Reliability of online demand / kVA calculator

30 Aug 2012
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United Kingdom
I'm in the process of getting a new electrical supply from UK Power Networks for my 4-bed, ~200sq.m barn conversion (not started yet!) and they're asking for a kVA rating. I'm planning to use an air source heat pump (maybe 12kW - 16kW output) and probably also one car charging point. There's no gas there, so it'll also be electric water heating and cooking. The max. that UK Power Networks say they can provide is 18kVA. I've run the figures through this online calculator and it reckons a diversified load of 14kVA should be sufficient. I've estimated 5kW input for the ASHP; it seems almost impossible to find the input rating for ASHPs - just the output, which is not much use for load calcs.
So my questions are:
* Does anyone have experience of online demand calculators such as this one and their accuracy/reliability?
* Anyone got any thoughts on whether 14kVA sounds about right as a diversified load for a property like this? (Not that you have much of an idea what the property will be like - but imagine an average, newly converted barn).
Thanks in advance for any thoughts/comments!
~ Paul
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it seems almost impossible to find the input rating for ASHPs - just the output, which is not much use for load calcs.
For a fag packet calculation for max demand, half the output rating. Think building regs specifiy minimum COP for new build of about 2.7? although this is at a relativly mild external temperature and so the actuall figure in colder outside temperatures will be a bit lower, so assuming 2 as worst case gives you a bit of headroom. If properly designed, it should be running quite a bit above 3 for most of the year.

18kva should be more than enough for a domestic dwelling, as long as you keep away from instanatous electric showers....
If I consider my own house, rather large three floors, and on a 60 amp fuse, which has never blown. So below 14 kVA. However heating is oil at 19 kW so that alone exceeds the 14 kVA, and a car charger likely 7 kW so if I went for heat pump and electric car it would need at least some load shedding when house using too much.

A consumer unit is normally rated at 100 amp, so that's the upper limit, and the supply fuse tends to be 60, 80 or 100 amp.

My boiler does not run continuously, and is either off or on being oil. The heat pump will be the same, so if 8 kW then it will use 35 amp when running, and the EV charging point another 30 amp when running, so you need around 65 amp more than me. So with a 100 amp incomer, you have around 35 amp for the rest of the electrics in the house.

Now I have lived in a caravan with a 16 amp supply which included heating, so it can be done, but I would be looking for either a petrol car or a oil or LPG central heating system. I am sure I could use a 8 amp supply to charge my car with the distances I travel, but you would likely not find you travel so little.

The economic way to heat a home is as required, so if not home, you don't heat it, same with a room not being used within the home, but to do that speed is rather important, using fan assisted radiators you can reheat areas fast, but then you need a large radiator, and boiler, or heat store.

My brother-in-law used a heat store, it was very good, solar, solid fuel, LPG, or electric all could heat the heat store, then when I wanted to use a room, or the whole house, he could use the heat stored in the two large tanks, seems a really good system, but needed the tanks to be on upper floor so thermo-syphon takes the heat from solid fuel, it does not rely on electric, so floors need to take that weight, on looking to fit it in new house, told starting at £24k, he is same age as me, at 72 he will never get that much back, so simply not worth fitting.
18 kVA (kW) = 78 Amps at 230 Volts single phase. Probably 80A DNO fuse.
7 kW will be used by a car charger (overnight). Faster chargers need 3 phase supply.
Some larger ASHPs are also 3-phase.

So check out exactly what you want (charger) and need (heat pump) from suitable designers.

One example ASHP 16kW output product has a max of 5.2kW input but also mentions a max 40A current (possibly startup inrush despite 'soft start' inverter electronics). So that one a COP of 3? 5.2 kW electricity in x 3 = 15.6 kW heat out. (@ericmark has calculated for resistive COP=1 heating)

Watch out for ASHPs with backup / booster immersion heaters - can be 2 x 3 kW jobs with a COP of 1 in coldest times. Many in the know disable them once the installers have left site.

You may need to be 'careful' occasionally with other loads. Solar panels and battery storage may be worth considering, even if costly?
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whether 14kVA sounds about right as a diversified load for a property like this? (
Good enough.
In reality, it will probably be a lot less.

air source heat pump (maybe 12kW - 16kW output)
Probably too big, but with a worst case scenario COP of 2.5, and it running at max output (which it won't be most of the time), that's about 6.5kW.

The EV charging is mostly irrelevant, as those can adjust the charge power when other large loads are in use.

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