6 Month Capacity Test

So, after 188 daily charge and discharge cycles, what's the score with these Lithium cells?

I decided to find out.

I waited until 3pm on Friday afternoon, when the bank had finished charging and had just gone into float stage and then hit the big red breaker on the PV array.  No charge input.  I then started recording the battery Voltage and AC kWh of energy consumed by the house from the Ofgem AC meter on the output of the inverter.  This meter is calibrated and modified to show down to 100Wh (0.1kWh).

I then did nothing special, other than going about daily life to see how long the battery would last until empty.  It never really gets empty as the protection cuts out the load before it gets below 3.00V per cell.  This leaves enough reserve for the battery to power the charge controllers and the battery monitors.

We did some laundry, which runs the inverter to over 2kW output and we did some vacuum cleaning and the fridge freezer did it's thing.  We even had the folks round for dinner and ran the power hungry video projector for a movie.

After 24 hours, the battery was down from the initial 27.00V to 26.09V and had delivered 5.6kWh of AC power.  So we kept going... And finally to bed.

Woke up the next morning (Sunday) and the power was off.  The logger laptop was still running on its internal battery and showed the inverter had shut down just after 7am.  So the house ran on battery power only with no charge input for just over 40 hours.  The battery was sitting at 24.40V off load and the AC meter read 9.0kWh.

Impressive for a system that I only rate as usable for 80% of the nominal 10.24kWh total capacity (if you take 400Ah x 3.2V x 8 cells = 10.24kWh DC power available).

Quite a bit of power is wasted by the inverter at low power output levels and at best it has an efficiency of about 92%.  So to get 9.0kWh out in terms of AC energy after all the conversion losses is pretty good.

Off the Scale!

July wasn't even that sunny but the generation figure was off the scale on the my production chart page. 

218.9kWh for the month!

Only 36kWh of grid power used in the same period, meaning 85.9% of demand was supplied by solar.

About 49kWh of gas was also not burned for water heating, as this energy came from solar electricity diverted to the immersion heater when the electric battery was full.

New Production Record!

March 2012 was the best month ever for energy production and utilisation on my solar system.

Solar AC energy used (measured through the inverter output OFGEM approved meter) was 183kWh for the month, beating even last year's record of 177kWh in May!  That just goes to show how phenomenally sunny it was this March.

And this total, compared to the grid energy consumed for the whole month of March of just 46kWh, means that 79.9% of all my electricity use was provided by solar power.

The weather wasn't the only factor though in this surge of utilisation.  The lithium battery bank is making a big contribution.  I don't have to worry about running the pack down on dull days and leaving it partially charged for any length of time, so I'm more likely to use it.

We've had free hot water on several days when the battery finished fast charging by about 11.30am and then the rest of the day was spent diverting between three and four kWh of energy to the water heater.

With the good weather, the pack has generally only been discharged by 20% per day and has only hit the "bottom" once since installing it in February.

Statistically Off Grid

Just looking at the recent generating and usage figures, I realised that for a few days I have been statistically off the grid.

These lithium batteries are 95% charge efficient.  This means that the losses are very small to put energy into the battery and get it out again.  For every 100Wh put in, you can take 95Wh out.  That compares well with lead acid batteries where the best you can get out is about 80Wh.

Another way to look at it is that by changing to lithium batteries, the effective size of your PV array increases by about 15%.  Or you effectively have 15% more charging hours in the day (as it still takes time to charge the wasted Watt.hours of energy lost in an inefficient battery).

As I had more power available with the new batteries, I moved a couple more loads to solar from being on the grid.  These were the garage CCTV camera and the central heating system.

The central heating system runs on gas but it also uses electricity for the boiler controller (valves, fans, thermostat controls), the heating timer programmer, as well as the zone valves and circulating pump upstairs.  But it was powered from one place in the airing cupboard.

With these things moved to run on solar power, that leaves very little in the house that runs on the grid:

The main cooker and oven in the kitchen;
The instant water heating shower (2 minutes a day);
A hair dryer (2 minutes a day);
The microwave in the kitchen (a few minutes a week);
A clock radio in the bedroom (24x7 load);
A radio and DECT phone in the kitchen (18x7 on a timer);
Two bedside lights (a few minutes a day);

On a number of days, the import meter did not increment at all (it only counts whole kWhs).  So on those days, the house statistically consumed no grid power and so the house appeared to be entirely powered from the solar batteries.

But it's not just a "rounding error".  The wireless energy meter spends most of the day now reading "zero" Watts and the import meter also stops.  This is because they cannot measure very small power loads of less than about 10-20 Watts.  Below that threshold, the import meter enters an "anti creep" mode where it stops counting and the LED that normally blinks to show the passing of Watt.hours lights continuously to show that it has entered the non-counting mode. 

This prevents the meter erroneously counting units that were just a calibration error (for example it might read 5W when the consumer unit is actually turned off).

The electricity company does not usually care about this small error, and in any case, who could run a house on just 10W of energy?

So it seems I don't have to bother with running wires to the bedroom for the clock radio or the kitchen for the radio and DECT phone, as they don't consume enough power to cross the threshold at which the import meter will start to count :D

The side effect is that the energy company doesn't believe my self-entered meter readings on their web site now.  They asked for a reading for the latest bill and I gave it to them.  Then a few days later they sent someone to re-read my meters, as they didn't believe my readings!

Unlike folk who have grid tied solar, it's entirely possible for my import meter to stop counting up.  People with grid tied solar still consume grid electricity in the evening that has to be paid for.  They just earn it back on the FIT for the energy they export (usually measured on a separate generating and export meter or "deemed" exports from the generating meter only).

Fuelly... Track and share your fuel figures

Like the new gadget on my page?

Click on it and you'll be taken to the Fuelly.com web site where you can sign up for a free fuel economy tracking database.  You can also compare how well your driving and car fare against other cars and drivers.

Can you beat my high-score?

2010 10:10 Challenge Results

So 2010 came to a close. How did I do on my 10:10 challenge?

.                  2009 kWh  Demand %  2010 kWh  Demand % 
Electricity (grid)  4379      94.2%     3448      83.0%
Electricity (solar)  269       5.8%      705      17.0%
Total Demand        4648                4153

So, from 2009 to 2010, I reduced my total demand for electricity by 10.6%. This was due to better efficiency in use of power (turning things off at the wall to prevent standby power leeching, running a laptop as a server instead of a desktop PC, using the coldest setting on the washing machine, etc.).

Looking at grid power only, I reduced my consumption by over 21%, compared to 2009 levels. This was largely driven by steadily increasing solar power capacity over late 2009 and early 2010, as I bought more panels, bigger charge controllers and bigger batteries to capture and store more power.

But that's only half the story. We use gas to heat the house and provide hot water. In 2010, I started using spare solar power in the Summer to pre-heat the water with the immersion heater; reducing the need for gas in the evenings. But even with the additional wall and roof insulation, the record-breaking severe cold weather we had in early and late 2010 pushed the total demand for gas up by 16.8% (10,295kWh in 2009, to 12,025kWh in 2010). So there's still work to be done.

Our windows aren't very good. The old double glazing isn't very good quality (old types have thin air gaps) and there are three small port-hole windows in the upstairs landing and bathroom that are only single glazed. Our boiler is very old and should be upgraded to a condensing type. We could build a front door porch so that when we open the front door, there's an 'air lock' to prevent cold air blowing into the hall and through the house.

More projects for 2011...