A refrigerator and chest freezer are the two appliances most worth protecting during a grid outage. Lose them for 48 hours and you lose $300–$500 in food — more if you keep a well-stocked deep freeze. A solar generator sized correctly can keep both running through daylight recharge cycles indefinitely, or cover a 72-hour blackout with no sun at all. The problem is that most buyers size by guessing: they see "2,000 Wh" on the box and assume that covers it. It covers about one night — if the battery chemistry is LiFePO4 and you have no other loads running. For AGM, it covers about half a night. If you want to understand what equipment actually makes sense for multi-day outages, start with the guide to best off-grid power for 3-7 day outages , then come back here for the sizing math that makes any system work.
The calculation has four steps: find your daily watt-hour load, apply a depth-of-discharge limit based on battery chemistry, add a surge headroom check for the inverter, and compute the panel array needed to recharge in one day. Work through those in order and the right generator selects itself.
What 'Watt-Hours' Actually Means for Your Fridge
Watts measure instantaneous power draw. Watt-hours measure energy consumed over time. A 150 W refrigerator compressor running for 8.4 hours per day consumes 1,260 Wh — but the compressor does not run continuously. It cycles on and off, maintaining temperature. The fraction of time the compressor actually runs is called the duty cycle. A well-insulated modern upright refrigerator has a duty cycle around 0.35 (35% of the time). A chest freezer, which loses far less cold air when opened because you reach down rather than letting cold air fall out, runs at roughly 0.25 duty cycle.
The U.S. Department of Energy's appliance energy estimator methodology accounts for this with the formula: Daily Wh = Running Watts × Duty Cycle × 24 hours. If your refrigerator's compressor draws 150 W while running: 150 × 0.35 × 24 = 1,260 Wh per day. If the nameplate only shows an annual kWh figure — say, 500 kWh/year — divide by 8,760 hours to get 57 W average draw, then divide by the duty cycle (57 ÷ 0.35 = 163 W running wattage) to recover the compressor draw when it is actually on.
Real Fridge + Freezer Daily Draw — Published Numbers
The U.S. Energy Information Administration reports that the average American household refrigerator consumed 448 kWh in 2020 — roughly 50 W average draw, or about 1,200 Wh per day. That figure covers older and newer units; ENERGY STAR-certified refrigerators use 9–10% less than the federal minimum standard, translating to roughly 400–420 kWh/year for a modern 18–21 cu ft top-freezer unit, or about 1,100–1,150 Wh per day.
For a chest freezer, ENERGY STAR data shows that a 5 cu ft certified unit consumes around 175–200 kWh/year, placing average daily draw at 480–550 Wh. A larger 15 cu ft chest freezer runs 300–350 kWh/year, or 820–960 Wh per day. Combined for a typical household — 18 cu ft upright fridge plus a 7 cu ft chest freezer — expect a combined daily load of 1,400–1,800 Wh. Use 1,600 Wh as a planning baseline for a standard-size combination. Measure your own units with a plug-in watt meter to get precision; a Kill-A-Watt meter costs $25 and gives you running wattage in 60 seconds.
Surge Wattage: Why the Compressor Kick Matters
Compressor motors are inductive loads. When they start, they pull 3–7× their running wattage for 100–500 milliseconds to overcome the rotor's inertia — this is called locked-rotor amperage, or the startup surge. A refrigerator compressor running at 150 W may surge to 450–750 W on startup. If your inverter's surge (peak) rating is below that figure, it trips on overload and the fridge goes offline.
The rule: your inverter's continuous watt rating must exceed the sum of all simultaneous running loads; its surge rating must be at least 3× the running wattage of the largest single compressor. For a 150 W fridge plus an 85 W freezer, continuous need is 235 W and surge need is 150 × 3 = 450 W minimum. A 1,000 W continuous / 2,000 W surge inverter handles this with room to spare. Where buyers get burned is on budget portable stations that publish a large continuous rating but a low surge multiplier — look for units advertising a surge rating at least 2× the continuous rating, and 2.5× for large compressors.
Depth-of-Discharge Math and Battery Chemistry
A battery's nameplate capacity is not its usable capacity. Discharging lithium iron phosphate (LiFePO4) below 20% state of charge accelerates degradation. Discharging AGM lead-acid below 50% does the same, faster. The percentage you can safely consume is the depth of discharge (DoD): 80% for LiFePO4, 50% for AGM. NREL research on photovoltaic system sizing confirms that battery cycle life drops exponentially below these thresholds — a LiFePO4 cell rated for 3,000 cycles at 80% DoD may last only 800 cycles if routinely taken to 95% DoD.
The sizing formula: Required nameplate capacity (Wh) = Daily load (Wh) ÷ DoD. Using the 1,600 Wh baseline: LiFePO4 requires 1,600 ÷ 0.80 = 2,000 Wh nameplate to survive one night. AGM requires 1,600 ÷ 0.50 = 3,200 Wh nameplate for the same one-night hold. For two nights without recharge, double those: 4,000 Wh LiFePO4, 6,400 Wh AGM. The math is why 2,000 Wh AGM units fail at this task and 2,000 Wh LiFePO4 units just barely make it on a single overnight. LiFePO4 delivers 1.6× more usable energy per nameplate watt-hour than AGM at this DoD ratio.
Step-by-Step Sizing Procedure
- Measure each appliance's running watts
Plug each appliance into a Kill-A-Watt meter and record the wattage shown while the compressor is actively running — not the spike on startup, and not the cord-tag maximum. Typical results: upright refrigerator 100–200 W, chest freezer 60–120 W. If you only have the annual kWh from the EnergyGuide label, divide by 8,760 hours to get average watts, then divide by the duty cycle (0.35 for fridge, 0.25 for freezer) to get compressor running watts.
- Calculate daily watt-hours for each appliance
Multiply running watts by duty cycle, then by 24 hours. Fridge example: 150 W × 0.35 × 24 = 1,260 Wh/day. Freezer example: 85 W × 0.25 × 24 = 510 Wh/day. Sum both: 1,770 Wh/day combined load.
- Add a 25% real-world buffer
Multiply the combined daily Wh by 1.25. This covers high ambient temperature (compressors work harder in a 90°F garage), aging compressor seals on older units, and simultaneous secondary loads like LED lighting and phone charging. Example: 1,770 × 1.25 = 2,213 Wh design daily load.
- Divide by DoD to get required nameplate capacity
Divide the design daily load by 0.80 for LiFePO4 or 0.50 for AGM. For one night of autonomy: LiFePO4 → 2,213 ÷ 0.80 = 2,766 Wh nameplate minimum. For two nights without recharge, multiply the design load by 2 first: 4,425 ÷ 0.80 = 5,532 Wh. Round up to the nearest commercial size.
- Verify the inverter surge rating
Confirm the unit's peak/surge watt specification is at least 3× the running wattage of the largest single compressor. Also confirm the continuous watt rating exceeds the sum of all loads you plan to run simultaneously. Reject any unit that does not publish a separate peak/surge figure — you cannot assume the rating handles inductive startup loads.
- Size the solar recharge panel array
Divide your design daily load by (local peak sun hours × 0.85 system efficiency). Use 4 PSH if you do not know your region's figure — it is conservative for the contiguous US. Example: 2,213 ÷ (4 × 0.85) = 650 W of panel capacity needed. Then verify the generator's maximum solar input wattage meets or exceeds this figure.
- Cross-check and select a model
Pick the smallest commercial unit that meets nameplate Wh, surge rating, and max solar input in a single package. For a standard fridge + 7 cu ft freezer combination, a 3,000–4,000 Wh LiFePO4 unit with 400–800 W max solar input and a 2× surge multiplier covers the math. If the solar input ceiling is too low, plan a supplemental AC recharge from a fuel generator as the fallback for consecutive cloudy days.
Solar Panel Input — Recharge Math
Peak sun hours (PSH) is the number of hours per day during which your solar panels produce at their rated wattage. The continental US ranges from 3.5 PSH (Pacific Northwest in winter) to 6.5 PSH (Southwest desert in summer). The Gulf South averages 5.5 in summer; the Midwest averages 4.0–4.5. Divide your design daily load by (PSH × 0.85) to get required panel wattage. MPPT charge controllers — standard on quality solar generators — achieve 93–97% conversion efficiency; the 0.85 figure accounts for cable losses, temperature derating, and realistic panel output below nameplate STC conditions. For a complete guide to matching panels to a portable power station, see the solar charging station for grid-down guide .
Outage-Survival Recommendations by Tier
FEMA's Ready.gov power outage guidance identifies food preservation, medical device power, and communication as the three priority loads during extended outages. A solar generator sized for the combined fridge and freezer load typically handles all three simultaneously — a CPAP machine draws 40–70 Wh per night, LED lighting runs 5–15 Wh per bulb per night, and USB charging is negligible at 5–30 Wh per device per night. If you built the 25% buffer into your calculation, those secondary loads are already covered. For a breakdown of which units make sense at each price point, see the off-grid power by budget tier guide .
Budget tier (under $1,000): a 1,000–1,500 Wh LiFePO4 unit with 200–400 W solar input covers a single upright refrigerator for one night. It will not cover a chest freezer simultaneously. Plan to cycle the freezer off during the day and run it only at night while the fridge runs around the clock. Mid tier ($1,000–$2,500): a 2,000–3,000 Wh LiFePO4 unit with 400–600 W solar input covers fridge plus freezer for one full night and recharges in 4–6 hours of good sun. This is the practical minimum for most households with both appliances. High tier ($2,500+): a 3,600–6,000 Wh unit with 800–1,600 W solar input covers 48 hours of autonomy without recharge and fully recharges in a single day. This tier removes the stress of multi-day cloudy weather from the equation.
Frequently Asked Questions
Can I run a chest freezer and refrigerator off the same solar generator?
Yes, if you size for both loads combined. Add each appliance's daily watt-hours before dividing by your DoD factor. The inverter surge rating must cover the larger compressor's startup at 3× running watts. Compressors rarely start simultaneously, but size for that scenario anyway.
What happens if my solar generator runs out of power overnight?
A full refrigerator holds 40°F for about 4 hours without power. A full chest freezer holds 0°F for up to 48 hours if unopened. If power fails mid-night, stop opening the appliances and connect AC shore power or a fuel generator first thing in the morning.
Is LiFePO4 always better than AGM for this application?
For a refrigerator backup application, yes in almost every case. LiFePO4 delivers 80% usable depth of discharge vs 50% for AGM, lasts 3,000-6,000 cycles vs 400-800 for AGM, and is safe indoors with no off-gassing. The only exception is very tight budgets where AGM cost per amp-hour wins short-term.
How many solar panels do I need to recharge a 3,000 Wh solar generator in one day?
Divide 3,000 Wh by your local peak sun hours times 0.85 efficiency. At 4 PSH that gives 3,000 divided by 3.4 equals 882 W of panels. At 5 PSH it drops to 706 W. Verify the generator's max solar input ceiling matches or exceeds that figure before buying panels.
Should I size for one night or two nights of autonomy without recharge?
Two nights is the safer target for most US households. Major storm outages average 24-72 hours of grid downtime according to EIA outage data. One-night sizing leaves no buffer for consecutive cloudy days. Two-night sizing covers the median outage duration with a full recharge day in between.