Battery & Solar Calculator
Need to find out how long it will take to charge a battery from solar panels, or a wall outlet?
Or maybe you want to calculate Runtime for a number of devices you want to run during the day, and you want to know how long you can run them all on batteries?
Check out our Prairie Light Press Battery & Solar Calculator.
Battery & Solar Calculator
Estimate how long a battery or power station will run your devices, and how long it takes to recharge from solar panels or a wall outlet.
Your battery
Your devices
Pick from the list or type your own. Watts are usually on the device label or charger. If it only shows amps, watts = volts × amps (120 V for wall plugs).
This is an example list. Replace it with your own devices.
Inverter (optional)
Used to check whether everything can run at the same time. Leave at 0 to skip.
Results
| Device | Wh / day | Share | Alone, nonstop |
|---|
How it works: usable Wh = volts × Ah × usable %. Each device's watts × hours is divided by the inverter efficiency (AC) or DC efficiency to get what the battery actually supplies. days = usable Wh ÷ daily Wh.
Your battery
How much to refill
Lithium can be drained almost to empty. Lead-acid and AGM should only go to about 50%, so that is all you'd refill.
Your solar panels
Peak sun hours are not daylight hours. Most of the US gets about 4–6 in summer and 2–3 in winter. Your local number is on solar maps such as NREL's.
Results
How it works: energy in = refill Wh ÷ battery charge efficiency. Real panel output = rated watts × real-world factor (heat, angle, dust, wiring) × controller efficiency. days = energy in ÷ (real output × sun hours).
Your daily use
Using the devices on the Run Time tab: — a day, including inverter losses.
Your location and panels
Plan for your worst season. If you need power in winter, use winter sun hours (often 2–3 in much of the US).
Your battery bank
How many days the battery should carry you with little or no sun. One day is fine for camping; off-grid homes often plan for two or three.
Results
How it works: panel watts = daily Wh ÷ battery charge efficiency ÷ (sun hours × real-world factor × controller efficiency). batteries = daily Wh × backup days ÷ usable Wh per battery. Controller amps = installed panel watts ÷ battery volts × 1.25 safety margin.
Your battery
How much to refill
Your charger
Use the charger's output rating. For a power station, use its AC charging input watts from the spec sheet.
Results
The last 10–20% usually charges more slowly, so a full charge often takes a bit longer than shown.
How it works: hours = refill Wh ÷ (charger watts × battery charge efficiency). With an amp rating, charger watts = amps × battery volts.
Your power bank
Power banks list mAh at the internal cell voltage, almost always 3.6–3.7 V, not the 5 V USB output. If the label shows Wh, enter that number as mAh × 1000 ÷ cell voltage, or use the Run Time tab in Wh.
Run a device
Charge a phone
The 4,323 mAh default is an iPhone 14 Pro Max.
Recharge the power bank
Results
How it works: Wh = mAh × cell volts ÷ 1000. About 15% is lost boosting the cell voltage up to 5 V USB. Device watts = mA × 5 V ÷ 1000. Phone batteries are figured at 3.85 V.
Assumptions used in every calculation
These are typical real-world values. Change them if you know your equipment's exact numbers. They apply to every tab.