Solar panel and inverter

[dakota]

If i have an 85 watt solar panel along with my 2 x 85watt batteries , and i use an inverter then im never going to need hook up , am i right ? my inverter would just power odd things like laptop and poss a small microwave .

[Mel E]

It depends on two things:

 

1. The amount of power you use - which will inevitably be significantly higher outside the summer months.

 

2. The weather and time of year. An 85 watt panel typically gives a maximum of 4.85 amps, but you only get this:

 

- in completely clear, bright sunshine

- with the suns rays at exactly 90 degrees to the panel surface

- provided the panel doesn't get too hot.

 

In practice, a roof mounted panel is never at right angles to the sun (unless you stray into the tropics!), so output is reduced. It is further reduced by any cloud, however light, and any shadows cast across the panel.

 

The table below gives the results of my own observations and calculations - it was part of an article I wrote for MMM some time ago.

 

Multiply your panel rating (watts) by a factor from the Table below to get an estimate of its daily output in amp-hours.

(e.g., A 75 watt panel in Spanish summer sun should give 41 amp-hours: 75 x 0.55)

 

Table - Roof mounted Solar Panel

Output Factors in order of Weather Summer Winter

 

South Spain

Full Sun - Summer 0.55 - Winter 0.27

Light Clouds - Summer 0.40 - Winter 0.16

Dark Clouds - Summer 0.15 - Winter 0.07

 

UK

Full Sun - Summer 0.43 - Winter 0.16

Light Clouds - Summer 0.27 - Winter 0.08

Dark Clouds - Summer 0.05 - Winter 0.04

 

(Note: The factors in this table are based on my own observations; they are good enough for working purposes, but not a guarantee of what you’ll get! You should expect less than the winter factor in December and January and may do better than the summer value in the long days of

June and July. Light clouds means high, wispy clouds, not heavy white stratus. The factors take account of the sun angle (the lower it is, the greater the light cutoff by polution), the weather and pollution conditions, and the time of year (which obviously affects day length)

 

In other words, your 85 watt panel in English summer dark clouds will give you just 85*.05 amp hours per day (ONLY 4.25 amp hours) into your two 85 amp hour batteries (each of which will only give you just over 60 amp hours of actual power before the voltage drops too low for use. But it's rarely dark cloud ALL day, so you should do better. But even under Cirrus (the whispy horse tail clouds that precede a weather front) you'll still do well to get better than .27*85 or 23 amp hours a day.

 

These figures are for mono or poly crystalline panels. Other panel types will give a better output in poor light, but require much more space (typically 3 times the area to generate the same maximum power - but you get more than the crystalline give in poorer conditions).

 

So add up your consumption. A single 10 watt lamp uses 1 amp hour per hour, a typical 60 watt laptop 5 amp hours per hour. It soon adds up.

 

Hope this answers your question.

 

Mel E

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[Bill]

You can get 12v microwaves, but you need to sit down before I tell you how much they cost. Somewhere north of £300, and then you only get 500 watts, which is awkward to use as nearly all recipes are for at least 650 watts.

 

On the other hand you can buy an 800w mains microwave for £24 at Tesco, and they work well. (We got one for my mother’s carers to use, and it survived them with aplomb.)

 

The bad news is that the 800 watts refers to the magnetron’s output, NOT to the power consumption. That is on a plate on the back, and typically will be about 1200w. An inverter capable of supplying 1200w will cost as much as the 12v microwave, though you do have the advantage that the inverter will be available for other uses.

 

Be aware that an inverter imposes an overhead of around 10%, even when it is “idling”.

 

A bigger problem is that your batteries will not enjoy that level of draw. In fact, if you are determined to do this you might be better off using engine batteries.

 

The other alternative is a generator. In RoB expect to pay at least £350 for a 1500w generator, however a few weeks ago I saw a 2200w job in Portugal going for €300 – about £210.

 

 

[Clive]

Everyone is correct .

But one item to keep an eye on is your MH heating system. If you have a diesel heater and want to use this regularly then you are on a hiding to nothing relying on solar panels to keep up with it, especially in the winter if wild camping.

Forget the microwave unless you are on hookup.

Otherwise in the summer you should with a little care be self sufficient.

P.S. I think that some of Mels figures are just a little pessimistic, we have seen better figures than his with our 80 watt BP Panel while attending Warners shows in the UK.

But if you want outside lighting then invest in one of those petrol powered lamps with two mantles. (Cannot think of the name at the moment! C C C?) Found it COLEMAN. OK

 

 

 

[dakota]

Thanks mel , bill and clive , well my heating is a gas fire , i very rarely use the fan , and of course the fridge is gas , all my lights are low wattage , i use a flat screen 15 inch L C D T V. my outside light is usually the awning light and a gas lantern , so if i forget the microwave , use the laptop sparingly and use them like clive says only on hook up , i should be self sufficient most of the year . Sounds good to me , so far with the 2 batteries but not using m/wave or laptop , ive been managing up to 3 days in winter and close to 5 days summer.