DIY PV System to Save Money AND for SHTF

[Tarrel]

I've been racking my brains as to what kind of solar PV setup we need at the cottage. On the one hand, I want to be able to generate power when the grid goes down. On the other hand, it would be good to have a grid-tied system that saves us money on the electricity bill. (I try to work it so that money invested in preps also has a payback during BAU).

At first sight these are incompatible. Commercial grid-tie systems (such as your average joe has on his house to get the feed-in tarrifs) have an islanding device fitted to the inverter, meaning that if the grid goes down, the system shuts off. This is to prevent power you've generated being fed down the line and zapping the poor fella who's trying to fix the problem.

The other problem is we live in a listed building, and even then, the only viable roof area is partly shaded.

So, I thought about the following system as a project over the winter:

8 x 80W 12v panels off eBay (£80 each), wired as 2 x four-panel strings, each generating 48V nominal.

These mounted on a wheeled carrier (timber construction, around 2m x 2m, angled to around 45 degrees but adjustable). This would be placed in a sunny spot in the garden, which happens to be near the shed which has mains power.

Connect the panels to a Soladin 600 plug-in grid-tie inverter, which would be plugged in in the shed. This would feed in a maximum of 0.6 kW, which would be enough to cover a lot of our base load requirements for most of the day, maybe more.

Totally separately, set up a bank of leisure batteries to be trickle-charged off the mains (say, 300 Ah), and have available a 500W inverter to produce mains elec from these batteries if needed.

So, in a BAU situation, the solar PV would be contributing to our power needs. Assuming an average of 4 hours power generation per day, all year, gives 1460 hours of generation. Assume the system delivers 500W, gives an annual generation of 730 kWh. At 15p per kWh, the saving is £110 per year. I estimate total costs for setting up the system would be around £1300, giving an annual return of 8.5%, which compares favourably with bank interest. (Here's the politically sensitive part; we have an old fashioned meter that I believe will run backwards, so all the power the PV system generates will be made use of, whether we are using it or not).

In a short-term power-outage situation (not uncommon up here), we would use the batteries to provide power for the essentials (e.g freezer and low energy lighting).

In a post SHTF situation, or prolonged power-outage, I would rewire the panels and connect them via a charge controller to the batteries and use this as an off-grid setup.

This system doesn't give me the benefit of the feed-in tarrifs, as it is not professionally installed by an MCS installer, but it does give me the flexibility to design it as I want, and the capital cost is lower, meaning I don't need such a big return for it to make commercial sense.

Any holes in the plan, or blindspots I haven't thought of?