The Glen Waverley Uniting Church PSALTERLink to FurtherEmissionsReduction2024 >
Day Night Electricity
1. Measurements
- Gauging the benefit and ideal size of a battery is complex.
- One measure is the number of days when the day time export exceeds the night time consumption, so that the tomorrow morning charge in the battery exceeds this morning's charge.
To gauge this, DavidMorgan downloaded the data for the church from United Energy. In this file B1 is export and E1 is import. We do not have any other monitoring of our panels. In particular, we have no measure of total daytime consumption, the sum of import and self consumption.
- The kWh in each 5min interval were summed for 6am to 6pm (Day) and 6pm to 6am (Night). As expected the night time export was zero except for small numbers during early morning in mid summer. Night time import therefore equals night time consumption.
- Day time export in green. Night time import and hence consumption in red, against the day in the year measured, ie March 2023 to March 2024
- Daytime export versus night time consumption. On days above the 45 degree line, the exports diverted to a battery would cover the night time consumption. Not surprisingly, days of high daytime export tend to have low nightime inport.
- A battery of 30kWh capacity would cover 61% of those nights. That is on 61% of days, the export was enough to fully charge the battery and the battery would be large enough to supply the evening load.
- A 20kWh battery would be fully discharged virtually every night, but only fully charged on 78% of days.
- Estimated saving would be about 61% x 365days/year x 25kWh/day x 27c/kWh, or $1500/yr for a 30kWh battery. Some extra savings would occur on intermittently sunny days, when the battery could supply energy during cloudy spells.
- The minimum overnight consumption of almost 20kWh surprised me, but it is consistent with the United Energy data for last January, which shows an average daily import of 36kWh, the sum of day and night, when most of the day time load would be met by solar and evening activities were few. Note that "Energy used" is actually energy bought.
2. What accounts for the night time consumption?
2.1. Boiling water unit (Urnie)
Monitoring device fitted by DavidMorgan 30 March 2024.
- It shows 260W apparently continuous background consumption.
- When re-heating after water is taken, this rises to 2.4kW. This also presumably happens intermittently if the standby element does not maintain temperature.
This web page and this one and this one suggest that 10W is normal, without giving a source. This one suggests 2.5W standby. There is a lot of confusion and misinformation on the web.
- I have not found any supplier that quotes standby energy consumption of their products.
- Consumption from 7pm Easter Saturday to 7am Easter day was recorded as 2.8kWh. Next night was 3.21kWh. Monday night was 3.28kWh
- 3kWh is enough to bring 33 litres of water from room temperature to the boil. (0.09kWh/litre)
2.2. Refrigerator
2.3. Freezer
2.4. Hot water system
- No easy way to directly monitor.
- Probably responsible for the blips in the overnight record. Note that "Energy flow" is kWh in half hour intervals, not hours.
- These appear to be about 0.8kWh twice per night, or 1.5kWh overnight.
