Powering a Home During a Multi-Day Outage: Battery Sizing

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Most portable power station buyers plan for a few-hour outage. Multi-day outages — the kind that follow ice storms, hurricanes, and wildfires — are a different sizing problem. You’re no longer bridging a gap; you’re actually powering the essentials for 24, 48, or 72 hours. This guide walks through how to size a battery system for a multi-day outage: what to run, how much energy each thing takes, and how to combine batteries and solar to get through days without grid power.

The core math

Multi-day battery sizing is straightforward arithmetic. You need three numbers:

1. Daily energy demand in watt-hours: sum of (watts × hours) for every load you want to run
2. Number of outage days to plan for
3. Daily solar recharge (if any) in watt-hours: panel wattage × 4 hours of good sun (a common average)

Required battery capacity = (Daily demand × days) − (Daily solar × days). Add 20% margin for real-world usable capacity vs nameplate.

Daily energy demand: what things actually cost

The single most useful table for outage planning:

• Full-size fridge: 800–1,200Wh/day (cycles ~1/3 duty cycle)
• Chest freezer: 600–900Wh/day
• Mini-fridge: 300–500Wh/day
• CPAP without humidifier: 200–350Wh/night
• CPAP with humidifier: 400–600Wh/night
• Wi-Fi modem + router (24 hr): 600Wh
• Laptop (8 hr work): 400–500Wh
• Desktop PC (8 hr): 1,500–2,500Wh
• LED lighting (whole apartment, 6 hr evening): 200Wh
• Furnace fan (gas furnace, natural cycling): 1,000–1,500Wh/day
• Small window AC (5,000 BTU, 4 hr): 2,000Wh
• Space heater (1500W, 4 hr): 6,000Wh — this alone breaks most systems
• Well pump (1/2 HP, ~20 cycles/day): 800–1,200Wh
• Phone charging (household): 100–200Wh

A typical “essentials” load — fridge, Wi-Fi, laptop, lights, phones, CPAP — comes out to about 2,500–3,200Wh/day. That’s the number to plan against.

Sizing scenarios

Scenario 1: 24-hour outage, essentials only

Demand: ~2,700Wh (fridge, Wi-Fi, laptop, lights, phones, CPAP for one night).

Battery needed: 2,700Wh × 1.2 margin = 3,240Wh.

Recommendation: single 2,000Wh unit + solar top-up, OR a 3,000Wh+ unit alone. The EcoFlow Delta Pro at 3,600Wh covers this with margin and no solar needed.

Scenario 2: 48-hour outage, essentials

Demand: 2,700Wh × 2 days = 5,400Wh.

Battery needed with no solar: 5,400 × 1.2 = 6,500Wh.

Battery needed with 200W solar (~800Wh/day): (5,400 − 1,600) × 1.2 = 4,560Wh.

Recommendation: single 3,000Wh+ unit with 200W solar panel, OR two 2,000Wh units. The EcoFlow Delta Pro + 220W solar panel covers this comfortably.

Scenario 3: 72-hour outage, essentials + summer AC

Demand: 2,700Wh essentials + 2,000Wh window AC = 4,700Wh × 3 days = 14,100Wh.

Battery needed with no solar: 17,000Wh.

Battery needed with 400W solar (~1,600Wh/day): (14,100 − 4,800) × 1.2 = 11,200Wh.

Recommendation: this is where portable stops making sense. You need either a fixed home battery (see our best solar batteries guide), a big expandable system like the EcoFlow Delta Pro with additional extra batteries (chains to 25kWh), or accept that AC comes off the list for outages this long.

Scenario 4: 72-hour outage, essentials only (no AC)

Demand: 2,700 × 3 = 8,100Wh.

Battery + 200W solar: (8,100 − 2,400) × 1.2 = 6,850Wh.

Recommendation: one EcoFlow Delta Pro (3,600Wh) + 200W solar + careful management, OR a Delta Pro with one extra battery (7,200Wh total) and no solar.

Solar is your force multiplier

A 200W folding solar panel adds ~800Wh/day in decent conditions. Over 3 days that’s 2,400Wh — often the difference between “just enough” and “we ran out on day 2.” Panels cost $300–500 and pack into a suitcase.

Key rules for solar during outages:

• Aim panels toward the sun; don’t just lay them flat
• Move panels 2–3 times per day to track the sun (adds 30% output)
• Clean cells daily — dust, dew, and leaves matter
• Match panel voltage to power station input spec (most portable panels are pre-matched to their brand’s stations)

See our portable solar panels guide for panel picks.

Load management: the second-most important skill

Battery capacity buys you a fixed amount of energy. Load management extends it by 30–50%:

• Fridge: keep the door closed. Every open door loses cold and forces a compressor cycle. A fridge that’s opened 3× per day uses ~1,200Wh; opened 12× per day uses ~1,800Wh.
• Freezer: pack it full (frozen mass = thermal ballast). A full freezer holds temp much longer than a half-full one.
• Cycle loads: run the fridge on the battery, then let the battery recharge from solar, then run again. Don’t leave the battery inverter idling on nothing — the AC inverter itself draws ~5–15W idle.
• CPAP: switch humidifier off during outages. Cuts consumption in half.
• Lights: LED only. One 60W bulb replaced with a 9W LED saves 600Wh/day if it was on 12 hours.
• Skip: hair dryers, space heaters, toasters, coffee makers, kettles. These pull 1000–1500W and drain a portable battery in minutes.

Combining batteries: expandable systems

Modern high-end portable systems chain together. The EcoFlow Delta Pro takes up to 2 external batteries (10,800Wh total), and with a Smart Home Panel integrates with home wiring. The Bluetti AC300 takes 4 B300 batteries (12,288Wh total). This is where portable and home-battery start to overlap; see critical loads vs whole-home battery for the crossover decision.

Common sizing mistakes

Multi-day sizing is where inexperienced buyers overspend or under-buy. The most common mistakes:

• Buying to run everything instead of essentials. The delta between running fridge + Wi-Fi + laptop + lights (2,700Wh/day) and running all of that plus AC and space heater (~10,000Wh/day) is roughly 4x the battery cost. Ruthlessly cut non-essentials from the load list before sizing.
• Ignoring solar recharge. A $400 solar panel functionally doubles the effective capacity of a portable system across a 3-day outage. Buyers who skip solar and instead spend that money on more battery come out behind on total runtime.
• Rating batteries by peak instead of continuous. A 3,000W surge rating doesn’t matter if the continuous rating is 1,500W. Match continuous output to your continuous load.
• Assuming battery capacity = usable capacity. Multiply nameplate by 0.85 for real-world usable. See our depth of discharge and cycle life guide.
• Forgetting inverter idle draw. Most inverters pull 5–15W with nothing plugged in. Over 72 hours that’s 350–1,000Wh gone before you’ve powered anything. Turn the inverter off when not in use.

Cold-weather multi-day outages

Winter storm outages are the hardest sizing case because heating loads are massive and solar recharge is weak. A few adaptations:

• Gas furnace fan only — if you have a gas furnace, the battery only needs to power the fan (~200W continuous). That’s manageable at ~1,500Wh/day.
• Electric heat is off the table for portable systems — a 1,500W space heater running 8 hours draws 12,000Wh. No portable unit will sustain that. Plan on wood, propane, or physical warmth (layers, sleeping bags, single-room heating with a small dedicated propane heater).
• Winter solar output is 40–60% of summer — a 200W panel that makes 800Wh/day in July makes 300–500Wh in January. Size solar accordingly.
• Keep the battery warm — LiFePO4 charging is disabled below 32°F. Store the unit indoors, wrap it in an insulated cover, or use the built-in battery heater if the unit has one.

The bottom line

For a realistic 48-hour outage with essentials, plan on 3,000–5,000Wh of battery + 200W of solar. That’s a single 3,600Wh unit + solar panel, or two 2,000Wh units, and it will cost $2,500–4,000. Going beyond 72 hours or adding AC pushes you into fixed home-battery territory or a multi-unit setup with careful load management. The three levers to pull are: bigger battery, more solar input, and stricter load management — the last one is free and usually undervalued.