnace blower at 400 to 600W heats the whole house for a fraction of that.
- Buying on capacity alone. A large battery with a small inverter still cannot start a pump. Check the output in watts against the loads, not just the watt-hours.
- Forgetting the start surge. Two to three times running wattage, for a second, is what actually trips a unit. Size the output for the moment the compressor kicks in, not for the steady state.
- Assuming solar fills the gap. A grey winter week returns a few hundred watt-hours a day. Size the battery for the outage and let the panels extend it.
Quick lookup: daily watt-hours to unit
| Your daily total | Voltage needed | Unit |
|---|---|---|
| Under 2,500Wh, one day | 120V | Jackery HomePower 3000, 3,072Wh |
| 2,500 to 4,000Wh, one day | 120V | HomePower 3000, or the 3600 Plus for margin |
| 2,000 to 3,000Wh, two to three days | 120V | HomePower 3600 Plus with expansion to 21kWh |
| 4,000 to 8,000Wh (furnace or window AC) | 120V | HomePower 3600 Plus with expansion |
| Any 240V load | 240V | Solar Generator 5000 Plus, 5,040Wh, 7,200W |
| Central AC, or 8,000Wh a day and up | 240V | Solar Generator 5000 Plus with expansion to 60kWh |
If your number sits on a boundary, go up. Nobody has ever finished an outage wishing they had bought less battery. For sizing tuned to your state’s outages, our state backup power guides do this arithmetic for each climate.
Keep reading: Best solar generators for home backup · Jackery HomePower 3000 review · HomePower 3000 vs 3600 Plus vs 5000 Plus · Backup power guides for every state