Solar + Battery Calculator

About the Solar + Battery Calculator

Sizes a solar array and battery bank to cover daily consumption with a target amount of overnight/backup storage.

How to use this calculator

1. Enter your daily electricity use in \(kWh\) and any target for how much you want to offset.
2. Add your location or average sun hours if the calculator asks for them, because solar production depends on sunlight.
3. Choose battery details such as backup hours, nighttime use, or desired storage days.
4. Review the suggested solar size and battery capacity, then compare them with your roof space and budget.

The formula explained

The calculator estimates solar size from energy needs and sun availability, then estimates battery size from the amount of electricity you want to store for later use. In simple terms, it converts your daily usage into panel capacity and storage capacity.

• E = daily energy use in \(kWh\)
• H = average peak sun hours per day in \(h\)
• P = solar panel capacity needed in \(kW\)
• B = battery storage needed in \(kWh\)
• \(\eta\) = system efficiency as a decimal, accounting for losses

Step by step method

1. Find your daily electricity use, for example \(18\\,\\text{kWh}\) per day.
2. Estimate the solar size with \(P = \frac{E}{H\\eta}\), using your sun hours and efficiency.
3. Estimate battery size from the energy you want to store, then adjust for usable depth of discharge if needed.
4. Compare the result with your roof space, battery budget, and whether you want full backup or only evening use.

Worked example

Problem. A home uses \(18\\,\\text{kWh}\) per day. The location gets \(5\) peak sun hours per day, and the system efficiency is \(0.8\). The homeowner wants \(9\\,\\text{kWh}\) of battery storage for evening use.

1. Compute solar size: \(P = \frac{18}{5 \times 0.8} = \frac{18}{4} = 4.5\\,\\text{kW}\).
2. Battery target is already given as \(9\\,\\text{kWh}\) of usable storage.
3. So the recommended size is about \(4.5\\,\\text{kW}\) of solar and \(9\\,\\text{kWh}\) of battery storage.

Tips and common mistakes

• Do not size only for the average day if you need reliable backup, because cloudy periods can require more storage.
• Battery ratings can show either total capacity or usable capacity, so check which one the calculator is using.

Frequently asked questions