How to Calculate Your Home’s Backup Power Needs

ByStay Powered HQ Editorial Team

Quick answer: to calculate your home’s backup power needs, list everything you want to run during an outage, add up their running watts (that’s the power/output you need), find the single biggest startup surge and add it on top, then multiply your running watts by the hours you need to run them (that’s the energy/capacity you need). Add a 20% margin. For most homes backing up essentials, that lands around 3,000–7,500 watts of output and 2,000–10,000 watt-hours of capacity, depending on your goal.

This one calculation tells you what size power station, battery, or generator to buy — no matter which type of backup you choose. Here’s the simple method.

The two things you’re calculating

Backup power sizing always comes down to two numbers:

Power (watts, W) — how much you can run at the same time. This must exceed the combined draw of everything on at once, plus the surge of any motor. It determines the output of your power station or generator.

Energy (watt-hours, Wh, or kWh) — how long you can run it. This is your battery capacity or fuel supply. It determines the runtime.

Get power wrong and things won’t turn on; get energy wrong and they turn off too soon. You need both.

The 4-step calculation

Step 1 — List what you’ll run during an outage. Be realistic: essentials (fridge, lights, Wi-Fi, phones) — not your entire house — unless you specifically want whole-home backup.

Step 2 — Add up the running watts → your power (output) need. Sum the running wattage of everything that could be on simultaneously. Then check the surge: find the single appliance with the biggest startup spike (usually a fridge, well pump, or AC) and make sure your unit’s surge rating covers your running total plus that spike. Add 20% headroom for safety.

Step 3 — Multiply watts by hours → your energy (capacity) need. Take your total running watts and multiply by the number of hours you want to last. That’s your watt-hours. Add ~15% for losses (divide by 0.85).

Step 4 — Convert to what to buy. Your output number tells you the power station or generator wattage; your energy number tells you the battery capacity (Wh) or how much fuel/how big a generator you need.

Common appliance wattage chart

Appliance

Running watts

Startup surge

Refrigerator/freezer

150–200

800–1,200

Chest freezer

100–200

800–1,200

Furnace blower (gas)

400–800

1,200–2,350

Well pump (1/2–1 HP)

1,000–2,000

2,000–4,000+

Sump pump

800–1,050

1,300–2,900

Window AC

600–1,500

1,000–3,000

Central AC (2–3 ton)

3,000–4,000

6,000–8,000+

Lights (whole house LED)

200–500

—

Wi-Fi router / modem

10–30

—

TV

60–120

—

Laptop

30–90

—

Microwave

900–1,200

1,000–1,500

CPAP

30–60

—

Space heater

1,500

—

A worked example

Say you want to back up the essentials for a 24-hour outage: a fridge (150 W run / 1,200 W surge), furnace blower (600 W / 1,200 W surge), lights (300 W), Wi-Fi (25 W), and phone/laptop charging (150 W).

Power (output): – Running total: 150 + 600 + 300 + 25 + 150 = 1,225 W – Biggest surge: the fridge or furnace at ~1,200 W. Peak demand ≈ 1,225 + 1,200 = ~2,425 W. – Add 20% margin → about 2,900 W. So you want a unit rated ~3,000 W output (or a generator of similar size).

Energy (capacity): – 1,225 W × 24 hours = 29,400 Wh… but that assumes everything runs constantly. In reality, the fridge and furnace cycle (running maybe a third of the time), so your real average draw is much lower — perhaps 400–500 W average. – 450 W average × 24 hours ÷ 0.85 ≈ 12,700 Wh for a full day with no recharging.

That’s a lot of battery — which is exactly why, for a full day or more, people either add solar (to recharge), use a generator (unlimited runtime with fuel), or accept a shorter runtime on battery. For a shorter outage (say 6 hours), the same setup needs only ~3,200 Wh.

Reality check: cycling loads and recharging

The example reveals two important truths:

Motor appliances cycle. A fridge or furnace doesn’t run constantly — it cycles on and off, so its average power draw is far lower than its running watts. This dramatically reduces your real energy needs over time.

Recharging changes everything. A battery’s capacity only has to cover the gap until you recharge. With solar (a solar generator) or a generator, you don’t need to store the entire outage’s energy at once — you refill as you go. This is why a moderate battery plus solar can outlast a huge battery alone.

Converting to what to buy

Once you have your two numbers:

  • Power station: Match output (W) and capacity (Wh) to your figures. For essentials, often a 1,500–2,400 W unit with 1,000–2,000 Wh, expandable or paired with solar for longer runtime. See What Size Power Station Do I Need?.
  • Home battery: Size in kWh; essentials for a day often need 10–20 kWh. See How Many Batteries to Power a House During an Outage?.
  • Generator: Match the wattage; essentials need ~5,000–7,500 W portable or 10–14 kW standby. See What Size Generator Do I Need to Run My House?.

Limitations and who this isn’t for

A calculation is only as good as your inputs, and real usage varies day to day, so build in a margin rather than sizing to the exact number. Startup surge — not just running watts — decides whether a unit can start motors and compressors, and manufacturer runtime figures are optimistic. Recheck your assumptions if you add appliances or face colder weather.

How we source this guide: capacities, runtimes and prices are drawn from manufacturer documentation and reputable public sources and can change. Always confirm current specifications and pricing on the manufacturer’s website before buying.

Frequently asked questions

How do I calculate my home’s backup power needs? List what you’ll run at once, add up the running watts (that’s your output need), add the biggest startup surge on top, then multiply the running watts by the hours you need (that’s your energy/capacity need). Add a 20% margin. This gives the size of power station, battery, or generator to buy.

How many watts do I need to back up my house? For essentials (fridge, furnace, lights, Wi-Fi), around 3,000–7,500 watts of output covers most homes, including startup surges. Whole-home backup with central AC and heavy appliances can need 18,000+ watts (an 18–26 kW standby generator).

How much battery capacity do I need for a power outage? It depends on runtime: for essentials, roughly 2,000 Wh for a short outage up to 10,000+ Wh for a full day without recharging. Because appliances cycle and solar/generators recharge, real needs are often lower than a simple watts-times-hours calculation suggests.

Do I need to account for startup surge? Yes. Motors (fridges, pumps, AC) draw a large spike when starting — often 2–3× their running watts. Your backup unit’s surge rating must cover your running load plus the biggest single startup surge, or the appliance won’t start.

Why is my calculated capacity so high? A simple watts-times-hours calculation assumes everything runs constantly, but motor appliances cycle on and off, so real average draw is much lower. Adding solar or using a generator also means you don’t need to store the whole outage’s energy at once.

The bottom line

Calculating your home’s backup power needs comes down to two numbers: power (add up the watts you’ll run at once, plus the biggest surge, plus 20%) and energy (multiply those watts by the hours you need, minus cycling, plus losses). Remember that motor appliances cycle — so real energy needs are lower than they first appear — and that solar or a generator lets you recharge instead of storing everything at once. Nail these figures and you’ll buy exactly the right size backup, whether it’s a power station, a home battery, or a generator.

Related guides