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 batteriesBased 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.


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 inverterPower 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
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. So you can use little power pack like GOAL ZERO GUIDE 10 PLUS or big gerenator, it depends on you.


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.


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