Buying a battery by guesswork is a good way to either overspend or run out of power at 2 a.m. The sizing sums are not hard, though. They need a list of what you want running, a few numbers off appliance labels, and a decision about how long an outage you want to ride out. This article belongs to our batteries and backup power hub, alongside guides on cost and on comparing storage with a generator.
Two numbers, not one
Battery specs list capacity in kilowatt-hours (kWh) and power in kilowatts (kW). They answer different questions.
Capacity is the fuel tank. A 10 kWh battery can deliver 10 kWh before it is empty, though usable capacity is often a little less than the headline figure. Power is how much it can supply at one moment. A battery rated at 5 kW continuous can run a fridge, lights and a furnace fan with room to spare, but it will trip or shut down if you also start a well pump and an electric kettle together.
When you read a quote, check both. A cheap unit with plenty of kWh but low kW is fine for a few small loads and frustrating for anything bigger. If you need a refresher on how storage fits into a solar system, read battery storage basics.
Step 1: choose what to back up
Start by sorting your loads into three groups: must run, nice to have, and leave out. For most homes the first group is short.
- Refrigerator and freezer
- Furnace fan or hydronic pump, and any heating controls
- Lights in the main living areas
- Internet router, phones, a laptop
- A sump pump or well pump, if you have one
- Medical equipment, if anyone depends on it
Electric resistance heating, electric ranges, clothes dryers and central air conditioning are the loads that wreck budgets. They draw several kW each. Most households leave them off the backed-up circuits and use a wood stove, a gas range or blankets instead.
Step 2: estimate daily energy
Multiply each device’s power by the hours it runs. Labels and manuals give watts; where a motor cycles on and off, use an average. These are typical figures, so check your own appliances.
| Load | Rough power | Hours per day | Approx. kWh per day |
|---|---|---|---|
| Fridge-freezer (cycling) | 150 W average | 24 | 3.6 |
| Furnace fan | 400 W | 8 | 3.2 |
| LED lighting | 150 W total | 6 | 0.9 |
| Router, devices, charging | 100 W | 24 | 2.4 |
| Sump pump | 600 W | 1 | 0.6 |
That adds up to roughly 10.7 kWh per day. Not every number is precise. The point is to see the shape: the fridge and the fan dominate, and lights barely register.
Step 3: decide how long an outage to cover
If you simply want to get through a typical evening outage, 3 to 5 kWh may be enough. If you want a full day of essentials, the example above needs about 11 kWh. If your area sees multi-day outages, a battery alone becomes expensive quickly, and a generator or a solar array big enough to recharge the battery in daylight starts to make sense. We compare the two in solar battery vs generator.
In winter, plan conservatively. Panels produce less in short days and snow can cover them, which is covered in how solar performs in Canadian winters. A battery that is recharged by solar each sunny afternoon stretches much further than the raw kWh number suggests, but you cannot count on sun in January.
Step 4: check the power side
Add up the loads that could run at the same moment, and watch for motor start-up surges. A fridge compressor or a pump may draw two or three times its running power for a second or two. For the example above, simultaneous running power is perhaps 1.5 to 2 kW, so a battery rated at around 3 to 5 kW continuous would be comfortable. If you add an electric kettle at 1.5 kW or a microwave, the total climbs fast.
Your installer should read the surge ratings and tell you what the system can handle. If they will not give a straight answer, that is useful information about the installer.
Worked example in one paragraph
Take a two-storey house with a gas furnace, a fridge, a freezer in the basement and a sump pump. Essential loads come to about 10 kWh per day, peak power about 2 kW. A 10 to 13 kWh battery with a 5 kW inverter would cover about a day with a little margin, and the owner might add a small generator for longer events. Price depends on the market, so see home battery cost in Canada for ranges.
Reduce first, then size
Every kWh you stop using during an outage is a kWh you do not have to buy storage for. A home energy audit often finds easy wins: an old chest freezer, a leaky furnace duct, a wasteful pump. Cutting 2 kWh a day from the essentials list can take a few thousand dollars off the battery you need.
Finally, leave room to grow. Many systems can be expanded with a second unit later, which lets you start modestly and add storage if your needs or your budget change.
Quick answers
What size battery do I need for a typical house?
For essentials such as a fridge, lights, internet and a furnace fan, many homes land around 5 to 10 kWh for roughly a day. Electric heating or cooking pushes the answer much higher.
What is the difference between kWh and kW for a battery?
kWh is the amount of stored energy, like the size of a fuel tank. kW is how fast the battery can deliver it, like how wide the fuel line is. You need enough of both.
Should I size a battery for the whole house?
Usually not. Backing up only essential circuits is far cheaper, and you can add more storage later if your needs grow.
Where to verify
Rules and prices change. Confirm current details with the official bodies below.