When it comes to preparing for power outages, many homeowners are turning to solar-powered generators and battery systems as a cleaner, quieter alternative to fuel-based options. But one of the biggest concerns when relying on solar is: How long will the battery last without sun?
The answer depends on your battery capacity, how much energy you’re using, and whether your system is powering just a few essential appliances or your entire home. In this guide, we’ll explain how to estimate your backup battery’s runtime when solar recharging isn’t possible—such as during extended storms or overnight blackouts.
Why Sunlight Isn’t Always Reliable During Outages
While solar is an excellent renewable energy source, it’s not always predictable during emergencies. Severe weather, snow, hurricanes, or even extended cloudy days can limit or eliminate your ability to recharge via solar panels.
That’s why battery capacity is critical. A well-sized solar battery ensures that you can keep essential devices running even when the sun isn’t shining—and may be the only power you have for several hours or even days.
Understanding Solar Battery Capacity
To understand how long your system will last, you first need to know how battery capacity works:
- Watt-hours (Wh): This tells you how much total energy your battery can store. For example, a 2,000Wh battery stores 2,000 watts of power for one hour—or 500 watts for 4 hours.
- Kilowatt-hours (kWh): 1 kWh = 1,000 Wh. Larger home battery systems are often measured in kWh.
Important: Not all of the battery’s rated capacity is usable. Most batteries are designed with a depth of discharge (DoD) of 80–100%. Lithium-ion batteries usually offer 90–95% usable capacity.
How to Calculate Battery Runtime Without Sunlight
Use this simple formula to estimate how long your battery will last when solar input is unavailable:
Runtime (hours) = Usable Battery Capacity (Wh) ÷ Total Appliance Load (W)
Example:
- Battery capacity: 2,000Wh (usable)
- Total appliance load: 400W (e.g., fridge + lights + router)
- Runtime: 2,000 ÷ 400 = 5 hours
This estimate assumes constant load. If devices cycle on/off (like refrigerators), real-world runtime could be longer.
Step-by-Step: Estimating Your Off-Sun Runtime
1. Know Your Battery Specs
Check your battery’s capacity in watt-hours or kilowatt-hours, and determine how much of that is usable. For example:
- Jackery Explorer 1500: 1,534Wh (90% usable = ~1,380Wh)
- EcoFlow Delta Pro: 3,600Wh (100% usable)
- Tesla Powerwall 2: 13.5kWh (100% usable)
2. Add Up Your Appliance Loads
Estimate the wattage of each device you’ll be running. Here are some common examples:
| Appliance | Running Watts |
|---|---|
| Refrigerator | 150W (cycles) |
| Wi-Fi Router | 15W |
| LED Lights (x4) | 40W |
| Phone Charging | 10W |
| TV | 100W |
| CPAP Machine | 60W |
3. Calculate Total Load
Let’s say you plan to run:
- Fridge (150W avg)
- Wi-Fi Router (15W)
- Lights (40W)
- Phone charging (10W)
- Total = 215W
4. Apply the Runtime Formula
If your battery is 2,000Wh usable:
2,000 ÷ 215 = ~9.3 hours of continuous use
Since the fridge cycles on and off, you may get closer to 12–15 hours in a real-world scenario.
How Long Will My Battery Last Without Sun? Sample Scenarios
🔋 Small Solar Generator (1,000Wh – 2,000Wh)
Appliances powered: Lights, router, phone, laptop
Runtime: 10–20 hours
⚡ Mid-Sized System (2,000Wh – 5,000Wh)
Appliances powered: Fridge, lights, TV, Wi-Fi, CPAP
Runtime: 12–36 hours (depending on usage)
🏠 Whole-Home Battery (10kWh – 30kWh)
Appliances powered: Full-home circuit or essential loads
Runtime: 1 – 3 days (without sun)
Remember: once the battery is depleted, you must wait for the sun to recharge it—or use an AC charger or generator, if available.
What Happens When the Battery Runs Out?
Once your battery is fully discharged, your system will shut down until it’s recharged. To prevent unexpected blackouts:
- ✅ Monitor battery levels closely during cloudy days or nights
- ✅ Prioritize essential devices to conserve power
- ✅ Use power-saving modes and LED lighting
- ✅ Recharge whenever sunlight returns
Some systems offer smart load management or allow app-based control to turn devices on/off remotely.
Tips to Extend Battery Life During Cloudy Days or Nighttime
- Turn off high-draw devices: Heaters, microwaves, and A/C units will deplete your battery quickly.
- Use efficient appliances: Energy Star-rated devices and LED lights consume far less power.
- Ration usage: Run appliances in shifts to avoid overloading the battery.
- Keep fridge closed: Reduce energy consumption by minimizing door openings.
- Add more storage: Some systems let you expand battery capacity with add-on modules.
How Battery Chemistry Affects Performance
The type of battery inside your system affects both lifespan and usable capacity during sunless periods.
| Battery Type | Typical DoD | Cycle Life |
|---|---|---|
| Lithium-Ion (Li-ion) | 90 – 95% | 2,000 – 5,000+ cycles |
| Lithium Iron Phosphate (LiFePO4) | 95 – 100% | 3,000 – 6,000+ cycles |
| Lead Acid (AGM/Gel) | 50 – 70% | 500 – 1,200 cycles |
Recommendation: Choose LiFePO4 batteries for best performance, especially for long-term outages and solar storage.
Top Battery Backup Systems With Long Runtime Potential
🔋 EcoFlow Delta Pro
- 3,600Wh capacity, expandable up to 25kWh
- Smart app control and 3,600W output
- Can power heavy loads like refrigerators, sump pumps, and more
🔋 Bluetti AC300 + B300
- Modular system with 3,072Wh per B300 battery
- Supports up to 12,288Wh with expansions
- LiFePO4 battery chemistry for deep discharge
🔋 Tesla Powerwall 2
- 13.5kWh capacity per unit
- Whole-home integration and automatic switchover
- Designed for grid-tied and off-grid solar systems
FAQs: How Long Will My Battery Last Without Sun?
Q: How long will a 2,000Wh battery last during a blackout?
It depends on your usage. For a 400W load, it will last about 5 hours. With careful use (lights, router, fridge), it may last 12–15 hours.
Q: Will my solar battery charge during cloudy weather?
Yes—but at a reduced rate. Expect 20–50% of peak solar panel output on cloudy days. Full sun is required for fast recharging.
Q: What if I have no sun for 2–3 days?
Choose a system with enough battery capacity to store multiple days of energy—or one that allows AC generator charging as a backup.
Q: Can I add more batteries later?
Many systems (like EcoFlow, Bluetti, and Goal Zero) offer expandable battery modules to increase runtime as needed.
Q: Does cold weather affect battery performance?
Yes. Cold temperatures can reduce battery efficiency and capacity. Store your battery indoors if possible during winter.
Conclusion: Plan Ahead for Sunless Backup Power
If you’re relying on a solar generator or battery system for emergency power, it’s essential to plan for times when the sun isn’t available. With the right size battery, energy-efficient appliances, and smart load management, you can stay powered overnight—or even through multiple cloudy days.
Start by understanding your daily energy needs, then choose a solar battery system that gives you a safety buffer. In emergency situations, stored energy can be your most valuable resource—especially when the sun isn’t shining.
Need Help Choosing the Right Backup Power System?
Start with our expert guide: What Size Generator Do I Need for My Home? to calculate your power needs and explore solar and battery solutions tailored to your household.