To size a home battery, you need two numbers: how much energy (kWh) your essential loads use over the time you want covered, and how much power (kW) they draw at once, including startup surges. Most Canadian homes that back up essentials only, with gas or oil heat, land somewhere around 10 to 20 kWh of usable capacity. Homes that want to run electric heat or central air need far more. Here's a straightforward method you can do with a pencil and your utility bill.
Key takeaways
- Size for energy (kWh) to cover how long you need backup, and for power (kW) to cover everything running at once.
- Start with a list of essential circuits, not your whole-house consumption.
- Motors in well pumps, sump pumps and air conditioners can draw several times their running power at startup.
- Add a buffer of roughly 15 to 25 percent for cold weather, inverter losses and battery aging.
- If the goal is time-of-use savings instead of backup, size to your peak-period usage instead.
Step 1: Decide what the battery is for
Your goal changes the math:
- Backup only: Size to your essential loads for the number of hours or days you want to cover.
- Time-of-use savings: Size to the energy you use during expensive hours. See Is a home battery worth it? for the pricing side.
- Solar self-consumption: Size to your typical evening use or your daily solar surplus, whichever is smaller.
- Combination: Find the largest of these needs and start there.
Step 2: List your essential loads
Walk through your home and write down what you truly need during an outage. Use the nameplate or spec sheet for wattage where possible; the figures below are typical ranges.
| Load | Running power (approx.) | Typical hours per day | Daily energy (approx.) |
|---|---|---|---|
| Refrigerator | 100–200 W (cycling) | ~24 | 1–2 kWh |
| Chest freezer | 50–150 W (cycling) | ~24 | 0.5–1.5 kWh |
| Gas furnace blower | 300–800 W | 6–12 in winter | 2–8 kWh |
| Sump pump | 400–1,000 W | Varies | 0.3–2 kWh |
| Well pump | 750–1,500 W | 1–2 | 1–3 kWh |
| Internet + phone charging | 30–60 W | 24 | 0.7–1.4 kWh |
| LED lights (several rooms) | 50–150 W | 5 | 0.3–0.8 kWh |
| Microwave | 1,000–1,500 W | 0.25 | 0.3–0.4 kWh |
| CPAP or medical device | 30–100 W | 8 | 0.3–0.8 kWh |
Your numbers will differ. If you have a smart meter, your utility's online portal can show your actual hourly usage, which is helpful for spotting the baseline load you can't easily turn off.
Step 3: Calculate daily energy
Add up the daily energy column for your essentials. A typical gas-heated home's essentials might total roughly 6 to 12 kWh a day in winter.
Then multiply by the number of days you want covered without recharging:
Required energy = daily essential energy × days of autonomy
If you have solar, you can count on some recharge in summer, but plan conservatively for winter, when snow and short days can reduce output to very little.
Step 4: Check power and surge
Add up the running watts of everything that might be on at the same time. Then identify the biggest motor load and check its starting surge, which can be two to five times its running power for a second or two.
Compare that to the battery's specifications:
- Continuous power rating (kW): must exceed your combined running load.
- Peak or surge rating (kW): must handle your largest motor start on top of other running loads.
Well pumps and central air conditioners are the most common culprits for a battery that has enough energy but trips on startup. Soft-start devices can reduce surge on some motors, so ask your electrician.
Step 5: Add a buffer
Several real-world factors reduce what you'll actually get:
- Cold temperatures: Batteries in unheated spaces deliver less energy and may spend some on self-heating.
- Inverter and round-trip losses: Typically 10 to 15 percent.
- Aging: Many warranties guarantee around 60 to 70 percent of original capacity after about 10 years.
- Reserve setting: Systems often hold some capacity back for outages if you also use them for daily cycling.
A 15 to 25 percent buffer on your required energy is a sensible starting point.
A worked example
A family in a gas-heated detached home in Manitoba wants 24 hours of winter backup without solar.
- Essentials: fridge (1.5 kWh), freezer (1 kWh), furnace blower (6 kWh), sump pump (0.5 kWh), internet and phones (1 kWh), lights (0.6 kWh), microwave (0.3 kWh). Total: about 10.9 kWh per day.
- Days of autonomy: 1. Required: 10.9 kWh.
- Buffer of 20 percent: about 13 kWh usable.
- Peak running load: furnace blower (0.7 kW) + fridge (0.2 kW) + freezer (0.15 kW) + microwave (1.2 kW) + lights and internet (0.2 kW) ≈ 2.5 kW, plus sump pump surge. A battery with around 5 kW continuous power covers this comfortably.
Result: one battery of roughly 13 kWh usable and 5 kW or more continuous output fits this household. If they wanted two days, they'd need two units, or solar to stretch one.
What about heat pumps and electric heat?
Backing up electric heating is much harder. Electric baseboards or an electric furnace can draw 10 to 20 kW or more and use dozens of kWh on a cold day, far beyond one or two home batteries. A cold-climate heat pump is more efficient but still draws several kilowatts with high daily energy use in deep cold.
Options include backing up only a portion of heating (one zone or a single ductless head), adding a small fuel-fired backup heater, or pairing the battery with a generator. Improving your home's envelope also helps: a well-sealed, well-insulated house loses heat more slowly during an outage. Our home energy upgrade sequence explains why efficiency often comes first.
Frequently asked questions
Is it better to oversize or undersize a home battery?
Modestly oversizing gives you margin for cold weather and aging, but each extra module adds significant cost. Many systems are expandable, so you can start with your essential needs and add capacity later if needed.
Does usable capacity differ from total capacity?
Yes. Manufacturers often reserve a portion of the cells to protect battery life, so always size using the usable capacity listed on the spec sheet.
Can I just use my monthly bill to size a battery?
Your bill shows total use, which includes loads you won't back up. It's useful as a sanity check, but listing essential loads gives a much more accurate figure for backup sizing.
The bottom line
Sizing a home battery comes down to honest arithmetic: list your essentials, total their daily energy, check their combined and surge power, then add a buffer for Canadian conditions. Done carefully, it keeps you from paying for capacity you'll never use, or worse, a battery that can't start your well pump. For context on choosing a system, see the complete guide to home battery storage, and check local programs with our rebate finder.
Sundamentals is an independent, non-profit education platform. Program details and prices change often — check current programs for your address in our rebate finder.