Solar panels but increased energy usage?

Reed_Richards

Nick_Dr1 said:

PS A heatpump doesn't operate like a gas or oil boiler. It won't heat your house rapidly. It heats it slowly and then tries to keep it at that temperature by slowly dribbling heat in to replace that which leaks out. If you get a 1 degree rise in temperature in an hour then you are doing quite well!

That's half true.  You can get a heat pump to heat your house pretty quickly if you want it to; fix the leaving water temperature at the maximum that the heat pump is capable of.  The trouble is that that is the least economical way to operate your heat pump.  But if for some reason you come home to a cold house then that is the quickest way to get it warm again.  

mickyduck55

As stated previously changing Tariff may help plus in the summer you will probably have excess electricity that you can export to the grid and payment for this will offset your winter use.

Tiger_greeneyes

Nick_Dr1 said:

We also recently moved from oil to heat pump so are still learning how to best run it.

However, on quick calculations (details later) it should be the same or slightly cheaper overall to run a heatpump when compared with oil, but could be much cheaper. The actual savings (or not) will depend on the cost of oil and the cost of electricity, which obviously vary and change regularly.

With a heat pump you are taking heat from the outside and squashing (pumping) it into your house. This takes energy and so uses electricity. For every 1kW of electricity you will get 3 to 4kW of heat squashed into your house.

This heat would previously have come from the oil burner.
1 litre of oil contains about 10kW of heat (round numbers being used)

If a litre of oil costs 50p then the cost per kW of heat is 50p/10kW = 5p per kilowatt.

If a kw of electricity costs 25p (about the cap) and your heatpump gives (pumps) 4kw heat per kW of electricity then each kW of heat costs 25p/4 = 6.25p

But as pointed out, switching to a different tariff for electricity will change the balance. Lets say you go onto Octopus Cosy and manage to get your whole use into the cheaper periods so it only costs 12p per unit of electricity (unlikely but again lets use round numbers). Then the heatpump calculation looks a bit different. Now its 12p/4=3p per kW of heat.
Now, at 3p per unit, it is much cheaper to run than the oil boiler, which was 5p per kilowatt. Oil would have to drop to 30p a litre to compare.

As you have solar, your average price per unit of electricity could be cheaper still. For example, if you manage to generate all the electricity you use on a day, then your cost to use you heatpump becomes zero. Similarly if you manage to only generate half the electricity you use, then the cost to use the heatpump that day would be halved. So if the normal electricity price was 12p unit (so heat was 3p unit as above) your effective price per unit of heat drops to 3p/2 = 1.5p unit of heat. Oil would still be 5p unit for comparison. 

All this is grossly simplified (it assumes no other use of electricity  for example , and that a COP of 4 is achievable, and 100 % efficiency of the boiler amongst other things), but bottom line is the cost to run the heatpump should be the same or better than a boiler and because it driven by electricity, there are options with tariffs and solar panels to get the AVERAGE cost right down.

I hope that helps. I find its easier work these things out if you keep it simple and go step by step!

PS A heatpump doesn't operate like a gas or oil boiler. It won't heat your house rapidly. It heats it slowly and then tries to keep it at that temperature by slowly dribbling heat in to replace that which leaks out. If you get a 1 degree rise in temperature in an hour then you are doing quite well!

Hi Nick

That really is fantastic, thank you! It all makes so much sense and clarifies my thinking. I will look into other tariffs for sure. We have our whole roof covered with solar, 8 of them, so we generate as much energy as we can through that. We will also be getting a battery so that will help tremendously. I think we must have a pretty powerful HSP too, half an hour after turning on the heating, the house is toasty and that is why we have been keeping the thermostat on 16c, we don't want to use more heat than we need to.

I grew up with a standard gas heater to heat the whole house and eventually I had central heating. I loved the oil fired heating as we had far more control over it, it was cheaper and very efficient, A five minute blast and the house was warm. Solar and ASHP are pretty confusing to get to grips with but I'll get there. I'm very happy with it so far as my main concern was it would take forever to warm up but you're right, once it's up to temperature, it stays there.

Thanks again Nick, you've given me way more confidence in what I'm doing!

Nick_Dr1

A battery is really useful in winter if you are on the right tariff. You charge it at the cheap rate and run the house on it during the expensive rate. You just need to size it to allow this to happen.

In the summer its also very useful because you can charge it on all that free excess solar, but not quite as useful because it is possible to sell excess back to the grid at a good rate, effectively using the grid as a giant battery!

Don't get too big a battery though because it may not be possible to charge it fully in the cheap period. For example, our inverter can only supply 3kW max DC to charge batteries. This is ok because our batteries can only charge at a maximum of 3kW anyway. Under those conditions I can get 3kWhrs into my batteries per hour and as I have 6kWhr capacity it takes 2 hours to fully charge them. It won't go any faster, even with solar (only slower is possible!)
Likewise the maximum discharge from my batteries is limited to 3kW, so if my house asks for more eg I've got two kettles going for some reason, the house will still draw 3kW from the grid as well as 3kW from the battery.

Most situations is fine, but just be careful - there are limitations that are not immediately apparent.

badmemory

My problem is my 8.5kWh shower.  Even if the sun is blazing & the battery is full it still has to draw some from the grid.  It all helps though when you have a member of the household using around 4 kWh just for a long shower.  It does make you think about different things though.  My latest "oh I wonder" moment was whether when my kettle needs renewing would it be better replacing the 3 kWh with a 2 kWh as it would reduce the spike & therefore less likely to take from the grid & does that make sense?

Alnat1

badmemory said:

My problem is my 8.5kWh shower.  Even if the sun is blazing & the battery is full it still has to draw some from the grid.  It all helps though when you have a member of the household using around 4 kWh just for a long shower.  It does make you think about different things though.  My latest "oh I wonder" moment was whether when my kettle needs renewing would it be better replacing the 3 kWh with a 2 kWh as it would reduce the spike & therefore less likely to take from the grid & does that make sense?

I've now got a 2kWh kettle for just that reason and it definitely helps. Also better when hubby once again puts the toaster on before the kettle has finished boiling, grrrrr. 

badmemory

With me it is putting the kettle on when the dishwasher & washing machine are already on.  Thank goodness for the distribution board fitted a few years ago which only needs a switch flicking.

Martyn1981

badmemory said:

My problem is my 8.5kWh shower.  Even if the sun is blazing & the battery is full it still has to draw some from the grid.  It all helps though when you have a member of the household using around 4 kWh just for a long shower.  It does make you think about different things though.  My latest "oh I wonder" moment was whether when my kettle needs renewing would it be better replacing the 3 kWh with a 2 kWh as it would reduce the spike & therefore less likely to take from the grid & does that make sense?

Makes perfect sense. We have a 2kW kettle, but to be honest it really just sits there as a reservoir to refill the 1kW travel kettle. Been using one for 12(ish) yrs*. But the biggest advantage (I think) is actually that its minimum level is less than a mug full, whereas many (most?) bigger kettles need about 2 cups minimum.

So I only boil enough for one cup, and no waste.

*The first one lasted about 10yrs, but the 2nd, and 3rd only about 3 months each, but the 4th has been fine for over a year now. [3rd and 4th were free replacements after paying for the 2nd.]


Regarding the shower, I'm pondering similar. This winter I'm experimenting to see if I can avoid using the GCH. If that works, then I need to solve the DHW (domestic hot water) issue. I thought about an inline heater for all DHW, but at 9kW+, it will mean pulling dayrate leccy, so I'm pondering a hot water tank, heated overnight on cheap rate, or from PV in the sunnier months.

Spies

Buy a kettle with a concealed element then the minimum is basically one cup.

badmemory

I have no cylinder & anyway the reason I have a power shower is because the pressure wasn't up to it.