Earth Notes: Saving Electricity and Gas (2019)

PVGIS suggests that an additional 5kWp (south-facing) at 35° (or maybe 4kWp at 70°) of PV would supply ~5kWh/d in December, so would cover house electrical loads when used alongside the current 5kWp even mid-winter. That would need storage (I would not be allowed to connect it to the grid and/or I'd want to cover poor-solar days) and planning permission (it would cover our garden) and some social licence. That could get us off the electricity grid, or to be export only. Wind&Sun has such a 5kWp off-grid kit for ~£10k for example, which is more than an order of magnitude cheaper than interseasonal storage of appropriate magnitude (300--500kWh). That new 5kWp does not cover space-heat demand at all in December, maybe partly outside December. We would not be able to export excess power without a special arrangement. But we only need kWhs not 100s of kWhs of storage, and maybe a month or two's heat storage (electrically charged) could cover space heat!

As we move into winter (and low solar generation) I have been fiddling with the Enphase AC Battery, simulating ToU (Time of Use) charging and load shaping. Rather less successfully I tried topping up in the wee hours with a night charge but the behaviour was very poor from my point of view!

At about 3pm (14:00Z) I brought the boiler controller into sync with the latest Radbots and test fired the boiler. Water pressure was a little low so I topped it up. I then let the boiler run for about 10 minutes until the flame cut because the return temperature was high enough. (~2kWh of gas burnt according to Loop.) Everything seems to be working.

Though about half of the Radbots were calling for heat, we were comfortable without the heating running. (We haven't had the heating on yet, and we usually try to do without until November.) This has prompted me to turn all the Radbots down one click (1°C) from where they were, mainly to 2 or 2.5 (17°C or 18°C), though 1.5 in the hall (16°C) and 3 in the living room (19°C). We may be happy at these lower temperatures and could earn a ~10% saving in space-heat demand if so.

Last night external temperatures were down to about freezing, but we managed without having had the heating on yet. Internal temperatures got down to a mean of ~15°C.

From today, the heating is on, set to run from ~06:30* to ~16:00, and ~19:00 to ~22:00. The early-evening gap is intended to simulate running an on-demand electric-based heating system that avoids drawing power during that peak period. The system could be pure resistive or a heat-pump, and might have storage or not. In any case, avoiding power to run the boiler during peak hours (eg fan and pump), is still helpful to the grid(s).

There are now three systems to reduce our imports in the peak 4--7pm slot: diversion of some load off-grid (~4W max currently!), avoiding the central heating boiler running (~170W max), and the Enphase AC Battery hoarding charge on weekdays until 4pm (~270W max).

* For this first Sunday morning I put off fire-up by an hour, to avoid having the entire house heave into life early! It actually fired up ~07:45 and burned ~5kW in the 8am Loop slot and ~2kWh in the 9am slot. After ~45 minutes Radbots were off. (There was a little DHW use also, and note that a subsequent shower took another ~2kWh+ in the 11am slot.)

At 19:12 the peak-time off-period was restraining the boiler from firing up, ie a couple of valves had just opened and the boiler would otherwise have started a few minutes sooner. Some simple grid support delivered!

(Straw poll on Twitter followers' heating status...)

Gas use for the 6pm--7pm slot on Loop was shown as 0.56kWh, which was one small hand wash of kitchen utensils. Against which, 0.8kWh electricity for our dishwasher's ECO (50°C) program for more than a day's dishes and cutlery and cups/glasses seems a reasonable deal, especially when grid carbon intensity is low.