Heat Pumps?

I have been asked why I haven’t considered a heat pump…
They are very expensive.
They need electricity in order to generate heat energy.
Admitedly the 4:1 energy gain (heat out:electrical energy in) is a good deal, however I will have a limited amount of electrical energy, and will be mostly burning (free) wood to generate heat.
I could use a heat pump during the day to turn excess electrical energy into heat, however I would have to store it.
The system would be even more complicated than my present scheme, and would no doubt be higher maintenance due to the high tech heat pump technology.
It is a technology to bear in mind, but doesn’t spring out as being the best option.

Project Updates

Update August 2014

  • All 4.5kW solar panels are up and running on outbuilding; pointing ~15deg east of south (best available) and at 16deg to vertical (for the winter)
  • Plans in hand to add an LE-600 wind turbine to top the power up in winter; it will feed directly into the batteries but monitored by the Sunny Island via a shunt.
  • Feeding spare summer solar energy to neighbours!

Progress to Easter 2013
Installation so far comprises the following…

  • 8 Rolls S600 batteries – 20kWh to 20% discharge
  • Sunny Island 5048
  • Batteries connected via 250A fused breaker
  • Earth spike for consumer unit and Sunny Island
  • Honda generator fed in to SI gen. input via 20A switch
  • Half of the Perlight 250W panels temporarily fixed to a frame in the garden (2.25kW max)
  • Bought SMA USB service cable which is required to program the Sunny Boy 3800 solar inverter to run in off-grid mode with the Sunny Island.  Also waiting for a Grid Guard Code from SMA as they restrict your ability to change what would be grid connection parameters on the Sunny Boy.


  • Our Honda generator under no load conditions exceeds 250V ac occasionally, which is outside the Sunny Island’s allowed generator input range, so it refuses to connect to it.  Our solution (until we find a better one) is to load the generator with a 2kW load to enable the connection to happen, and reduce the load afterwards to make more power available for the system.
  • Sunny Island menus are very hard to use and badly organised.  Screen has a tendency to display random characters – not sure if this is an issue or just a quirk…
  • Getting a Grid Guard Code from SMA Germany is taking ages (over a week so far) even though it is supposed to be an automated process…

Choosing a PV panel

If you have a limited roof space then the efficiency (Watts/square metre) is an important factor, however this isn’t really an issue for us, so if cheaper slightly larger panels will do the job, then that is more important.  Looks like we will end up with a single string of 16 250W panels or two strings of 9 slightly higher voltage panels…

Criteria for choosing our panels

  • Technology: Only sems to affect the efficiency and cost.
  • Lifetime and degradation: All seem to be linear degradation to -20% after 25 years.  Found some Chinese/Australian ones by Sunny Roo (!) which have longer warranty and 30 years to 80%.
  • Low light performance: All the graphs look the same, however still checking.  Have seen some claims that the thin film panels have better low light performance but most mono graphs look good so yet to be convinced.
  • Rugged: important for us as panels will be at low level so accessible to interference, also may get the odd branch landing on them.
  • Current – Voltage graph / range: All the panels I have looked at have very similar V & I at Pmax, and similar V opencircuit. However the power output per panel is important when matching voltages to the solar inverter input.  e.g. Sunny Roo ones above would require two strings of nine panels…
  • Temperature range: hopefully all the panels will cope with our climate.
  • MCS accreditation: important if we (as we probably do) intend to become MCS accredited for the generation tariff.
  • FINAL CHOICE: Tried to purchase the Sunny Roo 30yrs guarantee panels, however they pulled out of the UK market (due to lack of sufficient oportunities!!!) just when I tried to place an order, so had to give up on them.
    Sadly therefore it just came down to the best pice that I could find which was £122.50 per panel + VAT for Perlight 250W mono panels from http://midsummerenergy.co.uk/

Diagram the Off Grid System

Wiring Diagram

Here’s how we think it’ll look.

  • Panels: (cheap mono probably) 4kW of panels (that’s 16 x 250w panels) Sunny-Roo BBPV-250W looking favourite at the moment. May get 4.5kW and bypass two of the in the summer.  18 x Perlight 250W mono was final choice.
  • SMA Sunny Boy 3800: Rated for 4kW – Would have got a bigger one, but a bargain turned up ebay and we couldn’t refuse
  • SMA Sunny Island 5048: Also rated at 5kW. Apart from the antiquated DOS like operating system an excellent bit of kit.  Talks to the Sunny Boy by varying phase to modulate power being sent from panels. (49 to 52 Hz by default)
  • Rolls Batteries (8*s600): These are the best available; hard to know whether having spare capacity and so shallower discharging will really affect the lifetime.  Also can we avoid daily cycling with solar panels, and is there any point?
  • Generator (found one cheap on ebay): 4kW Honda Petrol engine, we added an LPG gas conversion for cheaper running.  Note that we have an issue connecting this to the Sunny Island as the voltage under no load sometimes peaks above 250V so SI refuses to connect to it – Temporary solution is to load it during connection with 2kW heater.  (Red diesel is cheaper I’m told)
  • SMA Smart Load: (going obsolete?) When the batteries are charged the excess will be dumped into an immersion heater for hot water and central heating.  Starting to look at microcontroller type solution for this function as well as system monitoring etc.


Solar Panel Angle – not just for Summer!

This graph was generated at
and shows the elavation of the sun at 51degrees north (where we are) throughout the day and at various times of year.

We will assume at this stage that the panels are fixed, i.e no facility for tracking the sun.  At winter solstice sun only reaches 16 degrees above the horizon and rises / sets about 50 degrees either side of south.  Optimising for that one day however would be a bit mad!!  2 months either side of this the sun reaches 28 degrees above the horizon, and sweeps south +/- 71 degrees.  Now at 71 degrees away from south (panels point south as we are lucky enough to be able to choose) we only have about sin 20 degrees (1/3) of the maximum output, so we should optimise more towards the middle of the day.

So it seems to me that our winter optimised panel should point south and be angled at about 20 degrees to vertical…


Basic Calculations on Photovoltaic Solar Panel Numbers

To start thinking about what’s going to be necessary it’s important to think about the numbers early on.The problem is that peak outputs from panels are not achievable in most/all months of the British year and so careful thought is required.

The following website is good to get an idea of the effectiveness of solar panels in England:
http://www.swea.co.uk/photovoltaics.shtml   (but panels not optimised for winter – see below)

The key point is that on the worst of weeks in the winter, their 6kW peak output panel array, produced 31kWh over the seven days. Christmas 2010 has weeks of 6kWh and 16kWh but I hope this is just due to the snow settling on the panels. 31kW a week is 4.4kWh per day. This is only 0.73 hours (44 minutes) of peak power the panels specified of 6kW. The outcome is:

A panel array that has a peak output of 1kW may produce only 730Wh per day in the British winter.

The panels that this data comes from look to be oriented at about 45 degrees, so angling for winter should gain about 10%; also they are 50% SE facing and 50% SW facing – need to consider the effect of that…at sunrise / sunset one set of panels is in shade!!  I’m think we should be able to exceed 1kWh average per day in winter.

The next consideration is how much power a house requires. For now I will just say that we have estimated 3kWh is what we will want to aim to use per day.

3kWh / 0.73 = 4.1kW peak output (staying pessimistic)

So in order to have an energy usage of 3kWh per day we need to have solar panels which total a specified 4.1kW peak output.

The batteries chosen will be sufficient to supply 2-3 days power at this rate, so also allowing for a bit spare power, and some averaging of the darker winter days.

Any questions, just ask. More to come…

Can you rely on solar power alone all year round in the UK?

Let’s find out…

We are buying an almost complete modern house, in a remote location that has no access to the national grid. The plan is to install 4kW (peak) of solar panels and a wood burning stove which between them will power and heat the house.

This project will take months. An outbuilding needs renovating and its roof altering to have the correct pitch for the solar panels in winter. We will be using a Sunny Boy, Sunny Island and Rolls Batteries to store the power and maybe a Sunny Smart Load to dump excess solar power into the hot water tank. The house needs underfloor heating laying and much more.