Building a Solar Power Generator

Portable solar generator This project describes how you can build a solar power generator to generate 240v mains voltage where-ever you want it.

Whether you want to run small power tools in a remote shed or provide emergency power in the event of a power cut, a generator can be extremely useful.

The problem with most traditional generators is that they are noisy, have to be run outside and, if you are to rely on them, need to be tested regularly and topped up with fresh fuel: all too often, a petrol or diesel powered generator hasn't been properly maintained or has stale fuel, and fails when it is required.

Petrol and diesel generators also provide very poor quality of power, with 'spikes' and power drops. This makes them unsuitable for powering a lot of electronic equipment and can damage sensitive equipment such as PCs and flat screen TVs.

The answer? A solar power generator to provide you with environmentally friendly, portable, easy to use and instant power.

What is a Solar Powered Emergency Generator?

A solar power generator is a battery based power supply that is charged up using solar panels. 12v lighting and mains power is taken from the batteries using an easily available inverter and this is then used to provide emergency power when needed.

Unlike petrol or diesel powered generators, a solar power generator can be used indoors - carried directly to where it is needed. It emits no fumes and is silent in operation. Depending on the size of the battery and the amount of power required, it can provide as much or as little power as you need.

There can be two types of solar power generators - an 'all in one' solar power generator where the solar panel is part of the generator, or a split solar power generator where the solar panels are fitted to an outside wall or roof, and the rest of the generator simply plugs into the panel to recharge.

All solar energy systems use the same basic materials, so no matter if you are planning a very simple system like this one, or a big off-grid house installation, the basic 'building blocks' are the same. This means that if you are interested in understanding solar energy without spending a fortune, the skills and knowledge you gain from a small system has a direct benefit for working on much larger systems.

In either case, the product is the same - only the packaging is different. A split solar power generator would work better if you are trying to provide electrical power to an outbuilding such as a shed or lock-up garage whereas an all-in-one solar power generator is a better solution for power on-the-move.

What you will need:

To build a simple solar power generator, you will need the following parts:

  • Solar photovoltaic (PV) panel - collects daylight and converts it into electricity
  • 12v Leisure Battery - stores the electricity for use later on
  • Solar Charge Controller - stops the solar panel from overcharging the battery
  • 12v - 240v Power Inverter - converts the power in the battery to 240v AC
  • 12v low-energy light (optional) - an efficient way of providing bright lighting
  • Tool case (optional) - useful for packaging everything neatly together

Components required for a simple solar generator

A useful kit of solar parts has been included at the end of this document. This allows you to buy everything on-line to build your own solar power generator.

Calculating the required size for your solar power generator

There are various ways to calculate the required size for your solar power system. For this project, I'm using a quick and simple method for working out what we need. This is fine for a simple project such as this. For bigger projects, however, you'll want to do this in a lot more detail. I cover solar sizing in a lot more detail in my book, Solar Electricity Handbook, but for now, for this project, we'll keep things simple.

The size of the various components depends on the amount of power you need to generate and the length of time you want to be able to run your generator for.

A typical power cut lasts for less than one hour, and is very unlikely to last more than four hours. During that time, most households would like to be able to power their fridge/freezer and lighting. This is well within the capabilities of even a basic solar powered emergency generator.

Depending on the amount of power you want to generate will depend on the size of the components you are going to require.

Small solar generator batteries Based over a 24 hour day, the typical household uses an average of between 800w-1kW per hour. For emergency only use, most households require between 75-200w per hour in order to provide lighting and essential power for items like a fridge/freezer.

As a general rule of thumb, for every 100watt-hours of power you require, you require 10 amp/hour of battery power. So if you require 200 watts of electricity for four hours, you will require an 80 amp/hour battery.

When looking for a battery, you are looking for a lead acid leisure battery (sometimes called caravan batteries). These look very similar to car batteries, but have a different chemical and mechanical make-up. Unfortunately, you cannot use a car battery for this job - car batteries are designed never to be fully discharged and will fail if discharged in this way. These batteries are available from battery specialists, caravanning shops and most motoring shops, including Halfords.

Once you have worked out the size of your battery, you need to calculate the size of solar panel you need to charge it up. The size of the panel will depend on how often you plan to use your generator, how large the battery is and how much sunlight the panel will receive.

If a solar panel is left outside in a south facing position at an approximate 45° angle to the sky, it is likely to produce around 2 times it's quoted hourly rating per day during the winter, and between 4-8 times it's quoted hourly rating per day during the summer.

If you want to be more accurate, you can use the solar irradiance calculator and the solar angle calculator on this site in order to get a better idea of how much power you can expect to generate and the best angle to place your solar panels at.

You will want your solar power generator to recharge fairly quickly so that it can be fully charged when it is next required. However, if you get too large a solar panel, you'll be paying far more than you otherwise need and wasting most of the power that you generate.

A fair compromise is normally to allow 10-15 days for the solar power generator to recover from a complete battery discharge. As you are probably only ever going to be using partial charges from the battery, this is a worst case scenario. You can always supplement the solar panel with an external power source to top up the batteries quicker if necessary.

To calculate the size of the solar panel you require, take the amp/hour rating of the battery and multiply this by the number of volts (normally 12). Divide this number by 2 (hours per solar charge per day in winter) and divide this number by the number of days you expect the solar panel to have fully recharged a battery. Then multiply the final figure by 1.1 to take into account losses in the system. The number you have left is the size of the solar panel - in watts - which you need to buy.

Example:

A 12v 80 amp/hour battery stores 960 watt-hours of electricity (12 x 80 = 960).

960 / 2 hours winter charge = 480 watt-hours

480 / 15 days = 32 watt-hours

32 x 1.1 = 35 watt solar panel

Power Inverters

Power inverter Power Inverters take the 12v voltage from your batteries and convert it to a 240v AC power supply. They are available in various different power outputs from 75 watts to 3kW and it is important to make sure you do not overload them.

Power Inverters can get extremely hot in operation, so if you are planning to build them into a case, it is important to make sure they have adequate space around them so they do not over heat.

When buying a power inverter, it is worth buying one that includes a low-voltage cut out. This means that when the batteries run low, the inverter switches off rather than totally draining the batteries. Draining lead acid batteries completely can damage or destroy your batteries, so this is best avoided.

Power inverters are available from camping and caravan shops, car accessory outlets and most electrical wholesalers. They are commonly sold to motorists who want a 240v mains outlet in their car for running laptop computers.

How do I measure how much power my appliances use?

You can measure your power requirements using a plug-in energy monitor. These are plugged into a mains socket and the appliance you wish to measure is then plugged into the energy monitor. Power drain is then monitored and shown as an average on the built-in display.

If you don't have a plug-in energy monitor, you can find out how many watts many of your appliances use by reading the power information often printed on the back of them, or by checking the output on power adaptors.

Often these figures are shown in volts and amps. A transformer for a laptop PC, for instance, may have a power output of 19.5v and a current of 4.5 amps. Multiplying these two figures together (volts x amps) will tell you how many watts are used per hour - in this case, my laptop consumes a maximum of 88 watts/hour of power.

It is difficult to produce a table listing all the likely electrical items you will have in a home and provide an exact list of their power requirements: power requirements vary dramatically depending on make and model. Highly efficient products can consume as little as one tenth of the power of a less energy efficient model. However, as a rough rule of thumb, here is a basic list:

Low energy light bulb
'A' rated Fridge
'A' rated Freezer
'F' rated Fridge/Freezer
Tumble Dryer
'A' rated Dishwasher
'E' rated Dishwasher
Laptop computer
Desktop PC with screen
24" CRT television
15" flat panel TV
32" flat panel TV
Small microwave oven
Large microwave oven
Energy efficient vacuum cleaner
Vacuum cleaner
Fan heater
Hair dryer
Energy efficient kettle
Kettle

8-11 watts/hour
15-20 watts/hour
20-25 watts/hour
150-180 watts/hour
1200-1400 watts/hour
30-40 watts/hour
150-200 watts/hour
70-100 watts/hour
350-450 watts/hour
80-90 watts/hour
50-80 watts/hour
240-300 watts/hour
600 watts/hour
800 watts/hour
800 watts/hour
2000 watts/hour
2000 watts/hour
1400 watts/hour
750 watts/hour
2000 watts/hour

As can be seen, there are a lot of electrical appliances that we all have around the home that use large amounts of electricity. Where possible, these need to be avoided when planning for emergency power. Heating and cooking, for instance, are best done with gas rather than building a bigger and more expensive solar power generator to cope with peak demands.

Lighting

One of the main benefits of a generator is providing lighting. Rather than running 240v lights from a solar power generator, however, it is more efficient to run 12v low-energy lights as the power does not have to run through a 240v power inverter. 8w low energy light bulbs running from a 12v power source are as bright as their 240v counterparts.

Choosing a Solar PV Panel

There are two different types of solar photovoltaic (PV) panels - amorphous and crystalline.

Amorphous panels are the larger of the two panels, as they are the least efficient in direct sunlight. They do generate the most power in poor lighting conditions, however, and can even generate power from bright moonlight or streetlamps.

Crystalline panels are about 1/3rd of the size of amorphous panels, making them more portable and easier to integrate. Prices tend to be higher than amorphous panels, although it does pay to shop around as it is often possible to get them for a similar price.

Working to a budget - where to find cheap parts

With careful buying, a small solar power generator capable of providing 1/2kW of power can be built for under £100. Shopping around, sourcing components from Amazon or eBay and using second hand (but good quality) batteries can all help to keep costs to a minimum.

A good source for second hand batteries are used ex-UPS batteries that are sold on eBay regularly and tend to only go for a few pounds. Beware though - they can be very heavy, so watch for postage costs! Alternatively, you can buy very high capacity used lead acid batteries from fork lift truck maintenance companies. These batteries will be old and will have reached the end of their useful lives as fork lift truck batteries, but will still give years of useful life for providing emergency power for household use.

Another alternative is to buy a cheap, low cost UPS. This will provide you with a battery (usually not big enough to be of any real benefit, but it's a start), a 240v power inverter and a means for recharging the battery from mains power if you need to recharge the batteries quickly.

These UPS cost less than £50 brand new (and are often available from around £20) and are usually capable of powering up to 10 amps (2400 watts) of mains electrical equipment - batteries permitting. If you can find a second hand one with a worn out battery, even better - these are often thrown away, or sell for a pittance on eBay.

The only other components you will need to buy are a bigger battery, a solar panel and a solar charge controller.

Maplin Electronics sell a solar lighting kit, comprising of a 5 watt solar panel, a solar charge controller and a 12v 9watt low-energy light bulbs and light fittings. List price is £79.99 but this is often discounted to around £60. Whilst the system does not include a battery, the equipment represents good value for money (at the discounted price) if you want to include lighting with your emergency generator.

Maplin also produce a 12v solar generator, including a case which is exceptionally good value for money. Unfortunately, this is not water resistant and has a low powered battery, which restricts its use. However, if you upgrade the battery and add a small 240v inverter, it could be useful for smaller applications and the cost of buying this generator and modifying it can be cheaper than buying the individual parts separately.

Assembling your Solar Powered Emergency Generator

In this article, I am assembling an entirely portable solar power generator using a second hand UPS I obtained free along with a few old lead-acid gel batteries that were being thrown out. Depending on the materials you use, you're generator may look very different to mine, but ultimately will do the same thing.

Handheld portable solar generator

Here is what I started out with:

  • A cheap second hand UPS with worn out battery that was being thrown away
  • A set of used 12v lead acid gel batteries that were being thrown out as surplus to requirements
  • An old plastic tool box that I had lying around.
  • A solar charge controller - £12.99 from Maplin
  • Total Cost - £47.98

Step 1 - wiring up the power generator

First of all, I opened up the UPS and took all the parts out of the UPS casing. You could choose to keep the UPS casing intact - it is certainly safer than having exposed 230v power cables lying around, but I wanted to fit everything into a compact plastic tool case for ease of use.

First step was to disable the UPS alarm that beeps incessantly when mains power has been switched off. This was accomplished by disabling the speaker using a small screwdriver.

I then connected up all the small 12v lead acid gel batteries I had in parallel, giving me a longer-lasting 12v power supply. I then wired this into the UPS.

A quick word about wiring up batteries in parallel

When wiring up batteries in parallel, it is important to wire the batteries correctly. If you don't, you will find that you will not drain all the batteries at the same rate. As a result, you can end up with some batteries becoming exhausted sooner than others and this is a sure fire way to damage your batteries. The answer is to make sure you wire up the batteries like this:

Parallel batteries diagram

Testing Step 1

Before getting to play with my solar panels, I decided it would be a good idea to make sure the power generation side worked as planned.

I had two 12v batteries, one with a 14 amp/hour rating and one with a 4 amp/hour rating, giving me a total of 18 amp/hours to play with. Taking into account battery performance and losses through the power inverter, I estimated that I would have approximately 200 watts of power from my emergency generator - sufficient to run emergency power in my house for two hours if necessary.

Typically, you don't want to mix and match battery sizes. Ideally you want both batteries to be the same capacity, and ideally the same make and model as well. In my case, I wasn't too worried: the batteries were free, so if they didn't last quite as long as a matched pair of batteries, I wasn't going to lose any sleep over it.

I plugged the UPS into the mains and charged up the batteries. Once they were charged up, I unplugged the mains supply, plugged a TV, a laptop computer and a table lamp into the UPS and left it powered on until the batteries were flat. Total power drain from all this equipment was 189 watts. The UPS managed to run the TV, laptop computer and table lamp together for 1 hour and 4 minutes on a single charge - a total of 201 watts of usable power.

Step 2 - connecting up the solar panel

So far, so simple. The next step was to connect up the solar panel. For this, you will need a solar panel (obviously) and a solar charge controller.

The solar charge controller is an important piece of kit - it stops the batteries from getting overcharged by the solar panel. If you don't have a solar charge controller, the batteries can get overcharged, which will destroy the batteries. In a worst case scenario, overcharging the batteries could lead to explosion or fire.

I used a low cost solar charger which I purchased from Maplin for £12.99 (part number L26BR).

My wiring now looked something like this:

Wiring diagram for a solar generator

Testing Step 2

I carried my solar power generator outside - thankfully it was a sunny day - and started prodding about with my multimeter, taking care to ensure I kept my fingers away from the 230v AC output from the UPS. My solar panel was sending between 720-780mA of power to the batteries, which equates to around 9 watts of power at 17.2 volts - a suitable voltage for charging up 12v batteries.

Based on these figures, I calculated that from a fully discharged battery, it would take between three and seven days to fully recharge my solar power generator, so long as my solar power generator was placed somewhere outside where it could receive at least some direct sunlight each day.

Step 3 - Tidying Up

The next step was to tidy up the installation and fit it into my box. I wanted a system that was going to be portable, so it was important to make sure the batteries were fitted down properly. I used thick Velcro strips (available from B&Q for around 3 a pack) to ensure everything was held firmly in place.

I mounted my solar panel onto the outside of the box and sealed it to the box using mastic. My plan was to be able to use this system outside, and to make the internal workings of the system waterproof. Unfortunately, I managed to break the box, so that is going to have to be put on hold for a while.

I then fitted everything properly into the case to ensure there were no exposed hazardous wires.

Testing Step 3

I double checked the power with my multimeter to ensure everything was working as it should. It was, so then I tested it properly by powering up the UPS and running my power drill from it. Everything worked as it should.

The Final Product

For something that cost me less than £50, I have to say I'm pretty pleased with the result. I have a portable solar power generator that I can take with me anywhere and use both inside and out.

My solar power generator can be charged up quickly from the mains if necessary, or trickle charged using solar power.

It can be used to provide a burst of power - up to 10 amps if necessary, or provide a smaller amount of power to keep me going in the event of a power cut for a couple of hours.

It's totally portable, so I can take it with me if I need power on the go - like using a power drill at the bottom of the garden, for instance.

And because it provides a clean power supply without chucking out noxious exhausts, it can be used inside exactly where it is needed, rather than with trailing power leads running across the floor.

Return from Solar Power Generator to DIY Energy Saving Projects page.

Did you know?

Solar photovoltaic prices are dropping by between a quarter and a third each year.

By 2020, it is widely predicted that solar will be the cheapest way of generating electricity, almost anywhere in the world.

 
Solar Electricity Handbook

A simple, practical guide to using electric solar panels and designing and installing photovoltaic PV systems.

The Solar Electricity Handbook assumes no previous knowledge of solar power systems, it explains how panels work, how they can be used and explains the steps you need to take to successfully design and install a electric photovoltaic system from scratch.

 
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