How Many Batteries Do I Need For a 250W Solar Panel?

250W solar panels have replaced 100W modules as the typical size used in homes and RVs, as they are more powerful and widely available. With the extra power comes the question of where do you store the extra power? How many batteries will you need to reserve the energy produced by a 250W solar panel?

A 250W solar panel does not need batteries if it is on a grid tie system because excess energy is collected in the power grid. 250W solar panels can produce 1200W a day with 5 sun hours, so you need a 100ah battery if you are off grid and have to reserve excess solar power.

250W Solar Panel Battery Requirement Calculation

The formula is: total solar panel watts per day / battery volts = battery size in amp hours.

Most 250W solar panels are designed to work with 12V batteries. In peak sun the panel can produce 250 watts an hour, but the current will drop below that as the day goes on. It will be highest at noon and drop off a little in the afternoon. You also have to factor in the seasons and geographical location, so allow for some fluctuations.

While it is possible for a 250W panel to produce 1200W a day (250 x 5 sun hours = 1200), it is more likely to be 1000W to 1100W.

Assuming you have a standard 18V solar panel, you’ll need 12V batteries. Using the formula given:

1100W / 12 = 91.6 amp hours (ah)

Rounded off that is 100ah, so a 100ah battery can store 1200W of solar power. If your panel produces 1200W the result is exactly 100ah. Theoretically you can use a 100ah battery, but it is better to buy a 120ah unit instead. It is never a good idea to fully charge or fully deplete batteries because it shortens the life cycle.

This conversion formula uses standard voltages for the panels and batteries. There are 24V, 36V and 48V batteries but they might not work with your 18V modules. Just as you want to make certain your inverter matches your battery bank, same thing for your solar panels.

The calculations assume 5 hours of sunlight, but in some parts of the US you will get more, and in others less. The same 250W panel will produce different yields depending on where it is, say, Texas or New York. If you are in the Lone Star State you will get a higher average daily production output compared to NY.

Do You Need Even Need a Solar Battery?

If your solar panel is on a grid tied system, you don’t even need a battery. A battery bank is only required if you are off the grid and want to store solar energy and use power during evenings and cloudy days.

All solar panels follow a basic principle in how they work , and the same with batteries. Solar panels cannot produce energy at night and when it rains. All the excess energy it produces will go to waste at the end of the day as well if not stored.

If your solar panel system is connected to the grid, you can use the grid as a battery. Extra power generated by your 250W solar panel (or array) are stored in the grid. You get credit from the power company that you can use anytime.

So if you have excess energy during the summer, store it in the grid. When winter comes and solar production drops, you can use your credit on the grid and avail of free electricity until your credit is used up.

That is how most solar powered homes in the US are set up. The drawback though is if the power grid goes down, you have no access to solar power. The grid shuts down the solar panels in case of a fire or electrocution, as the firefighters’ safety is a priority.

This is where solar batteries come into fore, because now you are independent from the grid.

Benefits of a Solar Battery

A battery performs the same function as the power grid on a smaller scale. Power generated by your 250W panel is stored in it. With an inverter, you can connect appliances to the battery and run them off solar power.

The difference between the grid and your battery bank is accessibility. When power goes out on a grid tied system, your solar power is out as well. But in an off grid setup, you are an independent power plant. When solar production stops during the evening, you can run your devices from the battery.

You use the solar panel to charge the battery, and the battery runs your appliances. That is how it works in off grid systems and in RVs. As long as you can keep the battery charged, you can keep running your appliances indefinitely.

Just to emphasize though, you cannot run appliances with just a battery bank and solar panel. You need an inverterand a charge controller. Plus you have to calculate how many watts you use so your system can provide for it.

A 250W solar panel will not be enough to run a house obviously, but even large solar panels will be insufficient to keep a large RV going. That is why most off grid systems have a power back up like a portable solar generator or others in case solar is insufficient.

The next question is how many batteries do you need? For a single 250W solar panel with 5 hours of sunlight, 100ah will do. But what if you have several solar panels?

How Many 250W Solar Panels Will You Use?

Add the total solar panel watts and divide it by the battery volt. If you have 3 x 250W solar panels, the total watt output will be 3600W. This requires at minimum a 300ah 12V battery.

You can connect two or more 250W solar panels to increase the power. There are two ways to join solar panels, in a series and in parallel.

• In a series, the solar panel voltages increase but the amps remain the same.
• In a parallel, the voltages remain the same but the amps increases.

To connect solar panels in a series, connect the negative terminal of a solar panel to the positive terminal of the next solar panel.

To connect solar panels in parallel, connect the positive terminal of a solar panel to the positive terminal of the next panel. Do the same with the negative terminals.

If you have 2 x 250W solar panels and each was 12V and 20 amps, they would be 24V and 20 amps if connected in a series.

If you have 2 x 250W solar panels and each was 12V and 20 amps, they would be 12V and 40 amps if connected in parallel.

Which set up you use depends on how much power you need. When we say 250W, that is the peak output of the module. As we pointed out, the actual output will be different from its rating due to various factors. It can reach 250W at noon, but it will fall down as the sun goes down. And obviously production is higher during the summer than winter.

Which Battery is Best For 250W Solar Panels?

Lithium ion batteries are more efficient than lead acid, have a better depth of discharge and lower maintenance. For off grid homes and RVs, lithium is the better choice though they are more expensive.

The calculations given earlier assume the solar panel will be filling up an empty battery. Lead acid batteries have a 50% discharge rate, so a 250W module only needs to supply 600W to top if off. This also means you only get to use 50% of the capacity, that is 600W of the 1200W the battery has. If you need the full 1200W you have to buy a 200ah or two 100ah batteries.

The more solar panels and batteries you use, the more power the system generates. Your charge controller must be capable of handling the amps being produced. The inverter must also be of sufficient power to handle the watts generated by the system.

For just one 250W panel, lead acid batteries may be sufficient. As long as you know its discharge limits and that’s all right with you, there will not be any problems. However if you are going to set up a battery bank and connect several 250W solar panels, better get lithium batteries. You will be surprised how quickly batteries can drop to the 50% mark, and recharging frequently can be a hindrance.

Conclusion

In summary, how many batteries you will need depends on your system setup. If you are on the grid there’s no need for it, but if you are on an RV or off grid house, a minimum 100ah battery is required. The more 250W soar panels you have the larger the battery bank must be, so always plan first before buying any components.