Dan G

I thought this question would get more interest 😄 MCS guidance recommends undersizing, and I was told the Solis design software does too. Just wondering what others think.

It seems to be something of an industry standard practice to under solar inverters relative to the power of the solar panels fitted, typically about 25% below the rated max of the panels. For example, I know of an 8 kW panel system facing due south with a 6 kW inverter (feeding home, EV, Powerwall, and grid); while at the other end of the scale (and country) the well-respected, long-established local company who installed my system insisted that a 1.6 kW inverter was the right size for 2.1 kW of panels (also south-facing). Both inverters are current gen Solis. It was explained to me that the smaller inverter would be more efficient when the panel output voltage is lower on cloudy days, which are obviously the norm for most of the time in the UK. But it just bothers me when I see the system output pegged at 1.65 kW with the sun out... When collecting lots of quotes for a larger rooftop project (20 kW) I noted that some installers were specifying undersized inverters and some were not. It tended to be the younger companies who were not. We've picked a company who happen not to be undersizing (on the basis of price per watt of panel, not that they didn't spec an undersized inverter). What do you guys think? Undersizing inverters, yay or nay?

I don't know if this was ever resolved but for the benefit of anyone wondering by via Google (as I am!), a ten-minute shower requires right around 3kWh of energy. You wouldn't get two such showers from a 3kWh Sunamp!

Hello! This thread is linked from at least one prominent place, and after reading it (twice! Once over a few days, once again quickly) I feel it needs a proper conclusion. The main issue was that ~2018 vintage electrically-heated Sunamps would only begin charging once about 50% depleted, which led to two problems: • With charging provided by a single 3ish kW element, "charging" a unit back to full could take several hours, and if a unit was at say 51% at the start of a day, the rest of the charge could be used up leaving no hot water for some time • Charging units using diverted PV was difficult as the units wouldn't accept charge unless significantly depleted. However this issue was fixed in 2019 with new firmware in the controller, as explained in this post: This meant that electrically-heated Sunamps would begin to accept charge after only a small amount of discharge from full, meaning they play much more nicely with PV divert. (There's still a bit of an issue with night charging on cheap-rate electricity: balancing the risk off running out of hot water against leaving sufficient room for PV divert energy. I think @DamonHD and others are looking at that.) My personal opinion of Sunamps is that they are space-efficient and vastly simpler than fitting an unvented cylinder system when replacing a combi boiler, although using grid electricity to charge is much less energy-efficient than a heat pump charging a UVC: heat pumps invariably achieve a CoP of at least 2 when heating domestic hot water. The lower rate of heat loss from a Sunamp than from a UVC only partially offsets that difference. But a primary reason this thread was started to discuss has been addressed.