How much backup capacity does your home actually need?

"How big a battery do I need?" comes down to one formula and a few honest assumptions. This guide states every assumption once, builds each example from them, and shows the reserve and the conversion loss as separate steps, so you can swap in your own numbers.

The formula

1. Daily energy (Wh/day) = the sum, over every appliance you want to run, of average watts × hours per day.
2. Rated battery capacity needed (Wh) = daily energy ÷ 0.85 ÷ 0.80, which is about daily energy × 1.47, for each day you want to cover.
3. Inverter output needed (W) = the largest combination of things that can be running at once, with surge headroom above the biggest motor start.

Steps 1 and 2 answer "how long". Step 3 answers "will it start". They are separate questions, and a unit can pass one and fail the other.

The assumptions behind every number below

If you would rather run the battery down to empty in an emergency, drop the reserve: delivered AC energy is then rated Wh × 0.85.

Step 1: find your average watts

Two numbers describe a load: running watts while it draws power, and surge watts, the brief spike when a motor or compressor starts. Daily energy uses average watts, so cycling loads need one extra step: average watts = running watts × duty cycle.

For a refrigerator or freezer you do not have to estimate. The EnerGuide label states yearly energy use, so daily Wh = label kWh per year ÷ 365 × 1,000. For a sense of scale, Natural Resources Canada reports that the average refrigerator shipped in Canada used 430 kWh per year in 2009 (NRCan appliance statistics). This guide uses a fridge labelled about 394 kWh per year: 394 ÷ 365 × 1,000 ≈ 1,080 Wh per day, an average of 45 W over 24 hours. A "150 W fridge" on a nameplate is its running draw, not its average, and 150 W × 24 h = 3,600 Wh would overstate the daily energy by more than three times.

Where there is no label, a plug-in power meter measures real draw for a few dollars. Measure where you can.

Step 2: the worked example

Every row uses the assumptions above. The loads are examples, not recommendations.

Now apply step 2, daily energy ÷ 0.85 ÷ 0.80:

What each capacity tier runs

Runtime = rated Wh × 0.68 ÷ average watts, from the same assumptions (reserve held back). Higher numbers need a bigger unit, a smaller load, or running the reserve down.

How to read it: a 1 kWh unit is a short-outage tool for a fridge and a few small loads. Around 3.6 kWh covers essentials for roughly a day. Scenario C, with furnace and sump running, needs more than any single tier in the table delivers per day, which is where expansion batteries or a different heating plan come in. Solar input can stretch runtime; see the solar pairing guide.

Step 3: surge, not just capacity

Capacity tells you how long things run. The inverter's continuous and surge output tells you whether they start at all. A fridge that averages 45 W still needs far more than that for the instant its compressor starts, and if the fridge and sump pump start together the spikes add. Read the surge figure on each appliance's label or manual, add the worst realistic simultaneous starts, and make sure the inverter's surge rating is above that total.

Resistive heat is the exception to avoid. A 1,500 W space heater or kettle draws 1,500 ÷ 0.85, about 1,760 Wh from the battery every hour, so it empties almost any portable unit in a couple of hours for little benefit.

Medical and life-support equipment

If anyone relies on electrically powered medical equipment, a portable power station is not a substitute for a medical-grade backup plan. Take the device's watts from its manual, follow its manufacturer's guidance on batteries and power converters (for example, ResMed publishes battery and power converter information for its CPAP devices), and talk to your clinician or equipment supplier about an outage plan. Do not assume a power station is compatible because the numbers add up.

Where plug-in backup ends and installation begins

Extension cords from a power station to individual appliances need no electrician. Powering circuits, such as a furnace, well pump or hardwired sump pump, means connecting to the building's wiring. That requires a transfer switch or interlock installed to the Canadian Electrical Code (CSA C22.1), normally by a licensed electrician, with a permit and inspection where your province requires one. Connecting a power source to a panel without one can energize utility lines and put line workers at risk. Check your provincial regulator, for example ESA in Ontario or Technical Safety BC, and your utility.

Large 240 V loads (well pumps, central air, electric ranges) need a unit with 240 V output; some larger systems offer it, and some pair two units with a manufacturer-specific kit. If whole-home coverage is the long-term goal, buy into an expandable platform and read the buying guide for how platforms differ.

Kits that match these examples

Sources and notes

• Refrigerator energy: Natural Resources Canada, appliance statistics (average 430 kWh per year for 2009 shipments); and the EnerGuide label on your own appliance.
• Conversion efficiency (0.85) and reserve (20%) are this guide's planning assumptions, stated above, not measurements. Check a specific unit's published inverter efficiency and its usable-capacity rating.
• All appliance wattages marked "assumed" or "example" are illustrations. Replace them with your labels and meter readings.
• Wiring rules: Canadian Electrical Code (CSA C22.1) and your provincial electrical safety authority.