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Based on a PICAXE 28X chip.
logging output, staggered dump
load and LCD display of volts,
amps, watts and watt peak.

12V sensor light

Converting a mains powered
sensor light to 12v operation.

 Off Grid System Sizing
 A off grid installation consists of a few basic components that work together to provide you with usable power. These components are usually rated in Watts or Amp hours. How do we work out the sizing of these components? We have our power source, and this could be solar, wind, hydro, a generator or even grid power. Then we have a storage system, usually batteries. An inverter to convert the stored energy into a more versatile forum, AC. And lastly the load, what we use the power for.

There are many other components, like charge controllers, fusing, etc, but we don't need to worry about those for this exercise.

The Load. First step, and the hardest to calculate correctly, is the load. The load is how much power we want to consume, in Watt hours ( Wh, see "What's the difference between Watts and Watt hours" ). To work out the Wh of an appliance, multiply the Watt rating with the number of hours it will be running during a day. Example, a 100 watt bulb running for 5 hours will use 500Wh, a 1kW ( 1k=1000 ) kettle running for 15 minutes will use 250Wh, a PC left on for 24 hours at 200 watts will use 4.8kWh !

Once you have worked out the Wh for all you appliances, total them up to get a Wh rating for the house. Less than 5kWh a day is very good, 10kWh is OK, 20kWh or more is not so good. The higher the figure, the more expensive you system will be to supply it.

Its much much much cheaper to buy a more efficient fridge, use efficient lights and turn off unused appliances than building a system to supply the extra power.

Say for example we worked out we need 10kWh a day. That's comfortable living, ample lights, TV, PC, fridge, microwave oven, using gas for cooking, solar hot water, minimal air conditioner use. Next we multiply that by 1.3 ( to allow for losses in the system ) to calculate the Wh needed from our power source ( solar, wind, etc ). So we need 13kWh from our power source.

Power Supply. We'll stick with solar for now, but the same rules apply to wind, hydro, etc. Solar panels are rated in Watts. To work out the Watt hours, we multiply the panels Watt rating by the time it's in full sunlight. For your average site, this would be about 5 to 8 hours per day, depending on the season and your place on the globe. So a typical 200 watt panel could make between 1kWh and 1.6kWh per day. We need 13kWh, so 12 to 14 panels ( 2.4kW to 2.8kW total ) should cover it at a minimum. Of course, this is assuming it never rains and we have full sunlight for every day of the year.

In reality, there may be days of cloud where our panels may make 25% of their full sun power, and we need to allow for that, either by adding more panels, reducing the load, or using alternative means of supplying power ( generator ). So it's a good idea to add as many panels as you can afford now, and set aside space to add more panels in the future. Solar panels are much cheaper than they once were, and should last for 20 or more years, so they are a good investment. You can also increase your panels output by 10% to 50% with tracking and MPPT's, but I wont go into these here.

The Inverter. The inverter needs to be sized on the total watt figure of the house at any one time. This is the peak Watts, not the Watt hours. An example may be while cooking dinner, watching TV, and ironing the cloths all at the same time. Running a iron, toaster, microwave, kettle, TV, lights and computer at one time could draw 4,000 Watts. The inverter needs to be able to supply that load and more, about 30% more. If the inverter cant supply the load, it will shut down and you could be left in the dark until its reset.

Inverters have a continuous rating and a surge rating. The continuous rating is the maximum the inverter can supply without overheating and shutting down. The surge rating, usually many times the continuous rating, is the maximum the inverter can supply for a few seconds ( or minutes, depending on model ). Most loads, especially large motors, draw more power at startup than normal running, so we need this extra surge capacity.

Inverters also have a battery voltage rating. 12, 24 or 48 is common, the higher the better. A higher battery voltage means you can use thinner wire to carry the same power, and as a rule efficiency is better. The simple rule with inverters is buy quality and the highest rating you can afford.

Storage. Batteries. We need batteries to supply our energy when our power source cant, like solar panels at night time. Sizing batteries needs to take into account how far we can discharge them before we risk damage, usually to around 70% of capacity is OK. That is, we can discharge 30% of its rated capacity, we leave 70% unused, the less we discharge the better for the long life of the battery. Its also important that we recharge the battery as soon as possible, next day is ideal, but within a few days is a must. We also need to allow for battery efficiency, around 85%. Next, using a little maths...

 House Wh per day * Days Battery Capacity (Ah) = Batt Efficiency * Depth of Discharge * Battery Voltage

 10,000 * 1 Battery Capacity (Ah) = = 816Ah 0.85 * 0.3 * 48

So we need a 48 volt battery bank rated at 816Ah. This is assuming you have no power coming in from your solar panels for 24 hours, or you used all your power at night and nothing during the day. If we needed the system to run for 48 hours without any usable sunlight, the capacity would need to be 1632Ah.

There is no such thing as a 48volt 816Ah battery, so we need to build one with smaller batteries. A common deep cycle battery used in off grid systems is a Trojan T105, a 6 volt 225Ah battery. 8 of these in series will give us a 48volt 225Ah battery, multiply by 4 ( 32 batteries ) for a 48volt 900Ah battery bank.

It's worth noting battery ratings vary for manufacturer and duty, and are usually rated over a 100 hour discharge period, but to be really accurate we should use the figures for a 20 hour discharge, these will be less than the 100 hour rating. The battery datasheet should have these figures. As an example, the Trojan T105 in our example has a 100 hour rate of 250Ah, and a 20 hour rate of 225Ah.

These days there are alternatives to lead acid, like Lithium. Lithium batteries are much more expensive than lead acid, but do offer some advantages, like less maintenance, and a better discharge capacity of 70% instead of lead acids 30%. If we used Lithium batteries instead of lead acid for our system, we would only need a 350Ah battery bank instead of 816Ah. Worth considering.

In review, for a 10kWh system, remembering 10kWh is comfortable living that many of us are used to, you would need at least 2.4kW of solar, a 4kW inverter and 900Ah of lead acid batteries ( less for lithium ). But this is only a guide for a 10kWh system in ideal conditions, you should where possible increase the solar panel and battery capacity as money allows.

You can bend the rules a little. In my own situation, I have limited funds initially, so have to cut a few corners. I bought the best inverter I could afford, a 7kW ( 20kW surge ) 48v Latronics, as I need to run a welder and other workshop tools. I have 12 190Watt panels to give me 2.28kW of solar, and, at this stage, will be using 8 230Ah batteries to give me a 48v 230Ah battery bank. Yes my battery bank is too small, but I'll use it intelligently until I can afford to upgrade it. Jobs like washing the cloths, vacuuming the house, using the workshop tools will be done during the day while the sun is shining. Night time will be just lights, microwave cooking and TV, so I should have the capacity to get through the first few months until I can upgrade. I also have the option of wind power, there is a constant usable breeze at night and I expect I could harness 1kWh to 2kWh most evenings. And of course I have a generator if the battery state gets too low.