How much solar capacity do I need

This article answers how much solar capacity do I need. It’s valid anywhere in the world that has enough sun. It can save you a lot of money.

The map below shows the amount you typically have available. Generally, solar is readily feasible where the daily amount exceeds 3.5. It is still feasible below that but needs a lot more solar capacity. The map shows the amount of sunlight in kilowatt/hours per day per square metre. This refers to any unshaded horizontal surface. The solar industry, however, in its non-technical publications refers to one kilowatt/hour per day per square metre as 1 Peak Sun Hour. This is usually abbreviated to 1 PSH. The concept is akin to measuring rainfall in a rain gauge.

How much solar capacity do I need – solar module alignment

Ideally, solar modules face due north (in the southern hemisphere) and due south (in the northern hemisphere). You do not need to take this too seriously but if you are more than 20 or so degrees out, adding about 10% more solar capacity will compensate.

In terms of tilt, having them at your latitude angle gives you the maximum yearly average. If you need more input in summer than winter tilt them closer to horizontal. If you need more in winter than summer, tilt them more steeply.

How much solar capacity do I need – assessing current energy use

Your next stage is to assess how much electricity you need per day (and also of any rare peaks loads). You can simply look at your electricity bill and see. A far better approach, however, is to see what can be done to reduce the draw.

Almost any existing home will have 30 or more so-called wall warts. These are little black boxes that enable you to turn run off appliances remotely. Many over a few years old (and all cheap ones still) draw 3-6 watts even when the related appliance is switched off. That may not seem much but if 30 of them (and some homes have more) that’s 90 to 180 watts twenty-four hours a day (i.e. 2.16 to 4.32 kilowatt/hours a day. Worse are items like 230 volt powered doorbells. One, personally experienced, drew a constant 40 watts – almost 350 kilowatt/hours a year – yet rarely activated.

Items to replace – lighting

Replace all incandescent globes by LEDs. These provide better light at only 25% the same watts. They last for many years: you recover their initially high cost over time. Be aware that ‘watts’ no longer indicates light produced: it is only the energy they draw. LEDs vary in this respect. Some are far more efficient than others. The light output is shown in ‘lumens’. Their efficiency is thus lumens per watts. Because of this, LEDs that are cheaper to buy are likely to use far more energy.

Items to replace – appliances

Recently made high-quality refrigerators draw far less energy. Replacing any over (say) 12 years old will save you money in terms of how much solar capacity do I need.

Air-conditioners likewise vary in the amount of energy they draw. Assess their efficiency by looking for (or asking for) their CoP (Coefficient of Performance). This is the ratio of energy draw and work done. The higher the CoP the better. By and larger smaller units have a higher CoP.