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Calculating Solar Panel, Inverter, Battery Charger

If you are designing a solar electricity system and don't have access to the grid, you will have to deal with solar batteries. After having decided which type of battery to use, it will be time to size your system. During this step, you are going to encounter a little math. Fortunately, SolarTown is here to guide you through the calculations. In general, the system should be big enough to supply all your energy needs for a few cloudy days but still small enough to be charged by your solar panels. Here are the steps to sizing your system.

Using a solar panel system at home is both economical and environmentally friendly. But how do you choose a suitable battery and inverter? What is the calculation of solar panels and batteries?  Also, how to work out all the precise calculations of the solar panel, battery, inverter, and charge controller may bother you a lot.

These questions have to be answered mathematically. In other words, you need some specific data of your power need and certain calculating processes to know the equipment and components' specifications.

The calculations are significant for you because they help you to build a better-performance solar system.

Sizing Solar Panels, Batteries And Inverters For A Solar System

A true off-grid solar power system includes solar panels, a bank of batteries for energy storage and one or more inverters. This kind of system has no connection to the utility grid.

It is possible to have home battery storage, even when normally using the utility company's grid connection. The batteries automatically come online if or when the normal electricity supply fails.

  1. Size the solar panels according to energy consumption
  2. Size the inverter according to the solar panel system power rating
  3. Size the battery bank according to how many hours you need it to run, i.e. autonomy

Solar panel size is found by dividing daily load kWh by the location's irradiance to give a solar kW rating. Inverter size is equal to the solar panel rating. Battery size is found by multiplying the daily load by the number of days autonomy required and dividing by system volts to give amp-hours.

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Frequently Asked Questions About Solar Panels

A rule of thumb is to choose a panel with roughly the same number of watts as the battery has amp-hours. Return to the battery article for a refresher on amp-hours if you need it — but a 55-battery like the Optima we used can comfortably pair with a 55-watt or slightly higher output panel.

If you went for a setup with a 100 watts solar panel, you might still have some questions about the size of your battery bank. Solar Industry studies estimate that a 100-watt solar panel can produce up to 30 Ah of battery charge with a 5-9 hour timeline sun exposition.

TA solar panel can be connected directly to a 12-volt car battery but must be monitored if it's more than 5 watts. Solar panels rated higher than 5 watts must not be connected directly to a battery but only through a solar charge controller to protect against over-charging.

A 100ah battery can provide 1 Amp for 100 hours, 2 Amp for 50 hours, and 3 Amp for 33 hours. A simple example, but the heavy user of power, is a hairdryer that is 10 Amps. A 100 ah battery will give you 10 hours of use.

Estimating Load Wattage

First, you will need to estimate how many watts of electricity you may require for the specified load.

Let's say you have a 100-watt load that needs to be operated for approximately 10 hours. In that case, the total power required could be estimated simply by multiplying the load with hours, as given under

100 Watts x 10 hours = 1,000 Watt-hours. This becomes the absolute power necessary from the panel.

Determining Approximate Solar Panel Dimension

Next, we need to determine the solar panel's approximate dimensions to satisfy the above-estimated load requirement. If we assume a roughly ten-hour daily optimal sunshine, the specifications for the solar panel could be simply and quickly calculated as explained in the following expression:

1,000 Watt-hours / 10 hours sunlight = 100 Watt solar panel.

However, you may notice that mostly during the summer seasons, you may normally get around 10 hours of a reasonable amount of sunshine. Still, the winter season may produce roughly around 4-5 hours of effective sunshine.

Contemplating the above scenario, you too might agree and recommend considering the worst possible sunshine hour into calculation so that your load keeps running optimally even on the weakest of rays of sunshine.

Therefore, considering the 4 to 5 hours of sunshine per day consideration, we calculate the true power for the solar panel, which would enable your load to keep running throughout the year.

1,000 Watt hours / 5 hours sunlight = 200 Watt solar panel.

How To Work Out Battery Amp-Hours?

Battery capacity is important for you to make sure that the solar panel system operates smoothly and effectively. If you can't access the grid, or you simply want to store the solar energy into a battery as a power backup for emergencies.

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Solar Panel Wattage

When you figure out the total load wattage of your house, you need to firstly rate all the parameters of a suitable solar panel to meet your electricity requirement. This is directly related to the best sunlight hours.

For example: Supposing the best sunlight hours in your area is approximately 9 hours a day, you can get the required dimension of your panel by the below formula:

1800 watt-hours ÷ 9 hours = 200 watts (a 200-watt solar panel)

Battery Amp Hours

Regarding battery amp-hours, you need to think about how much power you hope your battery bank to store as a backup. Namely, how many days do you want the battery to function without recharging? Generally, that would be 2- 5 days.

Since we have calculated the specific dimensions of a suitable solar panel, we can better estimate the battery's amp-hour (AH) rating. And it should support operations in any possible conditions.

And there are batteries with voltages of 12, 24, and 48 for various applications. If you choose to use a 12V battery, you can get a parameter by the given formula:

1800 watt-hour ÷ 12V = 150 Amp Hours

Usually, we would like to add the battery a 20% tolerance to guarantee perfect usage. So you then get the battery capacity as:

150 Amp Hours x 1.2 = 180 Amp-hours

To ensure there is always enough left power for extra days' use, the battery should handle double its normal usage. So you need to multiply the last result by 2 and again multiply by your days. Suppose you want your battery bank to keep running for extra two days. The final battery capacity should be:

180 amp hours x 2 x 2 = 720 amp hours

How To Estimate Inverter Specifications?

Charge Controller Specification

You may ignore a component like a charge controller, but it is also significant to help your solar system to perform perfectly.

A charge controller is used to prevent your model from overcharging. It interfaces the solar panels and the batteries. Thus it plays an important role in regulating solar power from solar panels to batteries.

You need the already figured out results from the above example to calculate how big your satisfying charge controller needs to be. Then, you can simply divide the solar panel's load wattage with the battery's voltage rating.

Charge controller sizing is the next step when sizing your system. As you have probably not yet encountered these components, we will briefly discuss them. If you wish to get straight to sizing your charge controller, skip to calculation.

Charge controllers regulate the power coming from the solar panels to the batteries. They are a key part of any off-grid system and prevent batteries from over-charging. We will discuss two kinds of charge controllers: PWM and MPPT.

PWM (Pulse-Width Modulation) controllers are cheaper than MPPT but create large power losses. Up to 60% of power can be lost. This is because PWM controllers do not optimize the voltage going to the batteries. This limitation makes a PWM controller a poor choice for a large system. However, in smaller systems, their low price makes them a viable option.

MPPT (Maximum Power Point Tracking) controllers optimize the voltage coming from the solar panels to transfer the maximum amount of energy to the battery bank. The maximum power point, or the optimal conversion voltage, will fluctuate with changes in light intensity, temperature and other factors. The digital optimization process performed by the MPPT controller finds and adjusts to the maximum power point quickly. Sophisticated electronics are needed in MPPT controllers to do this, which explains their high price. However, there is a significant pay-off: MPPT controllers are 93-97% efficient in converting power.

And as we have done to the above parameters, we can also add an extra tolerance like 20% to this value, and then we get a rough idea of the charge controller specification:

1800 watts ÷ 12V = 150 Amps

150 Amps x 1.2 = 180 Amps

Inverter Specification

At the last stage, we need to evaluate the inverter specifications. Finding a capable inverter allows your solar system to operate well with the batteries and charge controllers. And to avoid any disorder situations, you need to calculate the specific dimensions of an inverter.

The good news is that after working out all these required parameters, estimating the inverter parameters is not a complex task anymore.

Still, supposing the peak load wattage of your home is 200 watts, and you need the inverter to handle such a wattage effortlessly, then you just need to buy an inverter rated at 200 watts.

Or, if you would like it to have a little more tolerance, simply add 20%-25%. Then the value will be 240-watt – 250-watt.

Well, calculating the inverter specs doesn't look difficult at this point of the discussion.

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Since we already know the maximum load wattage, which is 100 Watts, it implies that we simply choose an inverter that might be capable of handling a 100 watt comfortably.

That implies we simply need to get an inverter rated at 100 watts. OK, you may be thinking of adding some tolerance to this candidate also, not an issue; instead of 100 watts, you can opt for a 125-watt inverter, allowing all the gadgets to happily "shake-hands" and your house powered round the clock forever, free of cost.

Battery wiring - putting it all together.

Wiring is going to play a major role in determining the number of batteries you need. The goal, in this final step, is to produce target AH and voltage. There are two methods of wiring components in a circuit: parallel and series. In the following diagrams, blue batteries are in parallel, and red batteries are in series. In a series configuration, the battery voltages add up while in parallel, currently adds up.

Series and parallel connections can be combined to produce the voltage and AH that you require. Just remember:                        

Series voltage adds current does not

Parallel → current adds, the voltage does not

We want to mention that parallel connections are to be minimized as they can decrease battery life. If a used battery is connected in parallel to a new one, it will degrade the fresher battery, and the lifespan of the whole system will decrease. This characteristic has concluded that an ideal battery bank would consist of a long line of batteries connected in series. Unfortunately, this is not always possible due to the voltage and AH requirements of a system.

As we mentioned earlier, it is not always easy to determine how many batteries you need to power your home. This is because wiring configurations have a huge impact on the output of a battery bank. So always design your storage system before you buy any components!


How big a solar system your house needs, what size the battery should be, or the capability of a satisfying inverter can always be scientifically and precisely answered.

What you need to do is calculate all these specifications by following the given formulas. Although that is a math issue, you can still work it out because the expressions and guides are simple and understandable.

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