Do solar panels make sense financially?

A typical residential narrowboat installation

A 64 Watt panel and a suitable solar controller from us cost less than £300. This is still a significant capital investment for most people, so can you ever expect to recoup the money?

A 64 Watt panel will provide around 20Ah per day during the summer months to a 12 V battery. That's about a quarter to a third of the capacity of one battery. The amount of electricity used per day varies a great deal with the appliances you run. However, 20Ah should be enough to cover use of lights, radio, and water pump for the typical user. Fridges consume rather more power, but on the other hand if you use energy efficient lights such as LED or fluorescent lighting, you may find your energy requirements are considerably lower.

A 40A alternator on a engine would take at least 1/2 hour of charging to provide the same amount of energy. So from spring to autumn (say 180 days) you would save at least 90 hours of charging. In the winter light levels are much lower, and you will have to rely on your engine or generator more. However you should still save at least 5 Ah or so through the winter, which corresponds to about 22.5 hours less engine use.

So in a year, we've saved well over 100 hours of charging from the engine. Red diesel at present costs around 50 pence per litre at most marinas, but the government is planning to prohibit its use on boats in the near future. White diesel is nearing 100 pence per litre these days, so that is the price boaters will shortly be paying.

At a conservative estimate, fuel bills should be down at least £50 per year. Wear and tear on the engine is also reduced, particularly as charging from an alternator provides a rather light load to a diesel engine. You should find that because the batteries are kept better topped up, they should need replacing less frequently. A lead acid battery won't last a year if it is frequently left flat - but will last 5 years if kept well-charged. Replacement of a battery bank of 150 Ah is well over £100.

It's rather hard to quantify just how much reduced engine maintenance and depreciation along with less frequent battery replacement will save you. It depends on factors such as whether you do the maintenance yourself for example, or get a yard to do it. But £50 per year is not an unreasonable typical figure.

So in total savings are about £100 per year, and you should recover the cost of the panel in about 4 years. Note that this estimate is for a boat in continuous use and without a connection to the national grid. If you only use your boat occasionally, payback time will be slower.

But remember.... it's not just about money! Even if for you it might end up being more expensive to use solar panels, remember that by doing so you are doing less damage to the environment. Your neighbours may also be grateful for not having a noisy, smelly engine rumbling away next door - and you can enjoy the peace yourself.

Grid-connected systems

Given how cheap electricity from the national grid is, the economics of attaching solar panels to a house are unfortunately less appealing than systems that rely on diesel generators.

Typical return from an 80 Watt panel is about 100 kWh in a year. At current prices a kWh costs about 8 or 9 pence, so in a year the panel will return about £8 or £9. Thats a payback time of about 30 years on an panel costing £250 - before installation costs are taken into account.

However, if you take a long term view you may decide that solar installations are still worthwhile - with no moving parts solar panels have little to go wrong, and it's not unreasonable to expect a panel to last 30 years. In comparison with other building costs, adding a solar installation may not be an enormous additional expense. You will have the satisfaction of knowing you are no longer relying on volatile energy costs, and of knowing that your home energy comes from an environmentally source. It may not be the cheapest option - but it doesn't cost the earth.