How Long Will My Solar Battery Last?
Solar batteries face a unique challenge: they must store enough energy during the day to power loads through the night, and survive extended periods without sun. The answer depends on your battery capacity, daily consumption, and how well the solar array replenishes what you use.
Daytime vs. Nighttime Operation
During the day, solar panels directly power your loads and any excess energy charges the battery. At night, the battery alone must supply all loads until sunrise. The critical runtime question is: how long does the stored energy last from sunset to sunrise?
This is fundamentally different from a simple battery backup scenario because the solar system is designed to cycle daily — charge during the day, discharge at night. The battery does not need to power your home for days on end (unless you are sizing for cloudy-day autonomy). It needs to bridge the overnight gap.
Solar Battery Runtime Formulas
Efficiency accounts for inverter losses during the night and charge controller losses during the day. Use 0.90–0.92 for modern equipment.
Worked Example: Overnight Runtime
Given:
- Solar battery: 200 Ah at 48V LFP (10,240 Wh total)
- DoD limit: 80%
- System efficiency: 92%
- Overnight loads: fridge (60W) + router (10W) + 4 LED lights (24W) + phone charging (15W) = 109W
- Hours of darkness: 12 hours
Step 1: Usable battery energy:
Step 2: Effective overnight load:
Step 3: Night runtime:
This battery can power these overnight loads for over 5 consecutive nights without any solar input — more than enough to survive extended cloudy periods.
Autonomy During Cloudy Periods
Cloudy-day autonomy is the number of days your battery can power your loads without meaningful solar production. This is the most critical sizing parameter for off-grid systems in regions with variable weather.
The formula for autonomy is straightforward:
A larger battery bank provides more autonomy days but costs more. The optimal balance depends on your local climate, how critical the loads are, and whether you have a backup generator.
Autonomy Reference by Battery Size
The table below shows autonomy days for common 48V LFP battery banks at 80% DoD and 92% efficiency, for different daily consumption levels.
| Battery Size | 5 kWh/day | 10 kWh/day | 20 kWh/day | 30 kWh/day |
|---|---|---|---|---|
| 5 kWh (50Ah 48V) | 0.7 days | 0.4 days | 0.2 days | 0.1 days |
| 10 kWh (100Ah 48V) | 1.5 days | 0.7 days | 0.4 days | 0.2 days |
| 20 kWh (200Ah 48V) | 2.9 days | 1.5 days | 0.7 days | 0.5 days |
| 40 kWh (400Ah 48V) | 5.9 days | 2.9 days | 1.5 days | 1.0 days |
| 80 kWh (800Ah 48V) | 11.8 days | 5.9 days | 2.9 days | 2.0 days |
The Role of Solar Recharge
Your solar array must generate enough energy during peak sun hours to both power daytime loads and recharge the battery for nighttime use. The minimum array size is:
For a home consuming 15 kWh/day with 5 peak sun hours, the minimum array is 3,000W (3 kW). Oversizing by 20–30% accounts for cloudy days and system losses, making a 4 kW array a practical choice.
If the array is undersized, the battery will not fully recharge each day. Over time, the SOC will trend downward until the battery is empty. This is the most common cause of solar system underperformance.
Factors That Reduce Solar Battery Runtime
Seasonal Variation
Winter has shorter days, lower sun angle, and more cloud cover. A system that runs comfortably in summer may struggle in winter. Size for your worst-case season if reliability is critical.
Panel Degradation
Solar panels lose 0.5–0.7% efficiency per year. After 10 years, output is roughly 93–95% of rated. This reduces daily recharge energy and extends the time needed to replenish the battery.
Load Growth
Adding appliances without upgrading the battery bank reduces autonomy. A new space heater or EV charger can dramatically increase daily consumption beyond what the system was sized for.
Temperature Derating
Cold batteries accept less charge from solar and deliver less energy at night. In cold climates, oversized batteries by 15–20% to compensate for reduced winter capacity.
Try It
Use the Solar Battery Sizing Calculator to determine the right bank size for your daily consumption and autonomy requirements.
Open Solar Battery Sizing CalculatorRelated
Estimate how long your battery powers specific loads with the Runtime Calculator.
Open Runtime CalculatorRelated Articles
How to Size a Solar Battery Bank
The complete method for sizing solar storage from consumption, autonomy, and DoD.
Read Guide →How Long Will a Battery Last?
The general method for calculating battery runtime from capacity and load.
Read Guide →Frequently Asked Questions
How long will a solar battery last at night?
A solar battery's nighttime runtime depends on its usable capacity and the loads it powers. A 10 kWh usable LFP bank powering 500W of overnight loads (fridge, lights, router) runs for approximately 16 hours — from sunset to sunrise with margin.
How many days can a solar battery power a house?
It depends on battery capacity and daily consumption. A 13.5 kWh usable battery (typical Powerwall size) powering a home consuming 30 kWh/day provides roughly 0.45 days of autonomy. Most homes need 2–3 days of autonomy, requiring 60–90 kWh of usable capacity.
Does solar recharge the battery during the day?
Yes, if the solar array is sized correctly. A properly sized array generates enough energy during peak sun hours to both power daytime loads and recharge the battery. If the array is undersized, the battery will never fully recover, reducing nighttime runtime.
What happens when the solar battery is empty?
If the battery reaches its minimum SOC (typically 10–20% for LFP), the system either switches to grid power (hybrid systems) or disconnects loads (off-grid). The battery remains in standby until solar production resumes and recharges it above the minimum threshold.