Friday, 30 March 2012

Not so Eco and not so cute

The electricity bill for the month of March just came through, and the good news is that we only used 147 kWh. The bad news is that we were away for all of the covered period--23rd February to 25th March--so most of this energy was being used while we were out of the house. Maybe not a big deal in the grand scheme of things. This only added about 1800 yen to our electricity bill, which is little compared to the repayments on the loan, or on the amount they are paying us for our solar electricity generation, which was 45,000 yen for the same period. 

It seems a lot compared to last month when we were in and used 573 kWh. 25% in fact. 29 kWh were used in at-home time, and 117 night time. Only one kWh was used during day time, but this is not surprising as the panels will have been producing electricity for most of this time.

At-home time is 7 am to 9 am and 5 pm to 11 pm; 8 hours per day. For the 32 days, that's a power consumption of around 110 Watts. Night time is 11 pm to 7 am, also 8 hours, and you'd expect the same consumption, but it averaged 460 Watts. The only difference is the eco cute, programmed to come on at night.  Taking 110 Watts as the background consumption of the house, mainly the ventilation system and the fridge, which we left running, but also the circuitry and leds on display panels, and maybe some phantom consumption on the light bulb sensors, that means the eco cute used 88 kWh over the month. That's almost 3kWh per day, and we didn't use any hot water, and the underfloor heating was switched off. The energy it was using was just making up for the heat it was leaking into the house.

I'd set the tank to low temperature while we were away, but when we came back the display said it had one bath and 50 minutes of shower. I suppose this is less than one bath and 90 minutes of shower that it usually has, but this still seems to be a lot of heat. The exact meanings of the high, medium and low temperature settings are far from clear or explicit, and it seems like it's impossible to set them to actual temperatures, for example having the water in the tank at 50 degrees. Also, having thought that I could vary the amount of hot water in the boiler, it suddenly clicked that the setting of full tank, 50 litres or 100 litres is just for the amount of hot water it will add to the tank when you press the re-heat button. It doesn't make any difference to the amount of heat in there.

Without knowing the COP of the heat pump, it's difficult to know exactly how much heat this represents. I can maybe get some ideas by looking back over the temperature each night, and the electricity consumption data, and work out the relationship between the two, assuming that it's losing the same amount of heat each day from the constant set temperature of the tank (whatever that is) to the more or less constant temperature in the boiler room. Heat loss is proportional to temperature difference.

In a way this is the worst possible conditions for heat loss, as the tank was left full. But it was set to the lowest setting, and the boiler is inside. Imagine the usual situation where the boiler is outside, and it is left set to high.