Batteries
How Big a Battery Do You Need? Essential Loads vs Whole Home
October 15, 2026
VerifiedOctober 15, 2026
Next reviewApril 15, 2027
AuthorGriffEnergy Research Team
The short answer
Size a battery twice: for power (kW), so it can run everything you switch on at once, and for energy (kWh), so it lasts long enough. For essential loads in a Canadian winter (fridge, freezer, furnace fan, lights, internet, a pump), plan on about 12 kWh a day: one 13.5 kWh battery covers about a day. Whole-home backup usually means two or three batteries plus a plan for big loads like the stove, water heater and EV charger.
Battery quotes often start with a size and work backward. It is better to start with what you want to keep running during an outage, and for how long. This guide walks through both numbers and the two common ways to wire backup.
kW and kWh in plain words
- Power (kW) is how much the battery can deliver at one moment. Too little and it trips when the well pump and furnace start together.
- Energy (kWh) is how much it stores. Too little and it runs out overnight.
A Tesla Powerwall 3 stores 13.5 kWh and delivers 11.5 kW continuously. An Enphase IQ Battery 5P stores 5 kWh and delivers 3.84 kW, so several are usually stacked. Check both numbers on any quote.
Step 1: list what must stay on
Motors (fridges, pumps, fans) draw a surge for a second or two when they start, often two to three times their running power. The battery must handle the largest surge on top of everything else that is running.
| Load | Running power (rough) | Energy per day (our estimate) |
|---|---|---|
| Fridge | Up to 700 W, higher at start | About 1.5 kWh |
| Chest freezer | Similar to a fridge | About 1 kWh |
| Gas furnace fan | About 700 W (1/3 hp) | About 6 kWh in cold weather |
| LED lights | 10 to 15 W each | About 0.6 kWh |
| Internet, phones, laptop | Under 100 W | About 0.5 kWh |
| Well pump | About 1,000 W (1/3 hp), double at start | About 1 kWh |
| Sump pump | 800 to 1,050 W | About 1 kWh in wet weather |
| Total, winter | Plan for 3 to 5 kW with surges | About 11.6 kWh |
| Total, summer | No furnace fan | About 5.6 kWh |
Running watts are from a utility generator-sizing guide; daily energy is our estimate for a typical home. Your own numbers are on appliance labels, and your utility's hourly data shows real use on a cold day.
Step 2: decide how long it must last
Divide stored energy by daily use, allowing about 10% for conversion losses. Winter is the hard case: short days and snow on the panels mean little or no recharge from solar.
| Usable storage | Essential loads, winter (11.6 kWh/day) | Essential loads, summer (5.6 kWh/day) |
|---|---|---|
| 5 kWh | About 9 hours | About 19 hours |
| 10 kWh | About 19 hours | About 1.6 days |
| 13.5 kWh | About 1 day | About 2.2 days, longer with solar |
| 27 kWh | About 2 days | About 4 days, longer with solar |
Most outages are short, so a day of essential backup covers the common case. If you are rural, on a well, or have lived through multi-day ice-storm outages, size for two days or pair a battery with a small generator.
Step 3: choose essential loads or whole home
- Essential-loads (backup) panel. An electrician moves the critical circuits to a small subpanel the battery powers in an outage. Cheaper and makes one battery last, but everything else goes dark.
- Whole-home backup. The battery's gateway disconnects the house from the grid and powers the main panel. Convenient, but large loads can trip it or drain it in hours. A typical Ontario home uses about 700 kWh a month, around 23 kWh a day, before electric heat.
Loads that make whole-home backup hard: electric ranges, electric water heaters (about 4.5 kW), EV chargers (7 kW and up), electric baseboard heat and central air with a large start surge. Smart load controllers can lock these out during an outage, which lets a smaller system cover the whole panel.
Heat pumps and electric heat
A battery will not heat an all-electric home through a cold night: a heat pump or baseboards can use 30 to 60 kWh or more on a cold day. If your heat is electric, plan backup around a gas fireplace, wood stove or generator, and use the battery for the fridge, lights and communications.
A quick sizing worksheet
- List essential loads and their running watts. Add the biggest start surge. That is your minimum continuous and peak power.
- Add up daily kWh for a cold day. Multiply by the days you want covered, then add 10%.
- Compare against usable kWh and continuous kW on each quote, not the nameplate.
- Ask whether rebates in your province pay per kWh (Ontario pays $300/kWh with new solar), which changes the cost of each extra battery.
See what a home battery costs, the best home batteries in Canada, compared and battery payback on Ontario's ULO plan.
How long will one Powerwall run my house?
About a day on essential loads in winter and about two days in summer, using our estimates. Whole-home use with electric appliances can drain it in hours.
Will my solar panels recharge the battery during an outage?
Yes, if the system has a backup gateway that lets solar run off-grid. In winter, expect little recharge on short or snowy days.
Can I add more batteries later?
Usually, within the same product line and the model's maximum. Mixing old and new units can be restricted, so ask before you buy.
Verified October 15, 2026. Load and runtime figures are estimates for planning; confirm sizing with a licensed electrician. General information, not professional advice.
