How to Use the Battery and Solar Calculators

• Battery Runtime: how long an existing battery can run a steady load in watts.
• Battery Size: how much nominal battery capacity you need for the days you specify without charging, starting from full.
• Solar Size: how much rated solar panel capacity is needed to replace your daily battery use, based on the sun hours and losses you enter.
• Solar Charging Time: how much equivalent sunlight you need to restore a known amount of battery energy, with no loads using the battery during charging.
• System Checker: whether estimated daily solar production covers daily battery use, and how a repeated shortfall affects your remaining usable battery energy.

Do not enter Wh/day in a field that asks for W. Battery Runtime uses a constant load; the daily tools work with energy used over a full day.

Find capacity in Ah or Wh and the nominal voltage in the battery bank documentation. Use the ratings for the whole battery bank, not just one battery when several are connected. Nominal voltage is the rated voltage, not the live charging or load voltage.

100 Ah × 12.8 V = 1,280 Wh of nominal energy. This is only a unit conversion; it does not mean all 1,280 Wh are usable. The Ah to Wh and Wh to Ah tools convert these ratings.

When you switch capacity units, the calculator clears the value so it is not accidentally reinterpreted.

Current battery level is the state of charge (SoC) shown by your battery monitor. Minimum battery level is the point where you choose to stop discharging, based on the guidance for your battery and system.

Runtime and System Checker use the difference between current and minimum levels. Battery Size starts at 100%. The tools do not choose a minimum charge level based on battery chemistry. Actual capacity also depends on condition, temperature and discharge rate; the model assumes energy scales linearly with the selected charge range.

In the battery and solar planning tools, DC device energy is used directly. For AC devices powered through an inverter, these tools divide device energy by the inverter efficiency you enter. Use a manufacturer figure or measurement that matches the load as closely as possible. An advertised maximum efficiency is not necessarily your operating efficiency.

For example, 400 Wh at AC devices with an illustrative 80% efficiency requires 500 Wh from the battery. Do not add the same loss into the device subtotal first. Idle draw is not separately modeled.

Rated panel watts come from panel labels. Peak sun hours describe a day’s sunlight energy as equivalent hours at standard intensity; they are not the number of daylight hours. Use an estimate matched to location, season and panel orientation.

In Solar Size and System Checker, system efficiency accounts for the losses you include between rated panel output and energy available on the battery side. Solar Charging Time uses a different efficiency: how much charging energy is actually stored in the battery. These represent different parts of the system, so Setup does not transfer one efficiency into the other tool.

The tools do not choose sunlight or efficiency defaults. If you do not know them, find relevant data before relying on the estimate. Avoid counting the same loss twice, for example, once in your sun-hours estimate and again in system efficiency.

Suppose you use 600 Wh/day of DC energy and 400 Wh/day of AC energy. At an illustrative 80% inverter efficiency, the AC portion draws 500 Wh/day from the battery, for a total of 1,100 Wh/day. With four peak sun hours and an illustrative 75% solar delivery efficiency, the estimated panel size is 1,100 ÷ (4 × 0.75), or about 367 W. The detailed value is 366.67 W.

These numbers are examples, not recommended defaults.

A surplus in System Checker means estimated daily solar energy is greater than the energy needed from the battery, including inverter losses for AC devices. It does not guarantee that the battery will last through the night or that all surplus energy can be stored. “Days to minimum level” assumes the same average shortfall repeats each day; it is not a clock-time simulation.

Save known values in My Setup, then use them in compatible tools. Review every imported value and complete missing assumptions. Return to the calculator list to choose your question.