In a previous article we outlined the main principles for sizing the battery system:

• size your battery system so that your grid offtake does not exceed 2,5 kW, over the whole year
• benefit as much as possible from your PV production
• subsidiarily try to shift your offtake (for battery loading) into the off-peak time window

Of course, depending on your situation it may not be possible to always satisfy them all, or to satisfy them all economically. But let us examine each point.

The very first thing, if you have an EV: it should not be connected to your system, but in parallel with it, otherwise it will empty your home battery very quickly. Also provide your EV charger with a power sensor, measuring the total offtake behind your meter. Most, if not all, charging poles are provided with this kind of sensor.

Of course, charging at 2,5 kW will take a lot of time starting form a nearly empty EV battery, but if you charge it each time it stands in your garage, it will be sufficient for most users. If possible, program the charging during the off-peak time window. If 2,5 kW it is too slow for you, charge it at 3,7 kW (16A with a 220-240V monophase) and take this value as minimum offtaken power because you will then reach it every month (except if you are abroad for the whole month). But below we’ll go further on a 2,5 kW basis.

Lowering your maximum offtaken power below, or rather equal to, 2,5 kW is quite easy. If you know how much your actual maximum is, you require your system to be able to deliver the difference of this maximum to 2,5 kW. If you don’t know this maximum, the power at which your main fuse will trip lies above that maximum, so you’ll never offtake more than that. Of course, you’d rather measure your maximum during a representative week.

This maximum to take into account is not the maximum of the instant power ever offtaken, it is the maximum of energy offtaken during a quarter of hour, multiplied by 4. This is quite lower, for the large majority of the cases it is even much lower, than the maximum of the instant power ever offtaken.

If your main fuse is not oversized, this maximum lays generally below 75-80% of its trip value. For a 9,6 kW fuse trip value, the maximum is very often below 7,4 kW. This situation is widely spread so that we will go further on this basis, thus your system must be capable of delivery 5 kW and take the eventually extra from the grid, which will de facto never exceed 2,5 kW, provided your battery is not empty during the quarter of hour of your peak.

Now, we have the power, but how much energy should the battery be able to store? This depends on two factors: your daily offtake and its profile along the day. I’m using offtake and not consumption, because if you have solar panels, at every moment they produce, you can benefit from this production to feed (partially or totally) your consumption, only the remaining need, if any, must be offtaken (from the grid). Conversely, only the remaining excess, if any, must be loaded into the battery.

If you don’t dispose of your maximum daily energy offtake, as a rule of thumb you can use your daily average energy offtake and multiply it by 1,25 (thus adding 25% to your average daily offtake is a good estimate for your daily maximum) if you have a single (total hour) meter and 1,65 if you have a double (peak / off-peak) meter. This difference comes from the consumption shift you achieve if you have a double meter, for instance to the 2 week-end days, that are full off-peak days.

But you don’t have to store all your daily energy need in the battery, indeed you can offtake during the whole day at maximum 2,5 kW and only consume the remaining out of your battery. Doing so, you will get the smallest and thus cheapest battery set. However, on an exceptional day, you risk not having enough and exceed the 2,5 kW offtake from the grid. Another drawback is that you will probably offtake during the peak hours (if you have a double meter).

You could also choose to reload your battery set when your offtake is low, e.g. when nobody is at home or rather only when low consuming appliances are used. With the same drawback.

Individual situations can vary widely, for instance, do you use a tumble dryer several times on some days, is your fridge small and of A+++ class or huge, do you have solar panels and then of how much power (kVA) …

To avoid an excess in investment, one would start with a 5 kWh battery set. Fully charged at the start of the day it would normally enable to afford a lot of situations. If you have a double meter, rather fully charged at the end of the first off-peak period of the day, to realize an offtake shift to the off-peak hours.

Besides, schedule your system to reload the batteries to 30 – 50 % in the middle of the day, surely in the week-end if you have a double meter, and monitor the state of charge (SOC, amount of energy remaining in the battery set) of your system during the first weeks to fine tune the scheduling to secure your offtake limit of 2,5 kW. Choose a system able to host an extra battery pack (of 10 kWh), for the case it would ever be necessary.

Solar panels play an important role during the sunny months (April – September), hence they can already have a significant impact from February till October or even November, depending on their power relative to your consumption.

If their yearly production (yearly kWh ≈ 1.000 × kVA, thus 1 kVA delivers roughly 1.000 kWh/year) is nearly equal to your yearly consumption, they will influence your scheduling from February till October, it is e.g. the case for a dwelling equipped a 3,5 kVA PV-inverter set and a yearly consumption of 3.500 kWh. From earlier to later in the yearly production exceeds your yearly consumption, and conversely from later to earlier in the opposite situation.

This means that you will have to adapt the SOC charging point in your schedule according to the expected solar production of the day. Your battery (management) system should make it possible to steer the scheduling automatically through an external signal (the expected/forecasted solar production).

Charging schedule, coupled with the SOC, is also the main point for shifting your offtake into the off-peak time window (or the cheapest hours if you are already subject to several time windows, as in Wallonia). The goal is to reach the desired SOC just at the end of the cheapest time window. E.g. if you have a double meter and your municipality has set the off-peak between 22:00 and 07:00 (from Monday to Friday, and on week-end days that are not feast days), you’d to reach the desired SoC at 06:59. If you don’t want to exceed 2,5 kW offtake and you have a battery set with 5 kWh storage, reaching 100% SOC will take a bit more than 2 hours, let us say 2h15, thus schedule the charging to start at 04:45, on not sunny days.

For the reload in the middle of the day, you can set the target SOC on 30 to 50%, between 12:00 and 15:00, surely in the weekend if you have a double meter.

The system must enable you to define the desired SOC for each day of the week and to adapt it from your computer and smartphone. To benefit from your solar panels production, you can adapt the target SOC, taking into account your daily (estimated) consumption and production, and eventually schedule an extra charging in the middle of the day if necessary and suitable.