Lesson 2: Buy your Solar Battery Charger Equipment

At this point, you know what your project array power will be or you have already acquired your panels (if not, read Lesson 1: How to Size Your Array first). In this lesson, we will help you with the following:

Making a schematic drawing of your system;

Where to get low-priced, high-quality PV panels;

What size and type of battery you should buy;

Whether you should invest in a MPPT charge controller;

Where to buy cheap Balance-of-System (BOS) components (cables, connectors, etc.) for your solar array.

Our 135 W solar battery charger. Follow these lessons and build one yourself! – Click to enlarge

Before we start spending money, it is important to make a general list of items you need for your array. Make a schematic drawing of your system, showing voltages and maximum currents you’re expecting on your wires and electrical components. Each component has its own range of voltages and currents in which it can operate, and you should therefore first check if all components are compatible with all the others. So read this whole page first, before you order your PV array parts!

Making a schematic array drawing

Keeping the ‘System Configuration’ theory in mind, make a nice drawing on a piece of paper where you outline your system. See below the drawings Garrett and I made for the example array we built. Each PV module has a voltage Vmp = 32.9 V. Since that is relatively high compared to our 12 V battery bank, we decided to parallel-couple the panels. Both the battery bank as well as the load operate at that 12 V level. With some cheap cigarette lighter converters you can get for a car, we built a very universal load bank, where people can plug-in their iPhones and Blackberries for a free green charge!

A schematic diagram of the small PV array that we built. The voltages at the charge controller are 47 V (open circuit panels), 12 V (battery) and 12 V (loads).

The charge controller is the brain of the system that regulates voltage levels for your array, battery and load. It is handy to give it a central place in your drawing as well as in your actual array.

Low-priced, high-quality PV panels

Watch out! Do not buy the very first ‘BIG DISCOUNT’ offer you see when you browse the web! Many solar panel retailers sell their products with huge profit-margins, and it is beneficial to spend a fair amount of time browsing the web for good-quality panels available at wholesale prices.

$/Watt, that’s what you think…

The main unit of comparison when buying your panels is $/Watt. It’s similar to buying beer, in terms of $/gallon, you get more bang for your buck when you purchase forties. With solar panels, you have to keep in mind that shipping and handling costs are never included in this $/Watt number when retailers promote their products. Besides that, they display ‘Pallet Prices’ instead of individual panel prices. It is therefore smart to continue to the check-out page of the online retail store and redo the calculation for $/Watt before you make your purchase.

Example: This retailer shows an ad for 0.98 $/Watt panels, but you would end up paying $547.19 for 320 Watts including the shipping. That is 1.71 $/Watt, still cheap, but almost twice as much as advertised! The altEstore has some well-priced panels, but shipping is relatively high for small orders ($175 for one $525-panel). Some vendors like Affordable-Solar.com don’t display a shipping cost: you have to call their office to get a shipping quote.

The best thing you can do is browse the web and try to find an offer for below $2.5/Watt delivered. Ecobusinesslinks.com provides a good starting point: it compares offers from multiple vendors. Unfortunately, the list does not include shipping costs in the $/Watt indicator.

One thing is important to note when buying panels for a small (< 0.5 kW) battery-connected system like ours: You should buy panels with a fairly low maximum power point voltage (Vmp), preferably under 35 V. This is because many charge controllers built for smaller arrays can’t handle PV modules generating power at higher voltages. Read more about this in the MPPT section.

What size and type of battery you should buy for your PV system

In the first lesson, you learned how to size your solar array by using PVWatts to find the average daily yield in each month. Of course, this electricity has to be stored somewhere unless your load is directly connected to the solar panels. So lets take a look at what size and type of batteries you should get. Again, you can save some money here on making a good decision.

Size (a.k.a. capacity)

Battery capacity is often denoted as ‘Ah’ or ‘mAh’, which doesn’t say anything about its storage capabilities without the voltage of the battery. Multiply the two and you get something more useful: ‘Wh’ or ‘mWh’.

It is obvious that you should at least get a capacity that exceeds the battery capacity of the electrical device you plan to charge with your solar charger. It makes sense to oversize it with a factor 2 or 3, so that you are prepared for possible battery performance loss and you can store more juice in those sunny summer days. The size very much depends on the application of your system. If you need to run some 5 Watt LEDs for 10 hours a day, you need (!at least!) 50 Wh of capacity in your battery. Garrett and I chose for a 40 Ah @ 12 V lead-acid battery that will allow us to run some lights and charge our laptops and phones during our Solar Journey from NY to CA. With our 135 Watt array, we can charge this battery in a day with approximately 5 full sun-hours, taken into account losses from the wiring, panels and the battery.

As per the type of battery to get, we would recommend buying either sealed lead-acid batteries or li-ion. Other battery types like NiMH and NiCd require special charging algorithms that are not supported in most charge controllers. These batteries are also more sensitive for high/low temperatures and NiMH has a fairly high self-discharge rate. Read more about comparison of batteries here. So the choice is between lead-acid and li-ion. Lead-acid is the cheap but heavy option. This is a great pick for stationary applications, where you don’t care about the battery’s weight. If you want to get all fancy, buy the more expensive li-ion batteries. They are very light and easy to handle, but keep in mind that not all charge controllers support this technology (like our Morningstar Sunsaver MPPT).

Whether you should invest in a MPPT

A Maximum Power Point Tracker (MPPT) is a special type of charge controller that optimizes the energy yield of your array by controlling the panel’s operating voltage.

As you already know, the PV panels produce a current at a certain voltage that is equal to the voltage of the load attached to the array. This operating voltage is very important: if it is not set right, you can miss out on the full potential of your panels. In the figure to the right you can see why.

If the operating voltage of the cell is set to 0.58 V, the current is about 2.6 A (Point A). The total output of the cell is therefore 1.51 Watts, which is equal to the area of the yellow rectangle in the graph. At Point B, the cell operates at 0.76 V and 1.9 A, generating a little bit less: 1.44 Watts (green rectangle). Yes, you got it, the MPP is the Maximum Power Point: the rectangle under the curve is at its maximum! The cell is producing 1.77 Watts at that point.

How a Maximum Power Point Tracker works: calculating the resulting power from several points along the operating line. – Click to enlarge

The MPPT arbitrarily sets the voltage when the system is connected and then it tries out different operating points until it finds the MPP. The device simply multiplies the current and voltage for a point and then compares it to the last operating point. Depending on your configuration and component specs, you can increase your yield up to 30% compared to a conventional charge controller. All these extra kWh come with a cost though: the MPPT version of a $50 charge controller can cost you $200.

For small systems (<100 W), it doesn’t make sense to buy an MPPT. It would be better to spend your money on a 30% bigger array. For large arrays (>250 W) it is worth the money. In our case (135 W), we decided to buy one so that we can get experience working with the device. After all, we are building a 7 kW system later…

The market for small MPPTs can be divided into two markets: Good quality and bad quality. There are a couple of good manufacturers of MPPT charge controllers out there although only three (Morningstar, Blue Sky Energy and BZ Products) build MPPTs for smaller systems:


Blue Sky Energy

BZ Products

Xantrex (>500 W)

Outback (> 500 W)

Apollo (> 500 W)

Midnite (> 500 W)

Cheaper MPPTs are typically from Chinese manufacturers like:

Juta Solar


Silicon Chip

Here’s a specs list with typical MPPT prices for <500 W systems:

BrandModelSystem Voltage (V)Max battery current (A)Max Open Circuit Voltage @ STC (V)Max PV input (W)Price excl. shipping ($)MorningstarSSMPPT-15L12/241575200/400200-250Blue Sky EnergySB1524iX12/2420/1545.6200/400~250Blue Sky EnergySB2512iX122528340170-220BZ ProductsMPPT 25012/242550250120-130Juta SolarMPPT 10A12/2410?200/400~70Juta SolarMPPT 15A12/24/4815?200/400/800~74WellseeWS-MPPT1512/241540/80?~31

This table displays small (<500 W) Maximum Power Point Trackers (MPPT) that are available on the market now.

You can see that there is a big price range between same sized MPPTs. We have bought two of these so far, the Morningstar SunSaver and the Wellsee MPPT-15. If you hold one in each hand, you can tell that the difference in quality is big. The Morningstar feels tough, it is a little heavier and not assembled in a simple plastic casing, held together by four screws. Lets not judge MPPTs by their looks and feels though, we should test and compare their performance!

We will post a comparative review of the Morningstar compared to the Wellsee and maybe later the Juta Solar MPPT as well. Stay tuned!

Where to buy cheap Balance-of-System (BOS) components for your solar array

Besides the main electrical components for your solar battery charger, you need to have cables, connectors, fuse boxes etc. Lets go over the parts we bought for our array and explain what type we got and why.

For sizing of battery and panels I refer to ‘Lesson 1: Sizing your Array’. We found a great online store that ships the battery for free (at least our shipment). For your panels, begin your search with the provided link in the table. Below, we give some smart tips on some important components.

The Morningstar MPPT charge controller (with Meterbus Adapter) we bought was quite expensive (one-third of the total). As we mentioned before, you can also get a conventional PWM charge controller that will regulate only the battery and load’s voltage. In our case, we wanted to have this more expensive controller because of the increased yield and also for the possibility to send data to our computer. This will be handy for educational purposes. When you have targeted your MPPT, download the manual and thoroughly read it before you buy other components. You might find constraints that will limit your freedom of choice in other components. Two examples: the MPPT we used only works with lead-acid batteries and the Wellsee MPPT only fits smaller cable sizes.

Morningstar Sunsaver MPPT 15A – Click to enlarge

The connectors you need depend on the type of solar panel you buy. Most panels nowadays come with MC4 connectors, but look at the manual to find out what type it is and figure out how many you need. The MC4 male connector of the panel is the positive side, the female is negative. You might want to buy a little tool to connect/disconnect the MC4s: like some good marriages, they are hard to separate. We used simple pliers for this task.

A female (left) and male (right) connector. On the PV panels, females are negative and males are positive 😉 – Click to enlarge

Our fuse holders are a little bulky, but they are tough and easy to use. It is impossible to get shocked when inserting the fuses, because they only connect after you close the lid. Make sure to buy the right size fuses for your fuse box, because there are different kinds.

Fuse box and a little 20 Amp fuse – Click to enlarge

The junction strips with jumpers from Radioshack were a little hard to find online. We didn’t really know what word to search for. The jumpers are little metal plates that connect multiple sockets together and allow for parallel-coupling. We decided to get some cigarette lighter sockets as loads, since these are universal connectors for electric components used in cars. We ended up buying 3 items 3-in-1 sockets from FocalPrice.com, which turned out to be a bad idea. When opening the box, I found that all package material of the product was opened already. Two out of three were not working at all, and the other had a malfunctioning USB charging port. Next time, we’ll get better quality.

Under ‘Miscellaneous’ you find items that you might already have: some screws, cable holders and double sided tape, all used for mounting the components to the plywood board.